Whilst Father Brown is visiting the Upcott seminary for trainee priests young student Thomas Potts confesses to him that the recent drowning of his friend Douglas Hitchens was no accident before he plunges to his death from the bell tower. Father Brown suspects foul play but is blocked by the sinister Father Lewis and so, with the connivance of the elderly rector Father Palfreyman, sends Sid in undercover to pose as a trainee. He soon learns of the Secret Fraternity, an elite group of students, and believes that they are covering something up - but which of them is a murderer?