The psychiatrist Marc Laxroix is a studious scientist of the brains of criminals. He has an unhappy and loveless marriage with his wife Marie Lacroix and they have a boy, Léonard Lacroix. Marc has also a mistress, Marianne, who is also his confidante. Marc has secretly built a machine to study the mind of a person to another one in his old house, but he has not tested it yet, and Marianne is the only person who knows. When the serial-killer Michel Zyto is sentenced to life in the institution where Marc works, he decides to study the mind of the psychopath. Then he brings Zyto to use the machine and their minds are exchanged. When Marc asks Zyto to press a green button to restore their minds, Zyto refuses and uses Marc's body to lock up Marc trapped in his body in the institution. Further, he goes to Marc's house to stay with his family. Will Marc retrieve his body?