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.

Jean-Louis Barrault
as Dr. Cordelier / Opale

Teddy Bilis
as Joly

Michel Vitold
as Dr. Lucien Severin

Sylviane Margollé
as Little Girl Attacked by Opale

Jacques Danoville
as Commissioner Lardaut

André Certes
as Inspector Salbris

Jean-Pierre Granval
as Boss of the Short-Time Hotel

Jean Bertho
as First Passer-By during the Aggression

Jacques Ciron
as Second Passer-By during the Aggression

Annick Allières
as Neighbor