Sherlock assists Gregson with a double homicide investigation that has the same modus operandi as a series of murders from 13 years earlier. That case led to the conviction of Wade Crewes, who is still in jail, and gave Gregson his big break. While Gregson looks for a copycat killer, Sherlock digs into the old case. He discovers that either Gregson or his then partner planted a mug with Crewes' fingerprints at a crime scene, which proved vital in obtaining Crewes' conviction. Sherlock questions whether Crewes is guilty after all. Crewes had no visitors during his incarceration, so an accomplice seems impossible, that is until Sherlock realizes a prison library worker named Sean Figuero taught the illiterate Crewes to read. Sean turns out to be Crewes' illegitimate son and his accomplice.

An under-the-weather Sherlock investigates the murder of a college professor who was shot in both eyes. Sherlock retraces the victim's steps back to the illicit gambling parlor in Chinatown where he was killed. Gregson is able to bring in the gunman thanks to video surveillance footage, but the killer claims that the victim himself hired him as his executioner. It turns out the professor had an agonizingly painful, untreatable cancer and had only a few months to live; his condition could have been detected by the appearance of his eyes. The professor had lured a Chinese woman to the United States, promising to marry her, but instead abused her physically and psychologically. The killer, by changing the location of the killing, messed up the victim's plan to frame her lover (his teaching assistant) for the murder.

Sherlock and Joan investigate the murder of Teri Purcell, the general manager of a high-end Manhattan hotel, whose body was stuffed into one of the hotel's industrial driers. The woman's background and family seem clean, but a call girl reveals that the victim secretly helped her and her cohorts to ply their trade at the hotel, and at no charge. Teri used the prostitutes to attract high-level diplomats and businessmen to her establishment, where she videotaped them to obtain valuable information. Sherlock discovers Teri was a Russian spy, as is her husband. Teri revealed all this to their daughter Carly, as well as her plans to make the teen a spy too. Carly objected and pushed her mother, causing her to fall and strike her head on a desk, apparently killing her. Teri's handler, Geoffrey Silver disposed of the body and blackmailed Carly into agreeing to become a spy. Teri, however, was not dead. Silver decided that Carly had much greater potential for espionage and killed Teri.

While getting prepped in a hospital for a kidney donation to his very ill sister, notorious serial murderer Howard Ennis (Terry Kinney) escapes from custody and resumes killing. Sherlock is brought in to consult, but he bristles when he learns that he must work alongside Kathryn Drummond, an FBI profiler who wrote a book about Ennis. She and Sherlock have a past, but their association ended when she published an article profiling Sherlock (called "the deductionist" in the article to protect his identity) that predicted, among other things, his forthcoming addiction. Even though Sherlock and Drummond do not get along, they both have a similar idea of what Ennis is likely to do next. The only problem is that Ennis is killing atypically. The experts are stumped until Sherlock realizes that Ennis is out to avenge himself on Drummond because the publicity from her false accusation that his father abused him devastated his family and led to the father's suicide. Watson leads Sherlock to realize that Ennis's sister is working with him. She stabs Drummond when the latter comes to apologize for her unsubstantiated claims; Drummond survives.

While driving home after a long day, Bell is shot at, and his car overturns. He is not able to get a good look at the shooter, but he recognizes the car as belonging to a recently released drug lord who had vowed to get revenge for Bell's involvement in the man's conviction. Then the criminal turns up dead, and all the evidence points to Bell. Sherlock finds the murder weapon, planted in Bell's apartment. Bell also struggles to reconnect with his brother Andre (Malcolm Goodwin), an ex-convict on parole who wants to use his underworld connections in order to help. Andre ends up with two bullets in his back, but survives. Sherlock finally realizes that it is not one of Bell's known enemies, but rather a colleague and former girlfriend who is after him. The career of Officer Paula Reyes (Paula Garcés) was derailed after Bell tipped off the department about a dirty cop whom she admired (and possibly abetted).

While helping Joan learn deductive skills, Sherlock is approached by Gerald Lydon, a wealthy man suffering from an incurable disease that is destroying his mind. The disease is genetic, but there is no family history of the disease, so Lydon is certain that someone gave it to him somehow. Sherlock is unconvinced and refuses the case. After Lydon kills his driver, however, Sherlock takes the case and learns that Lydon's claim is theoretically possible. He receives an email from Dr. Natasha Kademan providing details of how it was done, but before they can meet, she turns up dead. All the evidence, including DNA results from blood found at the crime scene, point to the killer being an ex-con with no connection to Sherlock's case. However, the blood turns out to have been manufactured specifically to pass the DNA test. Sherlock deduces the killer is Kademan's husband, who believed she was having an affair with someone named Lincoln Dunwoody. He was mistaken. The wealthy Lincoln and Dunwoody families each established a foundation to conduct research on Lydon's disease because a member of each family contracted the disease. It turns out that a brilliant scientist suffering from the disease poisoned all three wealthy individuals to obtain funding for research for a cure.

A security guard is killed during a robbery of unreleased mobile phones, but shoots one of the criminals before he dies. Sherlock and Joan work out that the robbery is a smokescreen to steal blueprints and head to New Jersey to prevent the East Rutherford Operations Center (EROC) from being burglarized during a snowstorm; the city is locked down, with roadblocks to aid emergency responders. Sherlock pays Pam (Becky Ann Baker) to drive the pair to EROC in her snowplow. Sherlock realizes the burglary has already taken place - $30 million in used notes about to be shredded - and the loot removed by ambulance. NYPD enquiries at hospitals put the shot criminal into custody, but she refuses to reveal the whereabouts of the loot-filled ambulance. Sherlock realizes that an escape route out of the city for the ambulance has been cunningly plotted, with roadblocks being removed one by one. The visiting FEMA official helping out with the city's emergency response to the snowstorm is involved in the heist. The remaining two robbers are apprehended.

Moriarty calls Sherlock to have him look into the several-month-old unsolved murder of a mechanic; in exchange, he offers to give Sherlock answers about Irene Adler's death. Sherlock learns the mechanic was under surveillance by a private security firm whose founder's sister was killed 20 years ago. After Sherlock tells the founder, Daren Sutter, that he knows Sutter avenged his sister and that someone has bugged his home, Sutter confesses to the mechanic's murder. Moriarty, however, tells Sherlock that he has not uncovered the whole truth. Sherlock discovers that Sutter did not see his sister's murderer's face as he fled. His wife (who was married to someone else at the time) did, but she could not come forward, as it would reveal her affair with Daren. When her husband's mental health began failing as the 20th anniversary of the murder approached, she convinced him that the lookalike mechanic was the murderer. It worked; Daren was finally at peace with himself. However, the mechanic was out of the country at the time of the sister's death. Mrs Sutter is arrested.

Sherlock receives a call from Scotland Yard about a missing inspector, Gareth Lestrade (Sean Pertwee), who worked with Sherlock. Sherlock and Joan travel to London, where Sherlock easily locates Lestrade at a bar and agrees to help solve the case that caused Lestrade to be suspended. Lestrade accused wealthy Lawrence Pendry of killing his wife, but had no proof. Pendry claimed an armed intruder shot his wife; the police arrived within minutes and the gun could not be found. Sherlock, Joan and Lestrade pore over evidence. With the couple following a diet that excluded milk, Sherlock is intrigued by the bottle of milk in their fridge and a line of artworks out of alignment. Sherlock and Watson visit the suspect's house, where Sherlock discovers a chipped nail, leading him to the conclusion that a (white) plastic gun was made using a 3D printer; Pendry later dissolved it in acetone in a milk bottle after shooting his wife. The only metal part was the firing pin; Pendry used the nail to rehang a piece of art, but in his haste misaligned it. Tracking 3D printer purchases leads them to Pendry's handyman, who is found stabbed to death. Plastic fragments found in a fruit bowl at the scene prove Sherlock's hypothesis that Pendry printed a second gun to tie up a loose end. The gun exploded when he fired it because he used the wrong ammunition, so he had to stab the handyman with a kitchen knife. Pendry's arm injuries from the explosion secure his arrest.

