On the Costa del Sol, a dentist and his wife have a fight. He leaves and returns to find her dead. When her look-alike sister shows up, she and the husband decide she should impersonate the wife. Each thinks the other has committed the murder, and they are trying to protect each other. The husband finally realizes the wife died of allergic reactions to medicines administered by her lover/doctor. He leaves with the sister, and the doctor runs upstairs to the unknown-to-him corpse.