When amateur detective Father Brown (Alec Guinness) is put in charge of shipping an extremely valuable religious artifact - a crucifix - from London to Rome, a master thief and master of disguise named Flambeau (Peter Finch) outwits the young priest and steals the cross for himself. Now Father Brown must track down Flambeau, try to reclaim the crucifix and also convince Flambeau not only to abandon his criminal career but also to save his immortal soul.