Dr. Cordelier, who lives in a Paris suburb, withdraws from society in order to conduct research on the functioning of the human brain. Maître Joly, Cordelier's lifelong friend, is concerned when he drafts a will that bequeaths his entire estate to a stranger, Monsieur Opale; he cannot understand why Cordelier defends him, given Opale's attacks on women and children. Following the death of a colleague, Joly confronts Cordelier and discovers the truth behind his friend's actions.