Marc Lacroix is a psychiatrist, a student of the brain, with a rocky marriage, a mistress, and a ten-year-old son. He also has a secret laboratory where he's built a machine he thinks will allow him to penetrate the mind of any person hooked up to the machine with him. He tests it with Zyto, a psychopath who has killed three women. The machine fails and exchanges the men's brains. Zyto passes himself off as Marc, and Marc (now in Zyto's body) escapes from the asylum to the flat of his mistress. She tries to warn Marc's wife, exposing herself to Zyto. All are now in grave danger.