DI Lewis returns to Oxford after two years' secondment to the British Virgin Islands, where he was sent to recover from his wife's death. He is reluctantly assigned by his new boss, DCS Innocent, to the murder of an Oxford mathematics student who is shot while participating in a sleep study. The key-code used to access the sleep lab is assigned to a fellow maths student, Daniel Griffon, but Daniel's maths tutor has provided him with an alibi. Daniel is a maladjusted young man who will soon inherit his father's automotive empire. He is disruptive and has no respect for his uncle, who now heads the company. The future of the company rests however on an impending deal with Japanese investors who insist that the family unity is all important at this time. When two other murders occur, Lewis must decipher a cryptic clue left in an old case file by his former boss, Chief Inspector Morse: "Polo not king after all".

Several other references are made to Morse: on his leaving London Heathrow Airport, Lewis is nearly run over by a red Jaguar Mark 2 (which also reminds him of his wife Val’s death in a hit-and-run); he knows the word "bruxism" (tooth-grinding) as a result of spending "15 years working for a crossword fanatic"; Trudi Griffon knew Morse and says: "He was a man you could talk to"; Griffon's daughter Jessica Pollock is being examined for the "Endeavour Award" which had been endowed by an anonymous donor for music that spoke to the soul.

DI Lewis and DS Hathaway investigate the murder of Dean Greely. As the investigation continues, they discover that Greely and three other men had formed a club during their Oxford student days, the Sons of the Twice Born, which refers to Dionysus, a hero of Nietzsche:

• Harry Bundrick, who runs a bike shop with his mum
• Sefton Linn, college master and prospective vice-chancellor
• Theodor Platt, last remnant of a wealthy family, failed author and cocaine user. He uses a wheelchair since being involved in a head-on collision when drugged, causing the second driver's death (thus Platt's behaviour eliciting Lewis’s ire)
• Dean Greely, a failed painter staying in a houseboat.

Currently, the men have little contact with one another and initially deny that their club existed. The group keep their ex-membership in this disreputable club secret, although they each have a gold ring bearing the image of Dionysus. When a second member of the foursome, Linn, is murdered, it becomes apparent that they have a secret from 30 years ago, and that someone is out for revenge.

It transpires that the murders have been masterminded by two women. Tina Daniels had been a prostitute and had recommended her colleague Patsy Worth to Harry Bundrick. Patsy got drunk or drugged with the Sons of the Twice Born, and they decided to murder her in order to take out her adrenal gland and extract the adrenochrome, which was reputed (by Hunter S. Thompson) to give a very good high. It was Platt who actually carried out the murder. Patsy had a daughter, Anne, whom Tina brought up. Anne trained as a nurse and got a job nursing Platt. When Platt chokes, Anne saves his life (in Lewis’s presence) although, as Platt tells her, she could have let him die. The couple seem to be reconciled, but in reality Tina and Anne are manipulating the Sons of the Twice Born to murder each other:

• First Tina tells Linn that Greely is near death and is planning to come clean, so he must kill him if the secret is to be kept;
• She then poses as a journalist named ‘Fury’ (an allusion to the avenging Furies of classical literature) to goad Platt into shooting Linn;
• However, Harry Bundrick is to be spared as he has shown remorse.

This leaves Platt outstanding – and he is the only one who knows where Patsy is buried. Anne therefore telephones him, pretending to be being held hostage and that her abductor will release her only if he tells her where her mother's grave is. He tells her she is buried by the statue of Dionysus in their park. She then reveals the plot and sets their two Belgian Shepherd dogs on him, which tear him to shreds. Patsy’s skeleton is disinterred.

Anne had sent Lewis a cryptic note reading "His best friends killed him because of a boast", which can be interpreted as meaning that, like Actaeon, he boasted of being a better shot than Diana, and his dogs killed him. Hathaway says: "If Anne sent the riddle, she is guilty of premeditated murder. Doesn't it make her just like Platt? Lewis replies: "Not in my book." Tina and Anne thus stand to face a low penalty, if any, and to become very rich women by inheriting the Platt estate.

A further erudite classical detail is that Dean signs his paintings in Greek. Lewis and Hathaway consult Professor Margaret Gold, who immediately deciphers them as telephone numbers written in Greek numerals.

