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.