Maigret and the Saturday caller

Inspector Maigret investigates a crime that hasn't yet been committed when a tense, fearful man comes to Maigret's home and announces that he intends to kill his wife and her lover. Maigret feels uneasy as he begins to investigate a crime that hasn't yet happened; what he uncovers in the bistros of Montmartre is a most peculiar menage a trois.