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. He recognizes Flambeau when he comes to his house under the pretense of having had an automobile accident outside, and cleverly hides the diamonds in his glass of milk until Flambeau reluctantly leaves without them. Flambeau continues to flirt with Evelyn from afar, but does not see her again until he appears at her house impersonating an insurance investigator to find out where the safe is hidden. She is upset that he has not previously introduced himself like a gentleman should. Later that night, he appears in her bedroom after he has stolen the diamonds from Fischer and tries to make love to her, but she is disgusted by his profession and refuses to have anything more to do with him. Later, Father Brown visits with Evelyn and realizes she is in love with Flambeau. Together they make plans to reform the beloved Flambeau. Flambeau attends the Eucharistic Congress in London disguised as a priest, in hopes of stealing Father Brown's gold cross. Father Brown recognizes him and, by behaving irrationally, attempts to demonstrate how certain behavior, such as thievery, disrupts the natural order of things. There is a method to his madness, however, because his "trail" of irregular behavior is noticed by many people and allows the police to find him and Flambeau. Before the police arrive, Father Brown allows Flambeau to escape without the diamonds, warning him that Evelyn will only pity him as a thief. Flambeau returns the Fischer diamonds to Father Brown, who elatedly reunites him with Evelyn, but when police arrive, Flambeau believes Brown has betrayed him and escapes. As Father Brown prays for his soul, Flambeau has a change of heart and turns himself in. Evelyn promises to wait for him, and Father Brown knows that God has heard his prayers.