Sherlock and Joan investigate the murder of mathematician Felix Soto, whose walls are covered in invisible ink with his work. A colleague of Sherlock's reveals the work is about the P versus NP problem, one that has bedeviled mathematicians. Holmes and Watson are directed to a professor, Tanya Barrett (Lynn Collins), who has written about the problem and all those who attempted to solve it, including the victim and his research partner, Cyril Nauer. Nauer is also murdered, within hours of Soto, and a mugger was shot and left for dead after seeing the killer leave Soto's apartment. Sherlock realizes Nauer's apartment was bugged and under CCTV surveillance and tracks down Roe, the man responsible. He tells Sherlock the proof that P equals NP would render all current encryption useless. It would be worth tens, maybe hundreds of millions to a company like his that could use the proof to develop new encryption methods before anyone else, so he had no motive to kill the men. Roe had Barrett evaluate the dead men's work; she told him that, while they had made great progress, they were only a third of the way to a solution. Sherlock comes to suspect Barrett has solved the problem herself, but needed more time to (criminally) cash in on it. A security video recording shows her at a bar with a friend at the time of the murders. Watson studies the footage and notices the drinks are too cheap; the video is from happy hour, not later in the evening like Barrett claimed. Barrett is arrested, but the federal government takes over in the interest of national security.

After leaking state secrets to the public, a government contractor named Ezra Kleinfelter (Christian Campbell) is on the run. A mysterious man hires Sherlock and Joan to locate him. Sherlock discovers that his new client works for Ezra's former employers and would most likely kill him if they found him. Sherlock identifies Ezra's connection with "Everyone," a group of cyber-activists, and deduces the identity of the female Everyone member harboring Ezra, but they find Vanessa Hiskie dead in her apartment. They suspect Ezra killed her, probably for resisting his romantic advances, but find a box he took from a bunker where he was previously hiding and use it to trace his new location. Sherlock steals a guard's phone, realizing he is passing messages to Ezra, but Everyone traces it to Sherlock and Joan and wreaks havoc on their digital lives. Meanwhile, one of Joan's friends posts her profile on a dating site. Everyone posts horrible things supposedly written by her, as well as her address. The hackers also disconnect her and Sherlock's cell phones, prompting a man she showed interest in on the dating site to show up at her house to check up on her. Sherlock catches up to Ezra as he tries to leave the country on a plane owned by a very wealthy former software engineer and Everyone member. Ezra threatens to expose 14 American spies unless they let him go. Sherlock contacts the man who originally hired him on the case, Mr. Honeycutt, and convinces him to release the list of names Ezra has to the authorities so they can get the spies to safe houses. Ezra is extradited and arrested, not for the leaks, but for Hiskie's murder.

At the morgue searching for a case, Sherlock discovers one of the dead, Leo Banin, is former Russian mafia (Bratva) assassin Vitaly Andropov, who garroted someone the day he died in a motorcycle accident that also claimed a young woman. Sherlock and Joan learn that he was working as a nurse and building a nursing home. Near the scene of the accident, Sherlock and Joan find the body of his victim, another hitman who targeted Banin for stealing $30,000 from the Bratva. They uncover bullet casings that indicate that Leo was being shot at, and that is what caused the accident. Finally, Sherlock discovers that it was Leo's wife who alerted the mobsters to his location. Banin was going to divorce her after discovering her participation in adult films to pay for her passage to the United States, leaving her nothing. Banin stabbed his first would-be assassin; Banin's wife's used fabric from her drapes for a tourniquet for the man's wound. It was she who shot at her husband after the second failed attempt.

An intruder invades Captain Gregson's home, terrorizing his wife and asking where he can find her husband. She manages to shoot him, and he runs away. Sherlock and Joan comb through Gregson's old cases for suspects. Then two murder victims surface, including Gregson's neighbor Lieutenant James Monroe. Sherlock realizes the killer went to the wrong house (due to a Google Maps error) and is not after Gregson at all. Tying the victims to their joint service in Afghanistan, they investigate an ex-soldier who attacked one of them before being discharged from the Army, but he has no bullet wound. They discover the men were assigned to guard an archaeological site. Sherlock questions Beth Roney, the archaeologist in charge, quickly deducing that she stole artifacts from the dig and is now killing her accomplices. They remember seeing a valuable bowl, one of a set found at the site, in Roney's home, but when they search her place the next day, it is gone. Sherlock realizes her ex-husband must have taken it; Roney's dog barks at unfamiliar men, but the neighbors heard nothing during the night. After being told of evidence incriminating him, the ex-husband confesses and implicates his wife.

Mycroft arrives in New York City, where he is opening a new restaurant. He asks for Sherlock and Joan's help. His ex-fiancée, Nigella (Olivia d'Abo), is in trouble. Sherlock had slept with Nigella to prove to Mycroft that she was after his money. So, instead of marrying Mycroft, Nigella landed a marquess, making her a marchioness. She had an affair with her stablemaster, which led to divorce. She fought hard to get a retired champion racehorse named Silver Blaze as part of the settlement. The stablemaster is killed by someone trying to poison Silver Blaze. Nigella is charging $100,000 stud fees for Silver Blaze's services. Sherlock proves that the horse died of natural causes, so Nigella substituted his lookalike brother. One of her clients, however, was Joaquin Aguilar, the local head of an international drug cartel; he found out and sought revenge using a cartel hitman called "El Mecánico". Sherlock sets a trap, expecting El Mecánico to take another shot (literally) at Nigella. It works. Sherlock and Mycroft promise not to turn her in as long as she pays back her victims, with the exception of Aguilar. Sherlock threatens to reveal that Aguilar sold his foal to another unsuspecting horse lover, unless Aguilar stops seeking revenge.

The NYPD investigates the death of Haley Tyler, who was stabbed before she fell off a balcony and landed on a truck. Haley was apparently the mistress of technology mogul Ian Gale (William Sadler), whom Sherlock and Joan are unable to locate until they learn that he is dying and in need of a transplant. Gale reveals to them that Haley was not his mistress, but rather his illegitimate daughter, the result of a one-night stand. She has the same very rare blood type as Gale, and has been donating blood to him. Evidence points to Gale's wife (Margaret Colin) due to a will amendment giving 20% to Haley, as well as the precise location of the stabbing, suggesting medical knowledge; Mrs. Gale was a doctor. However, Mrs. Gale suggested the will change herself. The detectives locate Haley's former boyfriend, and though the man is innocent, he informs them that Haley was ill the week before she died. Ian Gale dies, prompting Sherlock, Joan and the NYPD to confront Mrs. Gale at her home with the murder of both Haley and her husband: she gave Haley medicine that made her body overproduce antibodies; transfusions of her blood ultimately killed Gale, and Mrs. Gale murdered Haley to remove a loose end.

A schizophrenic young man, dressed as a knight, wanders into the police station with a gun and states repeatedly that he had to kill the queen. Sherlock manages to defuse the situation by playing along with his delusions. The story is being told by Sherlock in a hearing. After the man is identified as Silas Cole, Sherlock and Joan find his ex-girlfriend dead, shot through the heart, but Sherlock is convinced that Cole did not kill her; his "knight's code" would have prohibited destroying her heart, thought in medieval times to house the soul. In the course of their investigation, Sherlock unintentionally gets a man fired by revealing he is out on parole. Bell is shot in the stomach, jumping in front of Sherlock when the man comes gunning for him. Sherlock and Joan eventually discover that the victim's heart was enlarged; she was participating in a trial drug program, and her doctor killed her to cover up the drug's failure.

Hedge fund manager Donald Hauser is shot in the knee just as he is about to commit suicide after his Ponzi scheme is uncovered. He is tortured for information before finally being killed. The woman who finds the body is a former sober client of Joan's. She refuses to allow Joan to reveal their relationship, hindering the investigation when her former drug dealer becomes the main suspect. The reporter who broke the news of Hauser's fraud is murdered the same way hours after the first killing and her laptop is stolen. Sherlock and Joan surmise that Hauser turned over evidence to the reporter that contained information about another crime. Later, the body of the drug dealer is found in a dumpster behind an art gallery in which he is a silent partner, further confusing matters. It turns out that Jacob Weiss, a client of Hauser's who runs a nonprofit recovering money for the heirs of Holocaust victims, was embezzling the funds. He used the art gallery to launder the money. Hauser knew about it, but made the mistake of informing Weiss that he was going to reveal everything.

