Nestor Patou, a naive and honest policeman in Paris, is transferred to the red-light district of Les Halles, where he mistakenly raids a local brothel, believing it to be an innocent hotel. This action gets him fired from the police force. Disheartened but determined, Nestor takes a job as a bartender at a nearby café, a popular hangout for the prostitutes and their pimps. There, he meets Irma, a charming and spirited prostitute. As they form a connection, Nestor decides to become her protector, eventually replacing her abusive pimp. He quickly realizes the moral complexities and dangers of the underworld he has entered.

Determined to support Irma without compromising his values, Nestor devises a plan to keep her from working the streets. His scheme involves assuming a secret identity, "Lord X," supposedly a wealthy Englishman who becomes Irma's exclusive client. Nestor juggles his double life, managing to convince Irma while struggling with jealousy and exhaustion from maintaining the ruse. As complications arise, Nestor's elaborate charade spirals out of control, and he faces increasingly difficult challenges. From this point on, events unfold that test his resourcefulness and the strength of their relationship.