Father Brown receives a mysterious note suggesting that the church's priceless holy relic, the Blue Cross, will be stolen. In the church itself, he finds a handkerchief with the letter F, no doubt left there by the infamous thief Flambeau, wanted across Europe for the theft of priceless artifacts. Bishop Talbot is soon on the scene and makes arrangements to have the cross transported by the police to a safe location. Father Brown decides it would be safer in his possession and sets off on his own. He is out not only out to identify the thief but also to save his soul.
—garykmcd