Works of art are disappearing, stolen by a master thief, a master of disguise. Father Brown has two goals: to catch the thief and to save his soul.
After notorious jewel thief Flambeau meets Evelyn Fischer during a raid on a casino, he falls deeply in love with her. Later Flambeau sends notes to both Leopold Fischer, who unknown to him is Evelyn's uncle, and Father Brown, in which he vows he will steal from them the ten diamonds that comprise the "Flying Star." Flambeau intends to give these diamonds to Evelyn. Father Brown, whose gold cross contains some of the Flying Star diamonds, is determined to meet Flambeau before he is arrested, to reform and redeem his soul.
A Manhattan priest with a penchant for solving crimes goes to the aid of a young actress. She is becoming enmeshed in a series of bizarre incidents she can't explain, and her complaints to the police have gone ignored.