Dr. Cordelier, living in a suburb of Paris, withdraws from society to pursue research into the functioning of the human brain. His lifelong friend, Maître Joly, becomes concerned when Cordelier draws up a will that bequeaths his entire estate to a stranger, Monsieur Opale; he cannot understand why Cordelier defends him, considering Opale attacks women and children. After a colleague is killed, Joly confronts Cordelier and discovers the truth behind his friend's behavior.