Dr. Cordelier, a resident of a Paris suburb, withdraws from society in order to do study into the human brain's function. Maitre Joly is troubled when Cordelier bequeaths his entire estate to a stranger, Monsieur Opale; he cannot comprehend why Cordelier defends Opale, given that he attacks women and children. Joly approaches Cordelier and discovers the truth behind his friend's behavior when a colleague is murdered.