Father Brown is shocked when a nun dies before him, and discovers her death was due to cyanide poisoning. When a second nun dies in the same manner while being interviewed by Inspector Valentine, suspicions are placed on the victims' convent, especially when it transpires the nuns there are holding onto bottles of cyanide.

Father Brown investigates the disappearance of a parishioner, Charlotte McKinley, who believed she was being haunted by her sister who had disappeared nine years previously. Husband Victor (Andrew Havill), a struck-off doctor, is arrested as being responsible for the murder of both women.

Violet Fernsley (Emma Stansfield), convicted of killing her pig-farmer husband whose body has never been found, is given a temporary reprieve from the gallows when she claims she is pregnant. The pregnancy test gives Father Brown three days to find the real killer. When her husband's finger turns up in a pig's stomach, it points to a serial killer amongst the family. Father Brown's investigation concludes God's law is above man's law.

The Lannington Rosary, missing for 500 years, is a relic blessed with healing powers. Professor Hilary Ambrose (James Laurenson) has sent an ancient prayer book containing a clue to its whereabouts to his old friend Father Brown, before he goes missing and his home is ransacked. Father Brown receives unexpected help from the untrustworthy Flambeau, returned from the dead. A terminally ill fellow priest, Father Ignatius (Anton Lesser), also desperately seeks the rosary to heal both himself and others.

Edward Gerard, a prisoner of war who returns from North Korea sympathetic to his captors, greatly displeases his martinet uncle, Colonel Cecil Gerard (Nicholas Jones), who threatens to commit him to an asylum. When the Colonel is murdered, the suspects include Edward, the Colonel's daughter (an adopted Chinese girl) who was going to run away with Edward, the brother-in-law, and the victim's wife, Edward's mother. Father Brown finds the solution lies in the family history and the Colonel's sexual intentions to his adopted daughter.

An exhibition of valuable art at the Belvedere Museum includes a priceless artwork on loan from the Vatican. Father Brown spots his nemesis Hercule Flambeau, not realising he is a pawn in Flambeau's plan to steal the picture and wreak revenge on the museum's curator, a man who during the war had helped the Nazis loot paintings from Jewish families. Flambeau is shocked to find a woman from his past he thought dead seeking to avenge the death of her parents by killing the curator.

A body of a journalist about to expose an offshoot of the Freemasons, The Owl of Minerva, is found dead in the woods. Inspector Sullivan's investigations find him on the run for killing a fellow detective and seeking sanctuary in the confessional at St Mary's. Father Brown, with the aid of Mrs. McCarthy, Lady Felicia, and Sid, rally round to try to prove his innocence and protect him from a deadly organisation with members highly placed in the community.

Lady Felicia knows the Hollywood star Rex Bishop who is in Kembleford filming a "B" movie The Brides of the Demon. He invites Father Brown and Mrs. McCarthy, a fan of Bishop, to view the filming. Tensions are evident between the film's participants, the director, his actress wife, a starlet, Rex Bishop, and the writer. The director is found dead wearing a demon's mask and Inspector Mallory does not appreciate Father Brown's help.

Father Brown knows the Pope's coronation gift to the Queen of a priceless cross is too tempting to Flambeau and Father Brown convinces the Bishop to let him guard it. Flambeau arrives on Father Brown's doorstep seeking his help to steal the cross in order to save the life of his daughter whom he has never met, and who has been kidnapped by Flambeau's criminal partner he once left for dead. Before they can steal the cross it goes missing from the Bishop's Palace. Father Brown has underestimated Papal politics, Flambeau's daughter, and the consciences of men.

Baron Samdi is the voodoo spirit of the dead that Emmanuel Jannite invokes when he follows his love, Yveline Lafond, to Kembleford. Lafond is working as lead singer in a Haitian jazz band. The band leader, also in love with Lafond, is found dead in a swimming pool, poisoned, and Jannite dies following a police chase. Inspector Mallory believes the case closed but Father Brown's snooping uncovers the real killer. As he confronts the culprit, Father Brown collapses after being informed that he has already been poisoned that morning.

Robert Malmort and Joan Vanderlande are brought together by their respective parents with a view to marriage. When the maid Ruby is found dead, Lady Felicia's niece, Bunty, is suspected. Father Brown suspects the answer lies at the end of a Labyrinth and a sad secret the Malmorts are hiding. Following her latest scandalous headlines, Bunty's father insists she stay in Kembleford and Lady Felicia needs her to look after her home as she is leaving with her husband, Monty, who has been appointed Governor of Northern Rhodesia.

