Claude Chabrol's La Femme infidèle | The Unfaithful Wife (1968)

La Femme infidèle is seen by Guy Austin in Claude Chabrol (French Film Directors series) as the first genuine film in the Hélène series, and that particular Hélène is Hélène Desvallées (Stéphane Audran), the wife of Charles (Michel Bouquet), who is having an affair with Victor Pégala (Maurice Ronet): Chabrol himself stated that none of his Chabrolian characters (called Charles) could be seen to kill a person called Paul (a Gégauffian character), hence the name Victor.

And, of course, Charles kills Victor after learning from the private detective he's hired, Bignon (Serge Bento), that Victor is having an affair with his wife. Charles is seen wiping his finger prints from every object touched in Victor's place, including the huge cigarette lighter that had in fact been an anniversary present from Charles to Hélène, and then dragging the wrapped body to his car and dumping it in the river.

This surprisingly brings a lift to their marriage, things are more together (including the jigsaw of their son Michel (Stéphane di Napoli)), even if Charles does have to go to the police and presumably confess to the murder.