Dr. Cordelier, who resides in a suburb of Paris, withdraws from society in order to do study into the human brain's function. Maitre Joly is alarmed when Cordelier bequeaths his entire estate to a stranger, Monsieur Opale; he cannot see why Cordelier defends Opale, given that he attacks women and children. Joly confronts Cordelier and discovers the truth about his friend's actions following the murder of a coworker.