An assignment with the Demographics Unit leads Bell to discover a body in a barrel. Joan identifies the victim as a member of the Pardillo Mafia family who had been in hiding for years. She identifies a member of a rival family as a suspect. Sherlock and Joan get no information from him, but he dies from a car explosion as they walk away. Paperwork at the scene leads Sherlock to believe the National Security Agency (NSA) supplied the killer with information about Pardillo's whereabouts. Sherlock confronts an NSA agent, accusing the agency of having a leak. The agent later tells him that Bell's new boss, NYPD Deputy Commissioner Da Silva, requested the information. (Da Silva was trying to dispose of everyone who knew about him working for the Mafia for his entire police career.) After Sherlock informs Bell, Bell steals evidence linking Da Silva to the Pardillo family. They set a trap; when he tries to assassinate Robert Pardillo (Paul Sorvino), the victim's father and head of the family, the police are waiting.

While investigating one of Sherlock's cold cases, Joan spots an out-of-place rock in the murder victim's garden. She enlists a geologist to study the photo. Joan eventually steals the rock. It contains a very rare complete dinosaur fossil worth millions. The rock is later stolen from the police evidence room. Tracking down a very high-end smuggler, Sherlock and Joan find the man murdered and the fossil smashed to pieces. They eventually determine the curator of a natural history museum committed the murders to destroy proof of a scientific theory that would have ruined his reputation.

When a ballerina is murdered, evidence seems to point to the dance company's star performer, world famous diva Iris Lanzer (Aleksa Palladino). The killer steals a hard drive containing the venue's surveillance footage. Sherlock is convinced Lanzer is not guilty; he accepts her offer to sleep with her. An audio recording is leaked to the tabloids, it reveals Iris's intimate relationship with the victim, but Sherlock deconstructs the recording and deduces that Iris's lawyer is the murderer, using the publicity to further his career. The police find the stolen hard drive in the lawyer's home.

Gordon Cushing's wife disappeared in 2010; he was acquitted in a murder trial, but many people still think he is guilty. One day, he receives two severed human ears along with a ransom note. The DNA matches that of Cushing's wife, Sarah. At the ransom exchange, Cushing pursues the suspect and unintentionally kills him. Clues on the suspect's body lead Sherlock and Joan to an AA meeting, where they find Sarah, who is both alive and not earless. Sarah says that the DNA they collected when she disappeared was not hers and belongs to her husband's mistress; however, she died in 2011 in a car accident. Sherlock realizes that Sarah's plastic surgeon husband grew two exact replicas of her ears on her back and then cut them off in an elaborate attempt to collect the ransom from Cushing.

Sherlock and Joan investigate the murder, in the form of staged suicide by helium asphyxiation, of Barry Granger, a researcher who had been testing the "Hound", a breathalyzer for detecting cancer. His research had been alleged as fraud by an expert in the field under the pseudonym Adam Peer. The allegation looks set to ruin both Granger and the firm that employed him, a company run by Hank Prince. Sherlock discovers that the allegation is false, and Adam Peer was actually two people: Granger and another. Prince's wife is found shot dead, with Prince's gun the murder weapon. Sherlock realises that Prince really did murder his wife. Earlier Prince had tried to trash his company's value by masquerading as Adam Peer and impugning Granger, so that his wife would receive in their divorce only a share of the depressed value of the company, which would then rebound once Granger's research was vindicated.

Bell, recovered from his injury and returned to field duty, asks Joan to track down a missing witness to a street killing. She changed her mind about testifying after she became pregnant and is hiding with a former tutor, Manny Rose (Ron Canada), whose reputation as a community do-gooder is known to Bell. When Bell tells Rose to tell the witness that her testifying does not matter as they will get the perpetrator next time, he is summoned to the morgue where he finds the killer and Rose both shot to death - Rose had decided to dispense his own vigilante justice.

Bite marks on two murdered women echo the trademark of Aaron Colville, a man convicted as a serial killer who subsequently died in prison. This raises the question of Colville's possible innocence. The bite marks are traced to ex-con Alan Vikner with the help of hacker collective Everyone. However, when he is brought in, Vikner shows that he has dentures; eight sets were made from the same mold (of Colville's teeth) for inmates at Colville's prison. Eventually, Sherlock and Joan eliminate all the convicts as suspects. Finally, Sherlock realizes that the person who stands to benefit the most from exonerating Colville is his mother, who has sued the city for millions of dollars in his alleged wrongful conviction.

When a pickpocket collapses in jail, Joan starts to help, then realizes he seems to have contracted anthrax. He stole the anthrax from someone, thinking it was cocaine, then swallowed it when he was apprehended. The man dies. One of the pickpocketed items leads to Charlie Simon. He had the expertise to make anthrax spores, but they find him dead at his storage locker laboratory. From the empty trays there, Sherlock deduces he made 40 pounds of the stuff. Eugene MacIntosh, a member of an anti-government group known as the Sovereign Army, becomes the prime suspect. He is shot dead by his brother Bart when he goes to Bart's failing farm. However, it turns out that the brothers were actually trying to kill off Bart's herd of cows for a multimillion-dollar insurance scam.

A double murder appears to be revenge for the deaths of children poisoned by GHB, the inadvertent result of cost-cutting measures taken by the Indian manufacturer of toy beads. Each of the victims received five orange beads in the mail. The father of one of the children confesses, but Sherlock deduces that he is lying. FBI agent Vince Boden refuses to provide whatever information the agency has, citing policy. Evidence vital to the case against the murdered men has gone missing, which complicates figuring out the motive for the crimes. At first, Sherlock suspects the prosecutor, US Attorney Angela White, as the collapse of her case would end her ambition to be elected to a Senate seat. White admits to being blackmailed by one of the murdered men, but denies killing them. Then Sherlock realises who the killer is. Boden wanted the case to never come to trial, so he could then easily steal the no-longer-needed evidence: the GHB, an addictive drug worth millions of dollars on the black market.

[Warning: this episode contains instances of rapidly changing bright images mimicking an epilepsy trigger.] Software developer Edwin Borstein has created an artificial intelligence (AI) program called Bella, and he hires Sherlock because someone has broken into his company and stolen a copy of the program. Sherlock takes the case, not because he finds it interesting, but because he does not believe AI is achievable. He solves the theft case, but then Borstein dies from a fatal epileptic seizure, seemingly caused by Bella through rapidly cycling images displayed on Borstein's monitor. Sherlock finds a program hidden on a music CD death metal fan Borstein was given. Eventually he traces the criminal to a think tank that believes that AI is the greatest threat to human existence. He is confident that its leading light, computer science professor Isaac Pike, is responsible, but a devotee confesses, despite having no programming skills.

Joan returns from Denmark, and she and Kitty begin to get better acquainted. Kitty is called to a museum, where valuable old maps and a security guard are missing. She quickly finds the guard dead and hidden on the premises. Sherlock deduces that one particular map was the target; the rest were taken just to camouflage that fact. That map, "The County of King James, Virginia, 1794", was loaned to the museum by a now-deceased member of the Bray family decades before. The family's charitable organization still exists, and is run by Margaret Bray. Bray agrees to use her contacts and see if other institutions were also victims of map theft (but too embarrassed to notify the police). Sherlock identifies the thief as ex-cartography student Stuart Zupco, but Zupco turns up dead. The missing maps are found at his framing shop, but Sherlock quickly ascertains that the Bray map is a forgery. He discovers that it and the original (from an old photo) are almost identical, except the course of a section of a river has been altered. That one, almost-unnoticeable change provides the motive. A billion-dollar casino, envisioned by billionaire real estate mogul William Hull (Skipp Sudduth, see also Season 4's "Up to Heaven and Down to Hell"), is to be built on land affected by the alteration. It turns out that Margaret Bray's family owns property neighboring the site; the casino would send the value of the surrounding real estate sky high. Bray was the killer; without the map change, the project would have to be abandoned.

Sherlock, Joan and Kitty investigate the murder of Flynn, a Highway Patrol officer, whose sidearm had been replaced with an airsoft gun. Flynn was a drug addict. He stole guns from the police department's Rodman's Neck Armory, replacing them with airsoft guns, and sold the real weapons to fund his addiction. His buyer, Buros, a wanted arms dealer, already had buyers lined up for the prized military-grade police weapons, but Flynn, now clean, refused to obtain more for him. Buros killed Flynn, as he would be given an inspector's funeral attended by thousands of police officers, leaving the Armory poorly guarded, but Flynn's crimes came to light and his special funeral was canceled. So Buros kills another officer to cause another such funeral. He steals the weapons. Watson identifies the white fibers he left at both murder scenes as upholstery fibers - Buros plans to ship the weapons hidden inside furniture. He is arrested after his cargo shipment is traced.

