Spasmo (1974)

Christian (Robert Hoffman) and his girlfriend are taking a walk on a deserted beach when they discover a woman's body lying on the sand; a closer look proves that she's alive. The next day Christian meets her again at a yacht party and they fall in love. Later, at a nearby motel, they're preparing to go to bed together when an intruder breaks in and starts beating Christian, who accidentally shoots him with his own gun. A few hours later they finds out that the corpse is missing, and weird incidents start happening.