Sid Carter is released from prison after a year following his conviction of assault on Judith Miles, then a prostitute, now working in a solicitor’s office. Bent on revenge, Sid confronts his defence lawyer, Giles Foster, now a judge, with Father Brown at the home of a Edward Reese, who appointed Foster. When Foster is found dead from stabbing by a glass to the throat, Sid is accused by Reese's son of the murder. Father Brown becomes a target of a paid assassin and his only clue is a pail of ashes.

Hercule Flambeau, awaiting trial for murder, asks for Father Brown to act as his chaplain and professes his innocence. However, at his pre-trial hearing, he pleads guilty and is condemned to death. Father Brown realises he is part of Flambeau's plan to find a gold medallion hidden by the architect of the prison where Flambeau is being held. A plan that begins to unravel when Flambeau's victim fails to reappear after he is murdered by his wife intent on obtaining the gold medallion for herself. Father Brown has to engineer Flambeau's escape from the death cell.

A skeleton from a seven-year-old murder case is found buried in the woods, bringing doubts on the conviction of the confessed killer. Inspector Mallory and Father Brown reinvestigate the murder. Sid Carter brings letters for Father Brown and Bunty as well as a Christmas present for Mrs. McCarthy. Father Brown, Mrs. McCarthy, and Bunty audition for the local amateur pantomime, Cinderella, but do not get the roles they expected. Sid is recruited to partner Father Brown; Mallory and Sergeant Goodfellow get roles as the ugly stepsisters. When an accident happens during rehearsals, Father Brown realises the truth lies within the theatre.

British Railways proposes closing Kembleford Station due to non-profitability, causing its stationmaster Ben Webb, to have a heart attack. A fete to save the station finds Webb bludgeoned to death in a trunk on the platform. Learning Webb was a philanderer over many years gives Father Brown many suspects ranging from his pregnant wife Julia, a bus company boss, a British Railways manager, and the church's new cleaner Pandora. The murder is complicated by the fact that Webb died from drowning.

Lady Felicia and Sid return from Northern Rhodesia and visit Kembleford while her husband, Monty, is in London. Lady Felicia finds herself blackmailed by an MI5 agent, Daniel Whittaker, to exchange a roll of film one of her lovers, a known Soviet agent, has. At a party, her task goes wrong when a sculptor is shot dead, and Whittaker orders Mallory to find Lady Felicia as the prime suspect. Father Brown becomes involved with spies, British and Soviet, not knowing who is friend or foe, traitor or patriot. Lady Felicia has to choose between lover and husband; and Moscow get the film they do not expect.

Two items catch Father Brown's eye in the newspaper: Flambeau's death in Italy and the Iron Crown of Lombardy containing a nail from the True Cross on display at Gloucester cathedral. Father Brown opens a letter with a key that Flambeau had sent to him a few days before his death. A woman claiming to be his wife (Lisandra Flambeau) wants the key, but it is stolen while the presbytery is empty. Father Brown becomes part of an elaborate plan of Flambeau and his wife to steal the crown requiring a key and a combination to a safe. Flambeau double-crosses his wife; and to flush out her husband, she injects Father Brown with a slow-acting poison.

Opera diva Bianca Norman offers Lady Felicia and Mrs. McCarthy passage in her personal Pullman train carriage with her adopted children, latest husband, and staff. The carriage is stopped and robbed; the lights are shot out by two shots fired and a third kills Norman. To escape, the robbers kidnap Lady Felicia and Mrs. McCarthy. Father Brown and Bunty reconstruct the robbery with the family, while Lady Felicia and Mrs. McCarthy deal with the inept robbers to discover that family problems are the motive for both the robbery and the killing.

Alan Tylett, wanted for killing a policeman, is pursued by Inspector Mallory and Sergeant Goodfellow. Goodfellow is shot saving Mallory and both are hospitalised. Inspector Sullivan. posing as "Inspector Trueman". is sent from Special Branch to take over the case. With Father Brown's help, they capture Tylett, who protests his innocence. A wide-ranging web of police corruption is uncovered. Regaining consciousness at the hospital, Goodfellow might be in danger if he is able to identify the shooter.

