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.

In this crime drama, Michel Simon is cast as Pierrot, an elderly gangster who does not fit the stereotype -- he is soft-hearted. After a petty criminal betrays his cohorts by taking off with the loot from a big robbery, he is caught and sent to jail. Now he has served his time, and Pierrot is given the task of retrieving the stolen cash. The tyro criminal tries to use a pretty young woman who has fallen in love with him as a red herring for Pierrot's investigation. Everything backfires though, and Pierrot is left considering what to do with the loot, and with the criminal who does not yet realize he loves his attractive accomplice and could have a good life with her if he opts for walking the straight and narrow.