When a nurse Joan used to work with asks for her help in finding their missing acquaintance, the woman's trail leads Sherlock and Joan to an illegal human drug trial of a time-dilating substance known as EZM-77. Five test subjects were injected with varying dosages. The higher the dose the madder they went (one killed the missing nurse, who administered the drug) and they die or are murdered. Carlisle, on the lowest dose, survives and his interview leads to the name of a mysterious company called Purgatorium, which paid the human guinea pigs $150,000 up front and promised the same amount every year. The drug's inventor, Dwyer Kirk, takes full responsibility for the deaths, refusing to reveal who funded him. Sherlock identifies the sponsor - James Connaughton, who paid for Kirk's education and gave his aunt a stipend to help raise him. When Connaughton learned he was dying, he approached Kirk, who agreed to try to develop a "pharmaceutical fountain of youth" so that its time-dilating properties would make his last days feel like years. The trial subjects were executed to cover it up, but Connaughton refuses to confess or co-operate. Sherlock and Watson appeal instead to his new nurse, Brett Won. Shown photos of the dead victims and asked to give their families closure, Won connects Connaughton directly with Purgatorium and with the two hitmen he hired to murder the test subjects. However, when the police come for Connaughton, Sherlock finds the man non-responsive; he has injected himself with EZM-77, hoping to "extend" what little time he has left to live.

The wife of missing lawyer Steven Horowitz hires Sherlock to find him. Sherlock and Joan discover that he did not tell his wife he had been fired by his firm. They find out that he went to work for a debt collection agency and was given a debt package worth millions. Sherlock and Joan try to narrow down the pool of suspects from the thousands of debtors. It turns out that Horowitz had a change of heart after seeing the hardships suffered by the debtors and forgave what he had not yet collected. One of those released from their financial burden is a man named Downey. A company wants to create a ski resort; Downey is the sole holdout, refusing to sell his house, which happens to be at the center of the resort. Horowitz's former boss killed him because he stood in the way of the law firm collecting millions in fees from the developer.

Elana March tells Joan she had Andrew murdered in revenge. Sherlock enlists Detective Bell, who is on his day off, to help him find two missing pregnant zebras stolen from a zoo. A trademarked color provides the first clue; the color is only used on delivery vans for one company. They eventually find the zebras, who have given birth. They also find a veterinarian, shot dead; Sherlock believes he was forced to help with the induced premature births. They later find one foal, which Sherlock identifies as a quagga, which went extinct over a century before. Sherlock realizes the killer must be an employee of the zoo. The staff are gathered at the police station. A false accusation enables Sherlock to identify the real murderer, a PhD student, by his relaxation. The killer escapes police surveillance through an old speakeasy tunnel, but Sherlock, realizing the student needs to raise funds for his getaway, wins the online auction for the second quagga and sets a trap.

Sherlock and Joan investigate two murders in which the killer leaves envelopes of cash on the victims equating to their "worth". The probe leads the two to investigate an airline crash which killed over 80 people and the amount that each would receive as wrongful death compensation according to an equation formulated by attorney Arlen Schrader. When Dana Powell (Alicia Witt), the wife of one of the crash victims, reports being shot at from her back yard, Sherlock becomes suspicious, as the first two shootings were at short range. Powell gives a seemingly tone-deaf television interview, which forces the airline to drop its method of calculation to making the same flat payment to each next-of-kin. This is puzzling, as Powell's husband was a highly successful broker. Then they discover that the husband had a fatal disease, with only a few months to live. The police obtain a search warrant for her mansion, where they find incriminating evidence.

Detectives Demps and McShane, 35th Precinct, investigate the murder of Maria Gutierrez, who vanished three years before, in December 2011. Her body has only just been found. They suspect Sherlock killed her, as he wrote a signed note found in her purse, giving his address and a meeting time. The detectives do not believe Sherlock when he says he does not know her. He was a heavy drug user then, which caused blackouts and memory loss. Maria worked at her church's soup kitchen and as a cleaner at the offices of Councilman Robert Barclay.

Sherlock questions Oscar Rankin, a junkie often at the brownstone then. Oscar knew Maria, as he ate at the soup kitchen and had boasted of knowing Sherlock Holmes the great detective. Maria arranged a meeting with Sherlock and brought a bag containing bloodied clothes belonging to Barclay, wanting Sherlock to find out whom he had killed, for she had seen him take off the clothes, bag them, and put the bag in a dumpster. Barclay had been having an affair with Kelsey Prior, the wife of a friend, but when she tried to end it, he stabbed her, staging it as a robbery gone wrong. However, blood got on his clothes. Barclay must have decided to dump the bag somewhere more secure, but found it was missing. He must have guessed Maria had taken it. He lured her from Frobisher Motel in Bayside, where Sherlock often stashed witnesses whose lives were in danger. Barclay killed Maria and hid her body, and Sherlock, high on drugs, forgot all about her. Oscar assumed Sherlock wore the clothing when he killed Maria while high on heroin. Oscar kept the bag in case he needed leverage against Sherlock one day. Demps and McShane arrest Sherlock after a witness, Eddie Bynum, says he heard Sherlock threatening Maria just before she vanished. Joan takes the bag of clothes to the NYPD, telling Oscar they will exonerate Sherlock. Sherlock sees that the distinctive shirt style matches the shirt worn by Barclay in a photo. Told that traces of the killer's blood found at Prior's murder can be checked against his, Barclay confesses.

Galen Barrow, a driver for a ridesharing company called Zooss, is deliberately run over and killed by a city cab. It was staged to look like professional rivalry. Barrow was an internet journalist and had begun working for Zooss recently. Gordon Meadows, a registered sex offender, admits he killed Barrow but claims he was blackmailed into it after he broke his parole. Barrow's internet journalism employer, Lydia Guerrero, was having an affair with Barrow and was also being blackmailed by an anonymous texter. Despite her opposition, Barrow insisted on hunting the blackmailer. She and Meadows have the Zooss app installed on their phones. Zooss CEO Eric Frazier orders technician Brandon Felchek to hand over to the police the computer program (called Olympus after the home of ancient Greek gods) that records user details and tracks all rides. Mahra Kemp tells police her brother Patrick worked for Zooss until he was killed in a mugging a month earlier, and Barrow interviewed Mahra after Patrick's death - she suspects Barrow's death is connected. Sherlock examines Patrick's home, sees he was a sports gambler, and realises he was the blackmailer at Zooss. Sherlock reviews all Olympus data from the past week. Falchek is arrested: he had cut off Patrick's access to Olympus months before on discovering he was blackmailing users, not to protect Zooss or its users but to avoid drawing attention to his own crime; Falchek was using Olympus to stalk Felice Armistead and broke into her home where she surprised him and he broke her cheekbone.

Sherlock is visited by a member of Everyone, the activist hacker group. The visitor, username "Sucking Chest Wound", explains there is civil war in the group: his anarchist faction versus that of the more politically motivated "Species". Then Species, real name Errol White, is found killed by a now-missing samurai short sword. Chest Wound's hair is found under the body, and Species' blood is found in trunk of his car. He is arrested - real name Petros Franken. Rachel, his alibi, denies she was with Franken but later admits she lied. Sherlock reckons Franken is being framed and asks who benefits from White's death. Franken says White boasted to Everyone's 'inner sanctum' about a stash of data worth millions. Cabel Hill admits he stole the stash, but weeks before White's murder. He says the stash also contains every hack by White. From the stash, Sherlock identifies Species by message style ("fist"). But Sherlock detects two fists: Species was two people, a secret arrangement that allowed Species to be online 24 hours a day. Sherlock identifies the other half of Species as Tessee, another Everyone member. Tessee killed White when he rejected Tessee's Operation Right Nut, the hack of a right-wing think tank called the Atherton Foundation, because he felt it was too dangerous. Tessee is capitalising on Everyone's sympathy for Species/White's death to triumph in the Everyone rift, and the hack is imminent. The NSA says Atherton is providing services to US Intelligence. Tessee is identified as Bradley Dietz. When Sherlock and Joan confront Dietz, he is rescued by FBI Agent Branch, who provides an alibi. Sherlock reckons that, oddly, Branch wants Everyone to hack Atherton and, through a forensic accountant, finds that Atherton coordinates FBI and Homeland Security investigations with the internal security departments of major U.S. corporations. If Everyone hacks Atherton, the group's members would be arrested for treason and Branch would gain rapid promotion. Sherlock tells Branch he knows her secret (knowingly exploiting an illegal immigrant) and will report her unless she gives the police an anonymous tip on where to find the murder weapon. Everyone has been warned about Atherton, so Dietz is now useless to her. Branch caves in. Dietz is arrested and confesses.