Lady Felicia returns to Kembleford to sell their valuables after her husband, Monty, loses a fortune in a diamond mine. Nicholai Solevey who had offered to buy her necklace steals it and places it in an impregnable safe in his hotel room. Father Brown asks Flambeau for help who agrees because Solevey had betrayed him previously. Flambeau hatches a plan using Bunty, Mrs McCarthy, and Lady Felicia as distractions while he attempts to break into the safe. Father Brown finds his trust in Flambeau could be misplaced when he steals the entire contents of the safe and disappears and returns to Paris.

The Kembleford Choristers, this year run by Mrs. McCarthy, reach the final of the Three Counties Choir Competition 1953 at Worcester Cathedral. Mrs. McCarthy arranges a coach to take the choristers that include Sergeant Goodfellow and Lady Felicia, together with Inspector Mallory, whose daughter is singing in a junior competition, and Father Brown. Events conspire to prevent them reaching Worcester in time. Both Mallory and Father Brown suspect sabotage by one of the choristers and Mrs. McCarthy suspects a rival choir. A dark secret from the past proves them right.

Flambeau seeks Father Brown's help to catch a thief, Alexander Romanici, who is actually Flambeau's daughter Marianne Delacroix. She has been stealing items ahead of her father, including from an Italian crime lord, Vincenzo Murgida (Vincenzo Nicoli), who will kill to recover his property. Father Brown is asked to act as referee and organise a contest between the pair, with the loser having to give up criminal ways. Father Brown devises a contest to recover five chess pieces; the person with the most pieces wins. Neither Flambeau nor Father Brown realise Marianne has made a deal with Murgida; a deal he does not intend to honour.

Sid returns to Kembleford aboard a canal boat with his new girlfriend Maeve and her father, Pat Lochlin. Lochlin has an argument in a pub with local factory owner, Roger Barford, who is later found dead, bludgeoned with one of Lochlin's fishing tools, a Priest, and with a jeweler's receipt for a gold necklace stuffed into his mouth. Barford had asked Lochlin to collect a valuable necklace from a jeweler, but Lochlin replaced it with a cheap imitation. Father Brown discovers Barford got the barmaid her job at the pub after she was dismissed from his factory because she was pregnant. The original necklace is found in the possession of Maeve, who found it on the ground near her father's boat. Father Brown discovers that Barford truly loved his childless wife, though she suspected her husband of infidelity.

Reginald Brody is found dead at the bottom of a tower owned by Alistair Hemsley MP, an apparent suicide by jumping from the sealed roof. Chief Inspector Valentine returns and believes that a red scarf Brody was wearing is related to an eight-year-old murder in which Valentine arrested Brody's son, who was convicted and recently hanged himself in prison. The next day Valentine and Hemsley's brother William, who had purchased a red scarf the morning before Brody's body was found, go back to the top of the tower. William tells Valentine his family members are "evil" right before someone strikes Valentine over the head, knocking him out. When he regains consciousness, William is lying dead, a bloody knife is in Valentine's hand, and Inspector Mallory and Sgt Goodfellow break through the door, which had been wedged shut. Chief Inspector Sullivan returns to investigate Valentine's arrest, to the annoyance of Mallory, who had already been moved aside by Valentine. Father Brown with Bunty and Mrs. McCarthy have to solve three murders. They discover a secret room in the house left as a shrine to Hemsley's late father.

Phillip Darlington is stabbed to death, and Inspector Mallory arrests his son Lawrence after finding a knife in his bedroom – a knife that Sergeant Goodfellow saw Mallory plant. Goodfellow, after confronting Mallory, seeks Father Brown's help. Father Brown and Sid Carter discover Mallory's wife and children are missing, which complicates matters. Darlington's daughter, Arabella, also returns home from Oxford, having just married that weekend. Brother and sister claim they slept all night, the brother in the house and the sister with her new husband in Oxford. A stolen car and a man, known to Sid's prison contacts, that Mallory wronged ten years ago are potential keys to Mallory's recent behavior.

