A large house in a respectable suburb of Paris is the residence of Dr. Cordelier, an eminent psychiatrist who has withdrawn from society to pursue his secret research into the functioning of the human brain. His lifelong friend, the lawyer Maître Joly, becomes concerned when Cordelier draws up a will in which he bequeaths his entire estate to a stranger, Monsieur Opale. When Joly discovers that Opale is a misshapen, sadistic brute, who willfully attacks children and women, he cannot understand why Cordelier defends him, allowing him sanctuary in his house. After Opale kills a colleague of Dr. Cordelier, Joly has no other option but to confront Cordelier, convinced that he is under the influence of the evil Opale. In Cordelier's laboratory, Joly discovers the terrible truth of his friend's strange behavior.