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.