DI Lewis is less than pleased when he and DS Hathaway are assigned to protect Nicky Turnbull, a former convicted cybercriminal and "Rock and roll hacker" turned successful author. Turnbull is invited to speak at the Oxford Union. He is met by a welcoming committee composed of Caroline Morton, president of the Union; Jo Gilchrist, unscrupulous student journalist with ambitions to be a tabloid editor who intended to expose a professor's exam scam; Stephen Gilchrist, her brother, who is a serious scholar; and David Harvey, a rugby player who has had to retire because of an injury.

Turnbull had cheated two Oxford colleges in his computer scam and had received death threats. Lewis and Hathaway are tasked with babysitting him. Although Turnbull is everything Lewis dislikes - a criminal and a professional Geordie - he grits his teeth and does the job.

Turnbull gives his lecture and afterwards treats his audience to champagne in his hotel, which is housed in the converted prison in Oxford Castle. On the way there he is nearly run over by a jeep. Things take a more serious turn the following morning when Jo is found strangled in the hotel room next to Turnbull’s, which had been booked in Lewis’s name. Caroline emerges, having spent the night with Turnbull. She admits that after the speech, she and Jo planned a "honey trap" that would give Jo a juicy story for the press.

The next morning, after Turnbull is fatally shot by a sniper while talking to Lewis in the hotel courtyard. Turnbull’s wife Diane, who is also his agent, comes to identify his body. By an amazing coincidence, she was at school with Lewis, and he was indeed her first boyfriend. By another coincidence, David Harvey was at the same public school as Hathaway, and remembers him as the head boy. He tells him is nickname was ‘WC’, short for ‘Wolfgang Christ’, as they couldn’t work out whether he wanted to be Mozart or Jesus. It is revealed that Hathaway plays the guitar, specifically world music mixed with jazz, rock and medieval madrigals.

Chief Superintendent Jack Cornish, an ex-colleague of Robbie's, goes missing and a recently embalmed body is discovered near an isolated farm track. Lewis takes the awkward, young but bright DC Gray under his wing to investigate the case. It transpires that Cornish has gone to ground in a cottage in Croatia, where he manufactures crystal meth, which is smuggled into Britain inside corpses being repatriated to undertaker Brian Miller. Hathaway is conveniently on holiday in nearby Kosovo that week, helping to renovate an orphanage. Lewis relishes asking him to pop over to Split to dig him out, leading to the dialogue: "What’s the time difference?" – "About a century. You know Split is in a different country?" – "Only recently."

Peter Faulkner, with whom Lewis had previously had a run-in, had murdered the other members of the gang Johnny Jay (because he objected to his son Liam being involved) and Dr. Matt Whitby, and tried to murder Liam’s girlfriend Ruth Wilson in case Liam had told her anything. Cornish walks free as there is no proof of his guilt. On the way out of the police station he asks Lewis: "A drink for old time’s sake?" "There isn’t a spoon long enough."

The episode is interspersed with incidents in the culmination of Lewis and Hobson’s love affair: a longing look, her hands on his shoulders, holding hands and finally a passionate kiss in front of Innocent and Hathaway. Lewis has finally got over his wife’s death: at midpoint Hobson asks how Lewis is, and he says “Val is slipping away. I’m turning over the page.” She replies “Right!”

Hathaway returns from making the Camino de Santiago pilgrimage, and he starts work on his first murder case as an inspector with the help of a new partner, DS Lizzie Maddox. As the pair delve into the worlds of neurosurgery, blood sport and animal rights, Hathaway struggles to solve the case. Unwilling to delegate responsibility to an increasingly frustrated Maddox, he is overwhelmed by the work involved, and requests help from Innocent, who persuades the now-retired Lewis to come back on a one-year contract (to the annoyance of Laura Hobson, who had hoped for a quieter life with her retired partner). A well-respected neurosurgeon has been shot, and the case becomes still more complicated when his friend and business partner, the co-owner of the hunting estate where the shooting occurred, is also murdered -- apparently with the same weapon, though it was locked away in the police evidence room after the first murder. This second murder victim had been the prime suspect for the first killing, but there's no shortage of other suspects, as the neurosurgeon turns out to have made several enemies through his work, including a complaint of malpractice for a botched operation.

Lewis, Hathaway and Maddox's abilities as a team are severely tested when they investigate the brutal stabbing of American classics scholar Rose Anderson. The bizarre case that follows takes in the cocaine trade, astrophysics and ancient drama.

Anderson was killed while jogging. She had been having an affair with Felix Garwood (whose wife Philippa was her academic supervisor), an astronomy professor, but Garwood had recently dumped her. She retaliated by knocking him off his bike in her blue Mini. Garwood later meets a violent end in his office filled with antique astronomical instruments, including a telescope that had previously belonged to Isaac Newton.

