This deftly made French psychological thriller terrifies with its exploration of an experiment gone horribly wrong. Dr. Marc Lacroix, a psychiatrist specializing in brain functions, is obsessed with the link between mind and spirit. To find his link he studies the criminally insane. He builds a secret machine, known only to his mistress Marianne, in which he hopes to exchange minds between humans. He simultaneously wants to help the mentally ill recover, and he wants to experience their madness. He chooses the psychotic killer, Zyto, a man who stabbed at least three women, for his experiment. The initial experiment is successful and the two exchange minds. But trouble ensues when Zyto refuses to reverse the switch. There is little Marc can do when Zyto takes over Marc's life, and more ominously his wife and child whom are unknowingly in mortal danger. Marc, encased in Zyto's body, is returned to the asylum. Will Marc's wife recognize the danger? Will Zyto kill again?