Father Brown attempts to overturn a miscarriage of justice while rehearsing for the Kembleford Christmas pantomime.

Father Brown struggles to protect those closest to him as old adversary Katherine Corven is unexpectedly released from prison, thirsty for revenge.

The prospective closure of the local railway station leads to a fund raising event to save it, at which the station master Ben Webb is found murdered. Chief suspect is Deepak Chatterjee, the man in charge of rail closure, but he reveals a secret about Ben which allows Father Brown to unmask the real killer, in addition to acting as an impromptu midwife.

Mrs McCarthy's friend Freda Knight dies in her sleep at the hospital where Bunty's old nanny Ellen is also a patient but Father Brown feels her death was not natural, especially when the sole heir to Freda's will is also found dead. The father believes that a misguided person may be practicing euthanasia on terminally ill people and hurries to stop them before Ellen becomes the next victim.

Lady Felicia returns from Africa, having split from her lover Benedict Northam and is told by MI5 agent Daniel Whittaker that he is a Soviet spy, blackmailing her into preventing Northam from handing over vital documents to the Russians at an arts fund raiser. Father Brown notices her unease and follows her, coming to her aid when she is accused of murder.

Forced to reapply for his job Mallory is attending a local bowls tournament when Scotland Yard Commander Alec Frobisher, a former Nazi hunter is garroted. Replaced by the arrogant DI Ironside Mallory, for once, is grateful to Father Brown for helping him solve what proves to be a double murder.

Lucy Dawes is murdered at a ballroom dancing contest and her necklace stolen. Her boyfriend Oliver DeWitt is arrested but her blind dance partner Alexander believes their hostess Lady Rose is responsible and is guarding a guilty secret. Oliver holds another secret. Father Brown must work out who is telling the truth to solve the murder.

A girl is found near-term dead at a private school fete that Father Brown is attending. His observations and intuition set him against the conclusions of the police, after suspicion falls on a pupil.

Locals led by Hermione Harvey protest when brash American Raylan Reeve opens a golf club in the village and he receives death threats. During a golf tournament he is murdered but there is another corpse, that of a man Reeve's daughter Tamara clearly recognizes. Father Brown uncovers a shocking secret in the Reeve family to link the two murders.

Father Brown reads that Flambeau has been killed in Italy before setting off to Gloucester where the Iron Crown of Lombardy, made from a nail from the Crucifixion, is housed. He is approached by a woman claiming to be Flambeau's widow but a key Flambeau sent him has been stolen and Father Brown believes it is connected to the intended theft of the Iron Crown. A chance meeting on a train proves him right and he must stop the theft - at the risk of his life.