A man shoots two paramedics and steals their ambulance with the patient inside. Sherlock retrieves a bullet casing which is traced to Wallace Turk, who admits the murders, but refuses to say where the body of the patient, Maggie Halpern, is. Sherlock's examination of Turk's footwear leads to a salt marsh, where Halpern was murdered and her body eviscerated by Turk's partner. The stomach was finely cut and Sherlock determines she was a drug mule but, Joan finds, an unwitting one who had received what she thought was gastric bypass surgery in Brazil. Her surgeon, Dr Bruno Escanso (Turk's brother-in-law), had planted 40 pounds of stolen medical (high-grade) heroin from his hospital inside her, then vanished; the amount suggests drug cartel involvement. Bell and Joan visit the offices of dentist Dr Marty Ward, having learned that Janko Stepovic, a drug gang leader, uses the offices, which by law they cannot be bugged by the government. Ward admits to knowing Janko. Bell tells Janko that if he buys the heroin, the NYPD will devote all its resources to catching him. However, if he can help find the heroin for the police, it will not be in the hands of rival gangs. Janko sets up a sting, but is shot dead elsewhere. Ward and his lawyer, Sarah Penley, claim that a Chinese triad refused to buy the heroin, but cut off some of Ward's fingers to force him to reveal Janko's location. Ward wants immunity from prosecution and agrees to tell all, including the location of two other drug mules' bodies. The bodies were filled with rocks, sewn up and thrown into a pond. Joan notices the sutures are very neat and made of material used by dentists, and realises Ward has tricked NYPD into agreeing to immunity because he could not persuade Turk to keep quiet. Sherlock, Joan, Bell and Gregson trick Ward, pretending to have arrested a triad member who is ready to incriminate him. Ward gives up, only to discover the triad member is actually a police officer.

Judge Dennis Vaughn is found in the subway, fatally stabbed with a screwdriver. While in New York City for a fundraiser for the New Jersey governor's re-election, Vaughn had sex at his hotel with Loretta Nichols, one of the governor's staffers; her alibi depends on a friend, Andrea Schuster. Fingerprints on the screwdriver belong to Nikki Moreno, who escaped from a correctional facility in New Jersey days earlier and had been sentenced by Vaughn. Accounts of her are contradictory: at the for-profit prison, run by Reform Enterprises (RE), Deputy Warden Trey McCann says Moreno was a troublemaker. Later McCann is murdered with a now-missing 9mm pistol. Jeff Harper, a prisoner rights lawyer, says his Moreno had nothing against Vaughn, but hated McCann, and had no reason to escape or commit murder as he was close to getting her sentence reduced. He says that Moreno was forced into an unpleasant work detail recycling computers, but Moreno's prison record shows she had asked for the detail. Sherlock examines the recycling area and discovers her body in a recycling bin. McCann had previously worked for another prison management company, and their executive, Perry Franklin, reluctantly reveals that McCann was fired, officially for "excessive absences" but actually for corruption. Nicols' friend Schuster was the re-election campaign manager. Sherlock realizes the murders were intended to cause the governor to award the new prison contract to Franklin's firm, and he is arrested.

Following events of the previous episode, "A Controlled Descent", the NYPD will no longer consult Sherlock and Joan, and he is in danger of being charged with assault. Sherlock is determined to resume his sobriety. Sexual predator and suspected triple murderer Jonathan Bloom asks Sherlock to find out what happened to his Honduran wife Alicia, who went missing in 2010. Bloom confesses to the deaths of two other women, drug addicts who did not survive rough sex, gives the location of their bodies, and kills himself.

With the NYPD no longer employing him, Sherlock persuades FBI agent Gary Burke to hire him and Joan. They investigate a triple homicide at a neuroeconomics research facility, but chafe under Burke's close supervision. The researchers were working on brainwashing technique for DARPA. All the hard drives containing the research were stolen. They meet DARPA Deputy Director Meher and Special Projects leader Alta Von See. At first, Sherlock suspects a Chinese spy is responsible, but it turns out the researchers had realised that their method did not work at all. Joan eventually realises from media calls for Meher to resign that Von See would be in line to succeed him. Von See used another device developed at DARPA, a sonic pressure shield, to incapacitate the victims, then shot them at her leisure. Sherlock had noticed that a pet lab rat died at the same time as the victims, also from the shield's effects. A streak of rubber from the base of her crutch by the device's storage closet implicates Von See.

The department investigates the murder of two men who look very much like each other, Timothy Wagner and German visitor Otto Neuhaus. The only connection between the two is Dorion Moll, who spoke with both by phone. Moll runs a website that finds people's doppelgängers, using a stolen and modified facial recognition system. Moll is convinced that Countenance Technologies, the firm he stole the software from, is out to kill him; he barely managed to escape a masked man with a knife by kicking him in the shins. Sherlock and Joan meet Countenance executive Curtis Tofano. He admits to using Moll's website, but only to find out about Moll in order to break into his apartment to steal Moll's modifications. Then another man named Evan Farrow shows up who looks like the murder victims. He reveals that Wagner offered him $10,000 to take a DNA test, pretending to be Wagner. Farrow turned him down. It turns out that this has to do with an Oriskany Falls University fraternity pledge named Harold Cudlow, who disappeared years before; his body was found in concrete when a building was being demolished. Cudlow fought against his killer; scrapings were found under his fingernails. Every member of the fraternity, including Wagner, was asked to submit to a DNA test. An Oriskany Falls police officer, Cudlow's brother, eventually confesses to the two murders. However, Wagner's DNA does not match what was under Cudlow's fingernails. Sherlock and Joan go over the Oriskany Falls police original list of suspects and recognise a name: Curtis Tofano. Tofano also used Moll's site to hire a double to take his DNA test. Cudlow was pushed at a construction site, fell and broke his spine; Tofano then strangled him. Tofano also tried to kill Moll to cover his tracks.

The case of a dead unemployed archaeologist presents new mysteries, as the detectives find out that he was digging up an old landfill from the 1980s. The man was searching for Nottingham Knights, an old video game from the 1980s that was never released because of monumentally bad advance reviews. When it turns out a suspect already found and dug up the now-valuable video games years ago, Joan goes searching for another motive and finds that a chemical company was illegally dumping toxic chemicals. The landowner had the archaeologist killed to avoid blowing the multi-million dollar sale of the land and to avoid paying for the costly cleanup.

Alfredo tells Joan that Sherlock has been avoiding him, possibly because of his connection to Alfredo's kidnapping. After Joan informs Sherlock of Alfredo's visit, Sherlock realizes that Alfredo has been going to more meetings than usual, due to his relapse, and offers his support.

A sniper shoots nine people, killing four, in the financial district. Morland unexpectedly visits Sherlock at the brownstone to offer his help, as he knows the identity of the sniper. Although the shooting appears random, it is too precise, concealing the true target; Morland knows of a prior mass shooting with the same modus operandi, which benefitted a former client. Morland's contact at Interpol, Lukas, finds out that shooter-for-hire Gagnier was paid by a shell subsidiary of Dynastic Energies. Gagnier dies in a fall when the police attempt to apprehend him. The CEO of Dynastic Energies, Bill Wellstone, was having an affair with his lawyer's wife, Sarah; he found out that she had left him for plumber Frank Bova, the true target of the shooting. Wellstone used a golf bag to smuggle the sniper's weapon past the tight security protecting the building, but that left behind gun oil residue in the bag that implicates him.

