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 traveller, a Mr. Dobson, who Brown quickly determines is not the automobile salesman he claims to be. He does befriend another priest who 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.