Gerald Firth (Kalon – see Series 1, Episode 5) beseeches Father Brown to speak on his behalf at his parole hearing. Father Brown secures his release but is shocked when Gerald returns to rejoin The Church of Apollo now run by his disciple Tobias. Tobias's partner, Thelma, is unhappy to have Kalon back as she remembers Kalon's past actions with the girls of the church and she has a daughter, Clara. When Thelma is found dead in Kalon's locked room Inspector Mallory arrests Firth. Father Brown believes Firth is innocent even though Clara had written to him in prison, and delves into the various relationships at the church – but to solve the crime, he has to fish in murky waters...

Cardinal Ratcliffe tasks Father Brown to find a priceless Papal tiara hidden in a 15th-century castle. Father Brown takes Lady Felicia, visiting after a row with her husband, to pose as a prospective bride looking for a wedding venue, and Mrs McCarthy – reluctantly – as the bride's mother. Flambeau appears at the castle, having announced himself to the castle owner, Lady Cecily, as the groom wanting to surprise his fiancée. Flambeau declares his love for Lady Felicia, and that he is a reformed man. Circumstances dictate that Father Brown and Flambeau work together to find the tiara when a criminal partner whom Flambeau wronged in the past turns up.

Lord Arthur Hawthorne and his wife return to Kembleford when he retires from editorship of his newspaper and holds a garden party to announce his son Gabe would takeover as editor. At the garden party Hawthorne is threatened by Stanley Buchanan, a homosexual exposed in Hawthorne's newspaper, then a rifle shot wounds Hawthorne. Hawthorne had expected Father Brown to give him his personal attention, and when he refuses, finds himself accused of breaking the seal of the confessional and is suspended from his parish; his place taken by Father Featherstone, Hawthorne's London priest. Inspector Mallory arrests Buchanan, although Hawthorne suspects his wife or son. A second attempt on Hawthorne's life leads to Buchanan's release. Father Brown investigates with Mrs McCarthy's help, and uncovers Hawthorne's philandering, his wife's unhappiness, his son's unwillingness to take over editorship, and Father Featherstone's thwarted ambitions.

Father Brown, Sergeant Goodfellow, Inspector Mallory and his family receive an invite for the 20th anniversary of Chummy's holiday camp. During the visit, owner Marjorie Chummy is strangled with an apron belonging to Mavis Jug, maid and general dogsbody at the camp. Inspector Mallory arrests her while Father Brown and Sergeant Goodfellow look at the other staff, Jock McCudgeon, Griff Grimshaw, and Sandy Beauchamp, at the camp who Mrs Chummy kept under a tight rein. Father Brown discovers that a child, Billy Fairfield, had died in the past in the camp swimming pool that Mavis Jug, then an "Orangecoat", and Mrs Chummy should have been supervising.

Bunty, facing a speeding fine issued by Sergeant Goodfellow, agrees to give a talk at Langley Hall, a Borstal (British Juvenile Hall) for girls of ages 16-23 years. Bunty's compact mirror is stolen, and found in the possession of one of the girls, Brenda Palmer, who is placed in solitary confinement. Governor Cecilia Watson of the Borstal is found murdered, and Brenda is on the run after hiding in Bunty's car. Father Brown and Bunty return to the Borstal to help Sergeant Goodfellow's investigation; Inspector Mallory is on annual leave. They have to unravel the lies and ill feeling between two girls, Kate Goodall (who has a secret boyfriend) and Brenda, and tensions among the uniformed staff, as the governor's deputy, liberal-leaning Emily Harris takes charge over long-serving Helen Delaney who is as hardline as the murdered governor. Father Brown uncovers exploitation of recently released girls such as the recalled Brenda and the newly released Kate.

Sir Charles Hakeworth is garrotted at a New Year's Eve masked ball held at Montague Hall by Lord "Monty" and Lady Felicia Montague. A death threat, stating that the Red Death was coming, had already led to Inspector Mallory closing the estate, trapping the killer. Father Brown with the aid of Mrs McCarthy, Sid, and Bunty look to discover the killer from four people who knew Hakeworth would be present: his wife; his doctor; Bunty's friend Ruth Moulton; and Robert, Earl of Finchmore whose father committed suicide after Hakeworth's adulterous affair with Robert's mother. Father Brown sees a figure in a red mask escaping after stabbing Moulton. Lady Felicia, unhappy with her marriage, has a decision to make when a one-way airline ticket to New York arrives from Flambeau.