Retired FBI agent Robert Underhill is found pulverized in a woodchipper. Sherlock and Joan look into the old, unsolved cases Underhill was re-examining and find that of Mina Davenport (Ally Ioannides, see also Season 7's "Miss Understood"). Mina was kidnapped aged 8 and escaped her captor 14 months prior after 10 years in captivity and returned to her wealthy parents, Nancy and Richard Davenport (Raphael Sbarge). Sherlock is certain that the woman claiming to be Mina is an impostor, and believes that Underhill may have come to the same conclusion, providing a motive for his murder. The Davenports angrily refuse to cooperate. The fake Mina makes Sherlock and Joan an offer in private. She claims that Mina's father killed her, and if they will suspend their investigation for a couple of weeks (when she will gain control of Mina's large trust fund), she will help them prove it. Not believing her, Sherlock uses underhanded means to gain access to the Davenport mansion, finding Underhill's blood on a pedal in Mina's car. Richard Davenport confesses to Underhill's murder, but Sherlock realizes immediately that he is shielding "Mina". When the DNA test comes back, however, Sherlock is astounded that it proves Mina is the Davenports' daughter. Sherlock then figures out that the faker had actually found the real Mina and attacked her so she could cut off and steal some of Mina's hair. When the imposter staged her "escape" from her imaginary kidnapper, she locked herself in a gas station restroom, then shaved off all her hair and planted Mina's for the police to find. Checking police records for such an incident, Sherlock finds the real Mina and reunites her with her mother.

Ellen Jacobs, two months pregnant despite having had a tubal ligation, is found suffocated by a plastic bag in her car. She was leaving a voice mail message when she was killed; her last words are "Oh, my God. No. Stop it! No!" Sherlock and Joan discover that her husband is sterile. Ellen is the daughter of an imprisoned killer whose modus operandi matches her death. The father of her child is the realtor selling the Jacobs' house, whose mother was killed by Ellen's father; he is the prime suspect until he is attacked and hospitalised by the husband. The husband is arrested, but Sherlock deduces Ellen's brother Nolan is the true killer; the first "No" she said in the voice mail was actually her pet name for him. Nolan confesses - Ellen and he had had an agreement not to pass on their father's genes, so he killed her after she refused to get an abortion.

A bomb hidden inside a body explodes in the morgue, destroying the evidence for several crimes. Sherlock discerns which case was the target of the bombing. Despite the lack of a body, the victim is identified as Janet, a woman who made a small profit selling drugs. A secret camera in Janet's kitchen reveals Toby, one of her clients, was spying on her. Toby's journals are found to be full of descriptions of ways to kill Janet, but his alibi leads Sherlock to suspect his father of killing her to protect his son. When confronted with the possibility of Toby going to jail, his father confesses.

A colleague of Gregson's asks Sherlock for help infiltrating the heavily guarded headquarters of a drug-dealing biker gang. Undercover cop Ryan Dunning believes the gang leader stores everything about the gang's operations on a computer in his office. Sherlock plans the heist, but the inside man is killed before they can carry it out. The man's body camera records the moment of his death; he apparently grew impatient and tried to steal the computer by himself, but was discovered and shot. Sherlock spots discrepancies between the recording and the real office and concludes that the video was faked at a different location. The man and his partner wanted to incite a police raid on the gang, but his partner doublecrossed him. When the police search the headquarters, the computer is sent to police officer Lisa Hagen to crack the encryption. However, it turns out that Hagen was Dunning's partner. She is only interested in the gang's offshore bank accounts. She siphons off millions, but is caught.

A professor and a group of students are killed when they consume mushrooms tainted with synthetic deathcap mushroom toxin. Sherlock questions the professor's former associate, fellow academic Alston C. Harper, who said that although they had a falling out, they had made their peace. The professor's stash leads them to another body, Charlotte Koenig. Koenig gave the professor the mushrooms. Figuring that Koenig was the intended target, they continue the investigation and discover that Koenig was producing counterfeit erectile dysfunction pills to buy real estate. The property is land a pharmaceutical company has to have for a much-needed expansion. Koenig felt had stolen her biofuel process. She has the word "RACHE" (German for "revenge") tattooed on her back. Sherlock and Joan notice that the tattoo was originally just "ACH"; the "R" and "E" were added later and look slightly different. Harper's initials are ACH. They discover that Harper was married to Koenig. Koenig would not give Harper a divorce (due to a dispute over textbook royalties). When she suddenly dropped her resistance, he became suspicious and hired a private investigator, who found out about Koenig's valuable property.

Three members of a Chinese triad are murdered. Hoping to prevent a gang war, Gregson turns to Sherlock and Joan. Sherlock deduces the victims let their killer into an arcade late at night. The crimes are traced to a mortician who, using his skill at creating lifelike masks for his disfigured clients, disguised himself as an old woman the triad members trusted. The mortician is terminally ill and, masquerading as the triad's leader, orders the head of an assisted living facility (who is involved in a triad scam) to funnel the fraudulently obtained funds to a new bank account in order to provide for his family.

Sherlock and Joan investigate the suspicious fall of a wealthy octogenarian, Rosalyn Graham, from the penthouse flat, which kills both her and an unlucky man she lands on. An ottoman is found next to the railing of her balcony, but Sherlock realizes she did not have the strength to move it there; it was put there to make it look like a suicide. Graham leaves her estate to her dog, which would tie her estate up for years while heirs contest it. Billionaire real estate mogul William Hull ("Terra Pericolosa") is buying up properties' air rights so he can build a 63-story edifice designed by architect Malcolm Busquet. Graham's death deprives him of the air rights to her building. Sherlock deduces that Busquet or one of his staff made a career-killing design error - an average wind would topple it - and tried to cover it up by killing three people to ensure that the project could go ahead at a shorter, safe height, 40 stories.

In this adaptation of The Hound of the Baskervilles, venture capitalist Charles Baskerville is struck and killed by a truck while fleeing for his life from what an unreliable witness describes as a large glowing animal. Charles' brother Henry (Tom Everett Scott) believes it might have been murder. The animal breaks into Henry's mansion while he is on the phone with Sherlock; Sherlock deduces in time that the animal is actually a robot (developed for the US Army) and instructs Henry on how to save himself. The robot ends up falling into Henry's empty pool. The invention is traced to Stapleton Innovations. The CEO turns out to be the Baskervilles' sole surviving relative Roger; he stands to inherit a family trust if both Charles and Henry die. However, he does not need the money; he offers to commit a felony, which would disqualify him from inheriting. Sherlock suspects an unknown illegitimate relative is behind it all. He gets Henry to "kill" Roger. When the relative, an employee at Stapleton Innovations, steps forward to claim the trust, she is shocked to see Roger very much alive.

A comic book superhero impersonator who fights actual criminals is found dead. This leads the investigation to the publisher and the creator's grandson, Al Baxter, who is the editor of that comic. Baxter constantly voices his dismay over the fact that his grandfather was screwed out of his rights in the 1940s, irritating his co-workers. Baxter knew and supported the impersonator. Ironically, the impersonator was killed trying to stop Baxter from murdering his co-workers at a meeting at a restaurant with executives interested in making a movie about the superhero.

A survivalist doctor goes missing, leaving behind evidence that he was selling prescription drugs to dealers. When his body turns up, the investigation turns to the Keep, a deluxe shelter for wealthy survivalists. Sherlock exposes the Keep as a scam, determining that the victim attempted to cover his drug dealing by stealing the shelter's stock of medications, only to discover that they did not exist. The man's business partner, who helped him break into the shelter, killed him and framed the survivalists to keep their practice from going under.

Exiting a gym late at night, Phoebe Elliot spots a car she thinks is her ride share, but is shot and killed. Sherlock and Joan come across a selfie the victim took, which was later enlarged and sold as "art". The art gallery where the piece was exhibited was broken into, and several works were stolen, but not the selfie. However, Sherlock notices that the selfie has been replaced with another with some background details removed, details which seem to provide an alibi for a man imprisoned for the murder of Marissa Kagan. Sherlock now suspects that Elliot was killed because she spotted the alteration. Kagan was having an affair with a married woman; when Kagan pressed her lover to get a divorce, she was killed by her lover's wife. The killer wanted to turn herself in, but her wife and Kagan's lover, Assistant District Attorney Christina Pullman, talked her out of it and proceeded to murder Elliot to cover up the first killing.

Sherlock and Joan are investigating an apparent carjacking gone wrong, with both the driver and the carjacker shot dead. However, many details do not fit that story; for example, the alleged carjacker was left-handed, but the gun ended up next to his right hand. It turns out the carjacker was actually a hitman hired by the driver's partner in a struggling business. The partner was being pressured by the target to sell some seemingly worthless property. The partner, however, knew that it held rare "old-growth ginseng" worth tens of millions. The driver and the hitman decided instead to stage a phony murder so they could blackmail the partner. The driver's wife confronted him during the staged scene and killed both men because she found out her husband was planning to abandon her and start a new life in Tahiti. When she shot her husband, the recoil knocked her hand back against the car door, dislodging the diamond from her ring. The diamond remained undetected amid the shattered window glass, until Sherlock figured out what happened.

