An exhibition of valuable art at the Belvedere Museum includes a priceless artwork on loan from the Vatican. Father Brown spots his nemesis Hercule Flambeau (John Light) not realizing he is a pawn in Flambeau's plan to steal the picture and wreak revenge on the museum's curator; a man who during the war had helped the Nazis loot paintings from Jewish families. Flambeau is shocked to find a woman, he thought dead, from his past seeking to avenge the death of her parents by killing the curator.