The newly opened Kembleford Model Village features two-inch figurines of village characters such as Father Brown, Sergeant Goodfellow, Mrs McCarthy (who has returned to Ireland) and Bunty (on safari in Africa). A proposed housing development by Harry Grover is causing tensions in the village before a crucial parish council vote on the proposal. When Joe Telford and Jennifer Mossop, two of the council members opposed to the development, are murdered, Father Brown notices their figurines were moved to the locations where they were killed before their respective murders. Inspector Sullivan, temporary replacement for Inspector Mallory who has transferred to Scotland, arrests Grover found with Mossop's body, murder weapon in hand. Father Brown is aided by Mrs Devine, hoping to become the next housekeeper; a position Father Brown had given to Mrs Burns. When Father Brown and Mrs Devine find his figurine is missing, she is given the task of finding it while Father Brown looks for one of many suspects. Good news for Mrs Devine and bad news for Inspector Sullivan follow the apprehension of the murderer.

Brenda Palmer, whom Father Brown met at a borstal for girls, arrives at his door asking for help after being accused of theft by Jasper Granford, the owner of a gentlemen's club where she worked. Lady Felicia and Mrs Devine decide to accompany both to the club. Father Brown quickly discovers the real culprit; the owner's son Bertie. Lady Felecia is annoyed when Granford orders her and Mrs Devine out of the club. When Granford is murdered and his safe robbed, Inspector Sullivan arrests Bertie, who is desperate for money for a past indiscretion. A club member, Colonel Partridge, has disappeared leaving his jacket at the murder scene, the former club manager was bankrupted by Granford, and Granford's wife was abused by her husband. Father Brown, with Mrs Devine's help, uncovers Granford's shady business dealings, making money by bankrupting businesses. Brenda is befriended by night cleaner Moira, who worked in a theatre Granford bankrupted. The jacket left at the scene provides a clue with its odour of candlewax and turpentine. Brenda is taken on by Father Brown as housekeeper.

Flambeau believes he is being framed for thefts and murders across Europe when valuables he stole 25 years earlier are re-stolen and his handkerchief calling card is left at the scene. He asks for Father Brown's help, as the last item he stole, a gold bracelet, is up for sale at a nearby auction house. Having received an anonymouse tip about Flambeau's presence, Inspector Sullivan arrives at the auction house just as a security guard is murdered and the bracelet stolen. Flambeau is captured, claiming the auctioneer killed the security guard. Sullivan is swayed by the evidence and arrests Flambeau, which may prove to be his ticket back to Scotland Yard, despite Father Brown finding evidence to support Flambeau's claim. As Flambeau is led away, he secretly gives Father Brown the bracelet. Father Brown investigates the background of the bracelet, its theft 25 years ago and the suicide of a businessman. Father Brown is kidnapped by the killer and his accomplice. Flambeau escapes from Sullivan to find Father Brown and the bracelet aided by Mrs Devine and Brenda.

The 1955 Kemleston Olimpicks between the villages of Kembleford and Hambleston bring murderous tensions between the teams with accusations of cheating, and contestants unable to compete. Kembleford team captain, Dr Geoffrey Fleming, disqualifies his own brother, Brian, for cheating despite protesting his innocence. When a contestant is poisoned during the Spinach eating contest with a bowl meant for Dr Fleming, Inspector Sullivan arrests Brian for murder. Father Brown to prove his innocence has to find someone with a grudge against the doctor and is not short of candidates. Mrs Devine and Inspector Sullivan's not so secret affair is exposed when her son, Eddie Devine who idolised his dead father Ronald, arrives home unexpectedly. A photograph taken by Ronald Devine at the 1952 Olimpicks gives Father Brown a clue.

The Kembleford Arts and Crafts Fair organised by newcomer to the village Gaynor Garfield lets artistic rivalries to come to the fore, compounded when renowned artist Marmaduke Snell arrives. His obnoxious behaviour towards the other artists and craftspeople is tolerated by Garfield. When he is poisoned by cyanide in a bottle of wine Inspector Sullivan arrest the wine maker who is none other than Sister Boniface who provided the wine from her convent St Agnes that she had bottled herself. Father Brown with Mrs Devine and Sergeant Goodfellow's help deceive Inspector Sullivan to free Sister Boniface so she can prove her innocence. The discovery of a wedding ring, a missing tin of metal polish and glucose tablets provide a solution.