As Sherlock is reviewing known evidence which leads him to the conclusion that his assumption that Moriarty's organization would fall apart without her leadership has proven naive. Someone else has taken charge. Morland's office is bombed by the Russian hitman. This leads Sherlock to an economics professor, Joshua Vikner (Tony Curran). Not only is he the head of Moriarty's organization, he is also the father to her daughter. New evidence on the hitman surfaces, after which an angry Morland appears at the brownstone trying unsuccessfully to coerce Sherlock into giving up the name of the caretaker. The Russian hitman is arrested, but does not share any useful information. Sherlock notices corrosion on his belt and a distinct smell, identifying what propellant was used in the bombing. This points the team to the upstate theft of a pesticide barrel and a missing person. Meanwhile, Vikner summons Sherlock to discuss a possible truce with Morland, but Sherlock informs him that Morland will only be satisfied with his death. As the team prepares for an interview with the DA, the prisoner is assassinated in a murder-suicide by a rookie cop who turns out to be one of the psychopaths flagged in the DANTE survey. When Sherlock and Joan return home, they come face to face with a bomb identical to the first one.

An IED hidden in a soccer ball kills a man; this is similar to a string of explosions from half a decade earlier. While Sherlock and Joan are briefed at the scene, Sherlock notices someone suspicious looking and chases him. Sherlock is knocked down by a car, but the suspect leaves a hand print and is found in the system. It turns out he was in prison for vehicular manslaughter. Sherlock immediately likes him as the suspect, but some things do not add up. Joan asks Shinwell Johnson (Nelsan Ellis), one of her last patients as a surgeon and also someone who was in the same prison as the suspect, for information. He points her to the suspect's best friend in prison. Eventually, this friend admits to being responsible for the earlier bombings. Sherlock figures out that they were working together in a scheme to help the first suspect, a real estate developer, win a very valuable contract by targeting the neighborhood of a rival's site for a massive new entertainment center.

Powerful gang leader Halcon (Jon Huertas) has Sherlock kidnapped and orders him to find out who killed the gang's favorite smuggler. Sherlock offers to do so in exchange for the identity of a drug dealer selling poisoned drugs. (Halcon instead has the dealer killed.) The investigation reveals that the murderer was after a smuggled treasure thought to have been lost long ago, the Imperial Jade Seal of China, an artifact so valuable it was once traded for 15 cities. Representatives of both mainland China and Taiwan approach Sherlock, the latter offering him $50 million for it. While examining the site of the auction for the seal, Sherlock notices two indentations on a metal rail under a garage door; they come from a custom, heavily armored luxury car. This leads to Wayne Vachs (Ron Rifkin), the CEO of a large energy company. His company faces government fines that will ruin it. Vachs won the auction, but did not have the money to pay for it, so he had the smuggler killed. He planned to give the seal back to the mainland Chinese in exchange for a concession on a recently discovered, very rich Chinese mineral deposit, worth billions. Sherlock has no real proof, but threatens to reveal his identity to Halcon unless he confesses to the police.

Russell Cole is found pinned to a condo's wall, impaled by a poker. He was an investment analyst at the firm Barrett White Capital. Sherlock deduces the killer took Cole's laptop. Cole's boss, Mitch Barrett, is shocked, as Cole was his "mad genius" whom Barrett left to work independently, as it always paid off. Joan deduces from a photo that Barrett's wife Laurie was having an affair with Cole. Laurie provided a secluded cabin near Bear Mountain where Cole worked on projects. Cole had written a controversial astronomy article about the identification of asteroids that are potential threats to human existence. An asteroid's size is determined by the heat it emits, but the established method ignores what the asteroid is made of, which means those large enough to be dangerous may not be properly tracked. NASA was convinced by Cole's claim enough to cancel a $500 million infrared telescope project. Sherlock and Joan initially investigate to see who would benefit from the components contractors' losses, but Cole's paper turns out to have been deceitful manipulation. Grant Huber, a congresswoman's science advisor and Cole's secret research partner, was a lobbyist for asteroid mining companies and was receiving secret payments to ensure that government decisions would benefit them. The delay or loss of government funding for asteroid research would benefit the mining interests. Huber would also profit greatly from buying stock in the contractors' companies, temporarily depressed but fundamentally sound. When Cole got greedy and demanded more of a cut, Huber killed him. Sherlock had been suffering from coming in contact with poison ivy, but he is certain he did not. Then he realizes that oil from poison ivy from Cole's laptop got on Huber's office couch. Sherlock informs Huber that they may be able to match the DNA to the poison ivy growing near Cole's cabin. If not, someone who ate at the diner where Huber and Cole regularly communicated secretly will probably be able to identify him.

When Damien Novak, a vigilante who outed dating site sexual predators and publicly shamed them, is shot to death, Sherlock and Joan look into the men he humiliated. Among Damien's targets was Shane Fitzhugh, who fled to Indonesia after he was publicly shamed. Shane had engaged in many past sexual abuses and Sherlock realizes that one of his victims is Molly Parsons (Conor Leslie), an employee at the dating site Damien used. Molly admits she killed Damien out of anger because his actions caused Shane to leave the country before he could be arrested. Afterwards, Sherlock arranges to have Shane framed for drug possession.

Sherlock and Joan investigate a mass murder when a chef and several patrons are poisoned after dining on foie gras containing snake venom. The diners were all members of the IAO group, the internet's first line of defense. The sole survivor, a vegetarian, assures the detectives that there are many more defenses, as well as replacements for his deceased colleagues. A video is released, claiming credit for the poisoning and threatening to target the New York Stock Exchange (NYSE). In response, the NYSE shuts down operations for the day so its security can be checked by a team of outside experts. Sherlock realizes the whole thing is simply a cover for the theft of three paintings worth around $60 million displayed in the NYSE building. All the security measures in place were deactivated for the audit. Sherlock has an art forger create copies of the stolen works, and their "recovery" is reported in the media. From darknet chatter claiming the recovery was faked, he is able to identify one of the robbers - one of the security experts who checked the NYSE system - but he turns up dead, bitten by a coastal taipan. Sherlock then identifies his murderous partner, a cook who worked for the chef, by the distinctive bite marks. The cook took the precaution of injecting himself with small doses of the venom to build up an immunity, so he could "milk" the taipan; this proved a wise move, as he was bitten, though it later proved to be his undoing as well.

Sherlock and Joan search for two murderers after a base jumper is shot out of the sky by one killer and has his parachute sabotaged by another. The victim had bought a small plane and was smuggling people into the USA across the Canadian border. Both murderers' motives stem from the victim having an affair with an Afghan woman. His wife sabotaged the parachute, while the woman's brother shot him in an honor killing.

Joan worries Sherlock's sobriety is at risk when she learns he has been lying to her about attending recovery meetings.

Joan has her DEA number and identity stolen, leading to the discovery of two dead bodies, those of a rheumatologist who stole her identity and the mother of one of her patients. The crooked doctor was illegally writing drug prescriptions using five stolen identities. Joan realizes the patient had been secretly poisoned by his mother all his life for the attention she got to bask in. The doctor had stumbled upon this when the mother contacted one of her stolen identities and gave an entirely different medical history for her son. The doctor informed her patient. The patient murdered his mother as revenge and the doctor because she would have guessed he was the matricide.

While wrapping up a murder investigation, Sherlock discovers evidence of a woman being held prisoner for years by Ryan Decker and races to track her down before she is disposed of by her captor. Decker's ex-wife is not surprised, saying she divorced him after catching watching perverted porn. Blood found at a beach is feared to be that of the captive but is not. The NYPD rescue a captive, but it is not the one Sherlock is looking for, but a woman taken only a few months before. Joan notices that the brand of soy milk in the fridge at that site is the same as that preferred by the ex-wife. The couple were working together. The ex-wife is arrested before she could dispose of the captive. She had murdered her ex-husband at the beach.

