Father Brown is invited to a tea party to celebrate a new church clock tower, hosted by his friend Reverend Bohun. The serene atmosphere ends up being shattered, after his caddish brother Norman shows up and annoys most of the guests. After Norman's body is discovered behind the church, Father Brown investigates to stop an innocent woman being sent to the gallows.
Colonel Adams discovers that his wife Anne-Marie has started drinking again and he tells her that he wants a divorce. Lady Felicia finds out that Ruby Adams plans to get engaged to her American tutor John Van Ert but she won't give permission, sacking John in the process. Father Brown shows up with Mrs McCarthy and when Anne-Marie is found drowned in a lake he investigates the case.
Umesh Varma bursts into a study where Leonard Quinton is sleeping, to inform him that he is going to die that same day. Lady Felicia invites Father Brown to a poetry recital at the Quinton's. Mrs McCarthy joins them and after they show up she mistakes Violet for Leonard's wife. After Leonard's body is discovered hanging in the conservatory Father Brown investigates.
Lady Felicia is shocked to discover an injured man in his late 30s, stuck in a tree on her estate, wearing only his underwear. At Kembleford station, Father Brown and Mrs McCarthy meet their German visitor Father Franc. Father Brown witnesses the man being rescued from the tree and ends up surmising that he was pushed from a train carriage.
A truck shows up at a gothic manor in Kembleford, and people get out and unpack stacks of bibles emblazoned ?Church of Apollo'.Susie screams as howling wind knocks over a candle in her hut, which sets some photographs of her late parents on fire. Father Brown, Mrs McCarthy and some other villagers attend a welcome ceremony in the gardens of the manor where the group's founder Kalon regales the crowd about the astral spirits. He then takes a vulnerable Susie from the crowd and later asks her to greet the sun with them in the morning. After Dominique is mysteriously killed, Father Brown realises that Susie could also be in serious danger.
The clergyman sleuth is called in to investigate when two nuns die in unusual circumstances at St Agnes' convent. However, the mystery attracts the attention of Sister Boniface, a would-be crime-solver with a passion for detective novels.
Geoffrey and Emily are upset after Dr Evans gives up on treating their daughter Ruth , who has mysterious disfiguring warts on her body. Mrs McCarthy thinks that Ruth has atomic bomb sickness. A sceptical Father Brown goes to the Bennetts' house, where Ruth helps their ineffectual plumber Douglas. After Ruth goes missing, Father Brown investigates the mystery.
Lady Margaret receives a drunken call from Daniel who tells her that ?she'll pay for what she did'. The following day Father Brown discovers Daniel lying outside the church and he wants to know why Lady Margaret accelerated towards his dad, running him over.
William Knight, the new Mayor of Kembleford tosses a whisky glass at his wife Eleanor after she asks why a necklace was found under their bed. William later fires his secretary and ex-lover Matilda. As he makes a speech at the Kembleford Annual Village, William is interrupted by local councillor Edwin, who reveals that William has secretly purchased the land of the Polish resettlement camp, which has been wanted to be used for a new coal-fired power station. Susie is furious. William suddenly shudders and collapses, dead and Father Brown soon realises that William was electrocuted.
It is Saturday evening and Father Brown finds a note on the mat which says 'to catch a thief, look to the cross'.
After checking to see if the Blue Cross, an ancient holy relic, is still in the church, he then discovers a dropped handkerchief with a monogrammed letter ?F'...