在这部布朗神父喜剧/犯罪/剧情片中,Father Brown tries to transport a historically important cross to Rome engaging in battles of wits and faith with a thief and pursuing policeman. The thief, named Flambeau (Finch), is a master of disguise and is elusive, as Father Brown pursues him and tries to convince him to abandon his criminal career.
During the making of the film, Alec Guinness was mistaken by a small boy for an actual priest. This episode appears to have played a part in Guinness's later conversion to Roman Catholicism.
Amateur detective Father Ignatius Brown defies his Bishop and decides to transport to Rome a holy relic from his church - a cross that once belonged to St. Augustin - rather than allow the more elaborate plans to proceed. On the channel crossing he becomes suspicious of a fellow traveler, a Mr Dobson, whom Brown quickly determines is not the automobile salesman he claims to be. He does befriend another priest whom he takes into his confidence but soon realizes that his suspicions should have been reversed. The fake priest is in fact Gustave Flambeau a professional art thief and an expert at disguise. After he gets away with the cross, Brown refuses to work with the police, insisting that he wants to save the man's soul, not put him in prison. With the assistance of his friend Lady Warren, Father Brown sets a trap for Flambeau but Brown realizes that his work is only just beginning.
