Arrived in Naples to work in the Doumenec family's factories, engineer Stefano Mascardi meets Rita, wife of Junio Doumenec, eldest son of the industrialist Alfredo Doumenec, founder of the factory of the same name. A liking is born between the two but, soon, Stefano learns that Rita is involved in shady deals. The woman is in fact the lover of an unscrupulous man, Mr. Detoledo and, in agreement with him, has recently stolen an important industrial formula from Doumenec. When Mr. Detoledo is found killed, Stefano receives a mysterious phone call. A woman arranges to meet him in the very place where Detoledo's body is found by the police. It's a trap? Maybe... But in the meantime, next to Mr.'s body. Detoledo, a photograph portraying three girls is found. One of these is Rita Doumenec..