Meanwhile, questions arise regarding the classics professor Simon Flaxmore, who had made his name by apparently rediscovering the lost Euripides play Alcmaeon in Corinth (Ἀλκμαίων ὁ διὰ Κορίνθου, 405 BC). He is insistent that he hardly knew Rose at all and had never spoken to her, but this is considered unlikely, given that she was studying the subject on which he is the world expert. Hathaway works out that the "rediscovered" play is a fraud, as it refers to the "two lions of Nemea", whereas the constellation of Leo Minor was only so named by Johannes Hevelius in 1687. Flaxmore’s fingerprints reveal that he is Linus Cage, who was imprisoned for fraud and released in 1985. He was appointed an Oxford classics professor in 1992 on the basis of a fabricated CV. Anderson had discovered this and was planning to expose him.

Felix's wife Philippa’s sister, Jennie Brightway, has a daughter Tabitha with Fanconi anaemia, whom Anderson tutored. Jennie Brightway and her husband Paul, desperate to engender a saviour sibling via in vitro fertilisation, have been struggling to find the money for this expensive procedure, but Anderson had donated a large sum to help them. Could they be connected somehow to the murders of their tutor and brother-in-law? Chemical analysis shows that the telescope used to beat Garwood to death had been wiped clean with lens wipes, which points to Paul Brightway, who is an optometrist. It was only when they undertook in vitro fertilisation that Paul Brightway discovered that Jenny Brightway and Garwood had had an affair and that Tabitha is Garwood’s biological daughter. Paul Brightway killed Garwood because he threatened to take Jennie Brightway and the children away from him.

Lewis and Hathaway are called to investigate a body in a woodland. Realising three more murders may follow, the team must hurry to catch the killer. Hathaway is struggling to cope with his father’s illness, as he begins his search to understand his father's activities before dementia set in.

Three people are murdered, all members of a spiritual cult, the "Companions of Coinherence". This group revived the ideas of Charles Williams, a member of the Inklings, an Oxford literary discussion group that also included J. R. R. Tolkien and C. S. Lewis. Williams taught that it is not only God that can forgive; we can also take another’s guilt on ourselves through the ritual of "substitution". The members of the cult bear a tattoo in the form of a triskelion as used in Gothic tracery.

The murders follow the four stages of the alchemical process of the magnum opus:

1. Negredo – black – raven – decomposition: College dean and Tolkien specialist Phil Beskin is lured into the woods on Boars Hill, knocked out, semi-buried and covered with maggots.
2. Albedo – white – moon, purification: After undergoing the substitution ritual, Annapurna Kinneson is strangled and then tied to a post in the river so that the water can purify her
3. Citrinitas – yellow – sun, male, enlightenment: After his assistant Gina Doran is lured out of his tattoo parlour, Jay Fennell is killed and covered in a yellow substance
4. Rubedo – red – blood

Carina Beskin, Phil Beskin’s sister, confesses that eight years ago the four – Phil and Carino Beskin, Fennell, and Kinneson – were larking about, driving home late at night without headlights, and killed Francis Fisher, a practising alchemist. Carina Beskin was driving while drunk, but the group escaped punishment because they lied. Phil Beskin and Kinneson had used the doctrine of substitution as a way of coping with their guilt.

Lewis and Hathaway realize that student Sam Langton, who is in a relationship with Carina Beskin, is in reality Francis Fisher’s younger son John, who is wreaking vengeance. They track him down to a farmhouse near Burford, led there by traces of the local limestone from his shoes. They interrupt him in the process of concocting a red liquid and overpower him before he can drink it. Under Hathaway’s persuasion he tells Carina Beskin, who is dressed in red, that he forgives her. She asks what for, and he says “for murdering my father”. Suddenly she realises who he really is and berates him for killing her brother and her friends.

Three ongoing plot lines also develop: (1) Laura is persuading Lewis to look forward to their trip to New Zealand; (2) Moody overcomes his doubts and agrees that Hathaway will get on fine while Lewis is away; (3) Hathaway visits his old monastery school after 17 years’ absence, and learns from Father Placid that Hathaway's father had been talking to him about how to repair the damage caused by the father's being so distant from his son. Though he had previously angered his sister Nell by leaving all the filial duties to her, Hathaway rings up his dad and asks if he can visit him.

Part of Series 8 in the U.S. Shown in one piece on 14 August 2016.