Mr. Joly, doctor Cordelier's lawyer, is amazed to discover that his client and friend leaves his possessions to a stranger, Opale, a sadistic criminal. He needs this man to prove that people's behavior can be adjusted at will...

When the lawyer Joly receives the testament of his friend and psychiatrist Dr. Cordelier, Joly realizes that he is giving all his fortune and assets to his unknown patient Opale. When Joly learns that Opale is an evil man, he believes his friend is being threatened or blackmailed by Opale. Soon Opale murders a man on the street and the psychiatrist Dr. Lucien Séverin, who has a beef with Dr. Cordelier due to his experiments. After a party at Dr. Cordelier's house, his butler Désiré summons Joly since Dr. Cordelier has locked himself in the laboratory and now he is screaming in pain. Joly and Dr. Cordelier's employees break in the laboratory and find Opale inside. He asks to everyone but Joly to leave the laboratory and gives a tape to the lawyer to learn what happened with Dr. Cordelier.—Claudio Carvalho, Rio de Janeiro, Brazil