A body is discovered in a Milan apartment, and Inspector De Vincenzi investigates. The apartment happens to belong to an old university friend of his, Aurigi. When the body turns out to be that of Aurigi's banker, and a phial of prussic acid is discovered in the bathroom, suspicion falls on the apartment's owner, and De Vincenzi is agonisingly torn between his sense of duty and his loyalty to an old comrade...