Father Brown attempts to transport a crucial cross to Rome, engaging in witty and spiritual battles with a thief named Flambeau (Finch) and a pursuing policeman. The thief, a master of disguise, is elusive, and Father Brown struggles to convince him to abandon his criminal career. During the filming of the movie, Alec Guinness was mistaken for an actual priest by a young boy. This incident appears to have had a significant impact on Guinness's later conversion to Roman Catholicism.