Gerald Firth beseeches Father Brown to speak on his behalf at his parole hearing. Father Brown secures his release but is shocked when Gerald returns to rejoin The Church of Apollo now run by his disciple Tobias. Tobias' partner, Thelma, is unhappy to have Kalon back as she remembers Kalon's past actions with the girls of the church and she has a daughter, Clara. When Thelma is found dead in Kalon's locked room Inspector Mallory arrests Firth. Father Brown believes Firth is innocent even though Clara had written to him in prison, and delves into the various relationships at the church - but to solve the crime, he has to fish in murky waters..