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.

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.

A mysterious visitor to the confession box informs Father Brown that choleric Bishop Talbot will be murdered so Father Brown joins Talbot at a shooting party where a shot is indeed fired though the victim is the seemingly innocent Albert Davies. The bishop had argued with actor Peter Redhill over his divorce and there is certainly intrigue among Redhill and his fellow players whilst the bishop seems to inexplicably own a large collection of banned plays. In his investigation the father finds that the play's the thing and also uncovers a bizarre pact.