A Devil Under the Pillow (1968)

Original title: Un diablo bajo la almohada

Anselmo is a young anthropologist who is tormented by jealousy, and although his wife Camilla does not give reasons, keeps a close eye on her. After consulting a psychiatrist, the doctor says the problem is him and not his wife. So he plots to verify, once and for all, if his suspicions are true. By chance he meets an old friend, Lotario, who seems the ideal person to carry out his plans. What Anselmo does not know is that his friend is a real womanizer who ends seducing his wife.