Mount Pleasant Sheriff Malick (Debra Jo Rupp) has a dead clown, possibly killed with a crowbar, for Sherlock to investigate, along with regular night-time sightings of clowns. The murder victim is Dale Schmitt, employed by company AdRupt that used prank-vertising to generate social media coverage. Near the murder site, Sherlock and Bell find a manhole cover, which Sherlock opens with a crowbar - below is the pipeline delivering New York City's drinking water. The water supply is so clean, it requires only treatment by chlorine and UV exposure. The news sparks panic buying of bottled water, and Joan installs a water filtration unit at the brownstone, the only model recommended by the Department of Environmental Protection (DEP). The virus introduced into the water is analyzed and found to have been bioengineered specifically to survive both, but oddly, only causes diarrhea. The creator is identified as Raymond Thorpe, a virologist who as fled the country. Gio Bianchi is a suspect, as his construction company had repeatedly won the contract to build a water filtration plant for the city, a contract that would only be implemented if NYC's disinfection methods are no longer enough. But Sherlock realizes the one to gain immediately from the superbug is Wendell Hecht, Vice President of Water Integrity at the DEP, who holds the patent for only home water filter recommended by the DEP. Since it would take years to build the filtration plant, Hecht would have become very, very rich. Hecht hired Thorpe to contaminate the water supply. Schmitt saw him doing it, so Thorpe killed him.

O.G. Pwnzr, an eSports agent and popular commentator, is attacked while live-streaming. His body is found in a hotel room. He was killed by a blow from a sharp instrument, after being tortured for the location of Marcel Otolik, aka Tendu, an Inuit from northern Canada. Tendu recently signed with ProFine Peripherals' eSports team. Tendu and his girlfriend flee when someone breaks into their hotel room. The murder weapon is found in his room - a traditional Inuit seal-hunting tool - but Sherlock suspects he is being framed. Animal activist Rayna Carno has been working to ban seal hunting, and Tendu posted a social media photo of himself eating seal meat (a "sealfie") to promote seal hunting as a vital part of Inuit culture, so Carno posted threats. Carno, however, shows she negotiated a deal with Tendu: Tendu would oppose commercial hunting, while Carno would support much less extensive Inuit hunting.

The motive for the murder turns out to be unrelated to eSports. Law firm Mather & Kline was hired to secure Inuit land to build ports on, as global warming is opening the Northwest Passage to new, much more cost-effective shipping lanes. Sherlock and Bell rescue Tendu and his girlfriend from Kurt Godwyn, the law firm's "fixer". Godwyn's gun is proven to be the one used to beat Owen. Lundquist, of Mather & Kline and Carno's lawyer, is arrested: she had helped Carno pressure Tendu's village to end seal hunting to leave them destitute and thus more willing to sell their land. But when Tendu's "sealfies" turned public opinion against Carno, and the money he sent to Maniitok enabled the village elders to resist selling, Lundquist ordered the hit on Tendu. Godwyn implicates her.

Sherlock and Joan investigate Rohan Giri's death by lightning strike, discovering he was murdered by a laser. They quickly find the culprit, the financier of the laser. He confesses, but insists he was only trying to incapacitate Giri to buy time. He the confesses that black market plutonium he smuggled in to power the laser was stolen. It could be used to build a dirty bomb. National Nuclear Security Administration Agent Don Kohler then takes charge of the investigation.

A bomb maker is found murdered in New Jersey. Residual radiation readings show the plutonium had been there. The police then receive an anonymous tip from a woman indicating that a mosque in New York City is the target. A suspicious van is spotted parking near the mosque by a traffic camera. The authorities mobilize, but then Sherlock realizes that the threat is fake, merely a distraction for a $300 million diamond robbery, abetted or orchestrated by Agent Kohler.

Holmes and Watson investigate the death of a relocation expert, Leland Frisk, who was poisoned by wine laced with cyanide. They discover Frisk had a secret career as a contract killer. Talking to his assistant (Lesli Margherita), they identify Cal Medina (Joaquim de Almeida), an unscrupulous business man, as a suspect and his fingerprint is found on one of the wine glasses, but he has a popular politician vouch for his alibi. Sherlock and Watson set out to identify for what murder Medina hired Frisk in the first place. They identify the original victim, a scientist that had supposedly found another treatment for cystic fibrosis, while Medina had just substantially raised the price of the medicine that was being used to treat it. When they confront Medina, Sherlock deduces he was framed for the murder of Frisk by Frisk himself, who had neither killed the scientist nor worked for Medina at all. Frisk had a son suffering from cystic fibrosis and was obsessed with eliminating Medina, so the medicine his son needed would become affordable again. When the FBI was closing in on him, he committed suicide and framed Medina. Even though Frisk's plan failed, Sherlock and the NYPD manage to bring Medina to justice with a little help from the FBI. Meanwhile, Watson's path to child adoption suffers a setback caused by her sloppy lawyer and Sherlock enlists an actual king to help Watson leverage her lawyer into making things right.

Harlan Emple, one of Sherlock's Irregulars, wants to hire him and Watson because his mathematics doctoral student, Lily Zavala, has missed the oral defense of her thesis. He becomes the prime suspect in her disappearance; he was attracted to her, but had not told her because university rules forbid teacher-student relationships. Her parents receive a ransom call. The trail leads to the body of ex-con Jimmy Cantrell, one of the kidnappers. Fortunately, Lily escapes. Sherlock and Watson learn that she was being forced to subtly change the mathematical calculations used to update flood plain maps for the city. They eventually discover that Cantrell's landlord owns property that a company is negotiating to purchase for its new headquarters. The update would put the land in the high-risk zone, which would likely cause the company to look elsewhere, costing the landlord millions. (He killed Cantrell because Cantrell got greedy and came up with the ransom demand on his own.)

In their new careers as consultants for Scotland Yard, Sherlock and Joan investigate an acid attack on a popular ex-model, the latest in a series of such attacks by different perpetrators. With her face badly damaged, the victim commits suicide. Sherlock and Joan consider several suspects before finally realizing that her plastic surgeon was trying to hide evidence that he had performed the wrong cosmetic surgery on her.

Virgil Gwinn, the manager of a high-end storage facility, is murdered. The facility is a foreign-trade zone, which means that goods brought there from abroad are not considered to have entered the United States yet and, therefore, are not subject to customs inspection or duties, which makes it attractive to criminals. Sherlock and Joan discover that Gwinn was secretly searching his clients' units for valuable information to sell. Sherlock eliminates all clients but one from his list of suspects: heiress Aura Swenson. Swenson's now-deceased father had amassed a collection of religious artifacts through questionable means as head of an international construction company. Sebastian Florenti, an appraiser Swenson had hired, tells Sherlock and Joan that he quit after Swenson asked him to create forged provenance for the artifacts. However, Swenson proves she has no motive; she had negotiated with the Ethiopian government to secure her ownership of the objects in exchange for helping rebuild the country's infrastructure following an announced peace deal with neighboring Eritrea. Joan finds an archaeologist's journal entry suggesting there is oil on disputed land Ethiopia was ceding to Eritrea as part of the agreement. Ethiopia then cancels the deal after this is brought to light. Sherlock realizes that Florenti sold the information to the Eritreans and hired a hitman to kill Gwinn.

A van collides with a semi at a city intersection then explodes, killing the van driver and a good Samaritan trying to help. The driver was a member of the Mara Tres gang. Sherlock and Joan discover the traffic lights were manipulated to cause the crash. Initially, the fuel-laden van is suspected to be part of a terrorist attack, but then Sherlock discovers that more traffic lights were hacked. The route of the sabotage shows that the real target was the semi's cargo: ultra-high-speed elevators for a skyscraper under construction. This is only the latest act of sabotage, all of which delay work so much the construction company will be unable to start on another billion-dollar contract it landed. Rival Maranek Construction will get the project instead. Sherlock sets a trap, incriminating Cameron Maranek, the company's owner.

A woman is murdered during a race to checkpoints scattered in the woods and the bodies of a man and a pig are found in a pond near where she was stabbed. Sherlock and Joan try to find the link between the seemingly unconnected murders and unravel the perpetrator's full scheme. The male victim is moonshiner Renny Henderson. Sherlock believes he was forced to make the poison ricin; the ricin was tested on his pet pig and then the two were murdered. The woman just happened to be in the wrong place at the wrong time. Sherlock reconstructs a wine bottle from shards found at Henderson's shack, where he was killed. The trail leads to a specific wine vintage, Chateau Baptiste '57, the favorite of a rich hedge fund manager. The killer turns out to be a bartender who knew Henderson because the bar owner bought the victim's high-quality moonshine. The bartender is obsessed with the hedge fund manager's wife, a high school classmate.