The police raid a premises at night and find a priest at an open safe: he explains he is replacing the money for a parishioner. He is arrested and put in the cells but released when the Monsignor confirms who he is. Outside he meets the erring parishioner Bert (
Sid James) and convinces him to be a chauffeur to Lady Warren rather than drive get-away cars. He is chosen to go to
Rome carrying a 1,200-year-old priceless crucifix. He is aware that his rival, the arch-criminal
Flambeau, may try to steal the cross. He sails to France and then catches a train. The first stop is
Paris. He is accompanied by a priest he met on the ship. As they sit in a cafe they are aware that two policemen are watching. They wait until there are only two seats left on an excursion bus then grab it, leaving the police stranded. The two police get a lift in the back of a police van with a group of prostitutes. The excursion goes to the catacombs. There they separate from the group. Father Brown has worked out that the priest is Flambeau as he ordered a ham sandwich (on a Friday). Nevertheless, Flambeau overpowers him and steals the cross, leaving Brown tied against a pile of bones. Flambeau changes disguise and gets past the two police who wait at the entrance. Father Brown convinces his friend, Lady Warren, to auction an important silver chess set to lure Flambeau into stealing it. They expect him to appear in disguise at the auction. He does, but not as a bidder: Flambeau is the porter who carries the set out of the room after the bidding concludes. But Flambeau returns the set to Lady Warren to prove a point. When the police arrive at the door Flambeau and Brown remove all the milk from the back of a milk van to make it look as if they needed the space. This diversion makes the police follow the van and Brown and Flambeau escape. Brown starts researching Flambeau in the library (breaking his glasses in this task) and finds a link to Fleurency in the Burgundy region of France. He then goes there. At a wine festival, he finds Flambeau but Flambeau slips away. The next day he finds the old chateau and asks for "the duke." He is told he is not home. He slips into the inner courtyard disguised as an old woman on a cart of grapes. On entering the chateau he finds signs of habitation but the chateau is ruinous. He discovers a secret door in the back of the big kitchen fireplace and Flambeau asks him to enter. Inside he has a priceless art collection. He gives Brown the stolen cross. The police arrive and Flambeau flees through the window. His art collection is retrieved by the police and displayed in the
Louvre. Back in England, Father Brown gives a sermon on the
Prodigal Son. Flambeau enters and sits next to Lady Warren. ==Cast==