Dr. Cordelier, who lives in a suburb of Paris, withdraws from society to study how the human brain works. His lifelong friend, Maitre Joly, is worried when Cordelier writes a will that gives his entire estate to a stranger, Monsieur Opale. He doesn't understand why Cordelier defends Opale, since he attacks women and children. After a coworker is killed, Joly confronts Cordelier and finds out the truth about his friend's behavior.