A priest is found in an open safe during a nighttime police raid; he explains that he is replacing the money for a parishioner. After the bishop verifies his identity, he is freed from custody after being imprisoned and placed in cells. He meets Bert (Sid James), a mistaken parishioner, outside and persuades him to drive Lady Warren instead of driving getaway automobiles. He is selected to bring a precious crucifix that is 1,200 years old to Rome. He knows that Flambeau, his arch-criminal rival, might attempt to take the cross. After sailing to France, he boards a train. Paris is the first destination. A priest he met on the voyage is with him. They are aware that two police officers are observing them while they are seated at a cafe. The police are left stranded when they board an excursion bus and wait until there are just two seats available. A bunch of prostitutes and the two police officers are given a lift in the back of a police van. The catacombs are the destination of the expedition. They split off from the group there. Since the priest requested a ham sandwich (on a Friday), Father Brown has deduced that he is Flambeau. However, Brown is trapped against a heap of bones after Flambeau overwhelms him and takes the cross. Flambeau disguising himself, he avoids the two police officers standing at the door. In order to trick Flambeau into stealing a valuable silver chess set, Father Brown persuades his acquaintance Lady Warren to put it up for auction. They anticipate that he will show up at the auction disguised. Flambeau does, but not as a bidder; after the bidding is over, he is the porter who takes the set out of the room. To make a point, however, Flambeau gives the set back to Lady Warren. To give the impression that they required the space, Flambeau and Brown empty the rear of a milk van of all the milk when the cops arrive at the door. The police follow the van as a result of this diversion, and Brown and Flambeau manage to get away. In the library, Brown begins studying Flambeau (breaking his spectacles in the process) and discovers a connection to Fleurency in the French area of Burgundy. Then he goes there. He locates Flambeau at a wine festival, but Flambeau eludes him. He locates the old château the following day and requests "the duke." They say he is not at home. Disguised as an elderly woman pushing a cart of grapes, he sneaks into the inner courtyard. He discovers evidence of habitation upon entering the chateau, but it is in ruins. Flambeau invites him to explore the hidden door he finds in the back of the large kitchen fireplace. He has an incredible collection of artwork within. He hands the stolen cross to Brown. When the police show up, Flambeau runs out the window. When the police show up, Flambeau runs out the window. The police retrieve his art collection, which is then put on exhibit at the Louvre. Father Brown preaches about the Prodigal Son back in England. Flambeau walks in and takes a seat beside Lady Warren.