Lady Felicia finds an unconscious, stripped man in a tree, apparently having been thrown from a train passing on the viaduct above.

When two nuns are murdered, Father Brown investigates in the convent with the help of a young nun, very keen on detective fiction.

At a fête to raise funds for a Polish school unpleasant mayor William Knight is electrocuted by a microphone. Valentine arrests his long suffering wife Eleanor after a screwdriver is found in her handbag but there are plenty of other suspects, including his young daughter and the secretary he sacked after their affair was discovered, whilst he was also accused of bribery and corruption. As Father Brown solves the mystery a magician at the fête turns out to be the husband,presumed dead, of his housekeeper Mrs McCarthy, who walked out on her decades earlier. Now dying he wants to return to her but she refuses him.

When the notorious thief Flambeau declares his intention to steal a valuable blue cross from the church, Father Brown, against the advice of Valentine, decides to take it to safety to Newbury abbey by train. He meets a trio of fellow passengers, one of which he is sure is the thief but his efforts to flush him out lead to the theft of the cross. Facing dismissal for his rash action the Father agrees to meet with Flambeau and do a deal to return the artefact.

Charlotte McKinley asks Father Brown to exorcise her house, which she believes is haunted by the ghost of her sister Elspeth, who disappeared nine years earlier. The priest believes somebody is fabricating a ghost but then Charlotte disappears from a locked room. Inspector Valentine suspects Charlotte's husband Victor, a former doctor struck off for irresponsible prescription and previously believed to have killed Elspeth. A skeleton is unearthed but it is not Elspeth's and a coded message in a book leads Father Brown to discover the fates of both sisters along with the legend of the family's hidden treasure.

Wearing only a hospital gown Felix Underwood returns to the village and says 'Murder' before collapsing, apparently dead. At his funeral however the 'corpse' comes back to life but with a loss of memory. He was a patient at Danvers Retreat, a psychiatric hospital for service personnel, which Father Brown visits, along with new Inspector Sullivan, discovering a body hidden in a well. Father Brown gets himself admitted to Danvers Retreat in order to get close to a young woman who supplies clues in biblical texts and to solve both the murder and the mystery surrounding young patient Jeremy.

Impoverished aristocrat Sir St John Pryde plans to sell estate land, which antagonizes his tenants led by left-wing Alan Archer. On the day Pryde castle is opened to the public tour guide Audrey Diggle is murdered after claiming that she has evidence to state that the Prydes are not the lawful heirs to the title though St John's nephew Jago believes his uncle was the intended victim and St John's daughter Bunty is secretly in love with Archer. After an attempt to kill Jago Father Brown unmasks the murderer, in the process demonstrating that a supposed family curse has a basis in truth.

Professor Ambrose is abducted after sending his friend Father Brown an ancient prayer book which supposedly can locate the long disappeared Lannington rosary, said to hold healing powers. Aided by Lady Felicia's chauffeur Sid the priest uses the book to find clues which will lead him to the rosary. There is a deadline as a ransom note arrives, informing the father that the rosary must be delivered to save his friend's life whilst the quest is complicated by the arrival of supposedly reformed criminal Flambeau, who claims he is working as a mercenary for the Papacy to find the relic. And then there is a third party, the actual kidnapper, whom Father Brown must expose.

Alice Armstrong is receiving electric shock treatment for memory loss after her accidental shooting her mother. Six weeks after her mother's death her father Sir Aaron is also found dead from a blow to the head, his hands tied. Suspicion falls on Jim Magnus, the chauffeur and a former petty crook and on Peter Royce, the secretary, who loves Alice and was sacked after telling Sir Aaron not to send her back to hospital. But a conversation with debt collector Eddie Monk informs Father Brown that the dead man owed huge sums of money and another confession suggests that Sir Aaron tied his own hands. With the death solved Alice then recollects that she was not responsible for her mother's slaying.

Ex-colonel Gerard is poisoned after arguing with his nephew Edward. Edward is a former POW from Korea and the colonel claims he has been brain-washed but Edward sticks to his story that Gerard killed his father in order to marry his widow Ada. None of the household - including Ada's brother Rupert and his adopted daughter Jia-Li - seem upset at the colonel's death and each had a motive for murder. Then Father Brown is poisoned - hopefully he can recover in time to unmask the killer.