Drama based on the stories by GK Chesterton, about a crime-solving Roman Catholic priest.
Father Brown (Mark Williams) arrives at a police event and runs into Inspector Mallory (Jack Deam) who clashes with his former boss DCI Webb (Colin Mace). Mrs McCarthy (Sorcha Cusack) is shocked to see dioramas of murder scenes – what sick mind comes up with something like this? Wilber Lesser (David Yelland) says his daughter Agnes (Mariah Gale) does, as a teaching tool in the art of crime detection, they're remarkable. Later Agnes takes Father Brown to her workshop and shows him a diorama depicting the death of her mother, who fell down the stairs. When Agnes displays it for all to see, Wilbur loses his temper. As everyone files away DCI Webb, who was the original investigating officer, spots something in the diorama and takes it. Soon after he's found dead, a carpenter's awl stabbed into his chest. With blood found on his coat, the prime suspect is Inspector Mallory himself, who is arrested for murder…
Professor Ambrose (James Laurenson) goes to see his old friend Father Brown (Mark Williams) to ask for help - he's been followed ever since he came into possession of the diary of the university's architect. It claims that the location of the alchemist's secret is hidden in a riddle – the key to the lock to the door. Mrs McCarthy (Sorcha Cusack) and Bunty (Emer Kenny) accompany them to Gloucester University where Principal Malcolm (Danny Webb) allows student Timothy (Richard Southgate) to borrow a ceremonial key. When the absent minded Professor goes missing, Father Brown sees if he's with Timothy. He's saddened to find Timothy's body in his bedroom, his head bashed in with a polo mallet. When the key that Timothy took turns up in the Professor's room, Inspector Mallory (Jack Deam) arrests him for murder, leaving Father Brown determined to prove his innocence.
Recently released from prison, Sid buys a gun outside his caravan. The next day Father Brown reunites Sid with Bunty (Emer Kenny) and Mrs McCarthy (Sorcha Cusack), who insists on cooking him a hot meal. When Sid leaves suddenly, Father Brown follows him to a function where Edward Reece (Simon Shepherd) is honouring his latest judicial appointee Giles (David Reed). Sid confronts Giles, his former defence barrister, accusing him of burying a witness statement which suggests that his supposed victim of assault was coerced into incriminating Sid. Edward sees the commotion and orders Sid and Father Brown off the property. But Sid runs back and as Father Brown follows a scream is heard. Giles has been killed, a broken champagne glass in his throat. When Edward's son Randolph (Alex Felton) tells Inspector Mallory (Jack Deam) that Sid did it, Sid is quickly arrested. Can Father Brown clear his name this time?
Flambeau (John Light) is incarcerated under suspicion of murder. When Father Brown (Mark Williams) visits him he denies killing his cohort Flynn (Callum Dixon) and asks the priest to hear his confession if he's convicted. Flynn's wife Peggy (Emma Pallant) later tells Father Brown that she witnessed Flambeau dragging Flynn's body to the river and throwing it in. At the presbytery, Mrs McCarthy (Sorcha Cusack) thinks that Flynn is still alive and has framed Flambeau. Father Brown leaves the surveillance to her and Bunty (Emer Kenny) while he attends Flambeau's pre-trial hearing. He's shocked to hear Flambeau plead guilty then be sentenced to death by hanging. Meanwhile, Bunty and Mrs McCarthy spy Flynn through the window at Peggy's house and manage to escape after they're seen. However Father Brown has realised that Flambeau isn't the victim in all of this, he's the architect. But what if his plan backfires?