In medieval northern Italy, Baccello from Stimigliano loses the key to his wife`s chastity belt while on his way back from the Crusades. He soon discovers that his wife, Ubalda, has been unfaithful to him, thanks to a duplicate of the key made for her by the inventor Capoccione from Capena. Baccello decides to get even with Capoccione by seducing his beautiful wife, Genuflessa.