Paris. Bored demi-mondaine Zelie de Chaumet, preparing for an evening out, seems dissatisfied with her luxurious surroundings. Her rich lover and keeper, Stetz, leaves for the notorious White Coffin Club to arrange an after-theatre party to satisfy Zelie's taste for the sensational.

In a run-down area next to the Seine, a rat disappears down a hole. Meanwhile, Pierre Boucheron, 'the Rat', runs down an alley, pursued by Police. Ducking into a sewer, he confuses a policeman by cutting off his shoelace through a grill in the pavement. In a poorly-furnished room, Odile prepares a meagre supper for Pierre, daydreaming about a day they spent together in the country, where he gave her a ring made of grass. He arrives home in dramatic style, throwing his hat to the wall and pinning it there with his flick knife. He sees a notice advertising a society event, at which there will be much jewellery to be stolen. Leaving, he warns Odile not to hang around the White Coffin Club.

At the Club, the Parisian underworld are assembled, people dance and the whores fight over who is the Rat's main girl. Proprietress Mère Colline breaks up the fight. At the theatre, Zelie and her society friends watch the lavish but boring show at the Folies Bergère. At the Club, Odile catches Stetz's eye. He grabs Odile, but is caught from behind by the Rat, who ejects him from the Club, telling him never to return.

Later that evening, Zelie and her friends arrive at the club and watch the Rat win a staged knife fight. Zelie and Pierre engage in a flirtatious battle for dominance. He picks up the nearest girl and does a violent apache dance, ripping the girl's skirt with his knife. Zelie hands her man a bank note to give the Rat by way of a tip; the Rat lights his cigarette with it. She say that they are alike - he rules in his world, she in hers. She looks for her cigarette case, which he hands back to her, having stolen it earlier. She looks enquiringly at the pearls and he says he'll get them later.

Pierre receives a note from Zelie asking him to meet her at Stetz's apartment. He tells Odile he is going for the pearls. Stetz questions Zelie about her obsession with the Rat. She refuses to deny it and he tells her to move out while he goes to tell the good news to her successor. He leaves and goes to find Odile, who he plans to make his new mistress. Pierre and Zelie express their fascination with each other; she promised to make him the most attractive man in Paris. He realises she has been Stetz's mistress and they both realise that he has engineered their meeting so he can pursue Odile. Pierre runs through Paris to reach Odile, trapped in her room with Stetz, who is about to seduce her, with jewellery or, if necessary, by force. Pierre arrives as Stetz is carrying her limp body and stabs him. Odile tells the Police that it was her, realising that Pierre would certainly face the guillotine, while she might claim self-defence. Inspector Caillard, however, disbelieves her story. Mère Colline visits Odile in prison and tells her that Pierre is in a state of great anxiety about her. Zelie arrives to see Odile, on her invitation. Odile asks her to testify in court that Pierre was with her when the murder took place. Zelie is aghast, but eventually agrees when she realises that she will only lose her reputation, whereas Odile could lose her life. Inspector Caillard tells Zelie that for Odile to have confessed to the crime on behalf of The Rat, there must be some good in him. Meanwhile, Pierre roams the streets of Paris in a hysterical state. As the trial begins, Pierre sees Zelie and, ignorant of her plans, berates her for her cruelty. He is taken away by the Police and returns to the rooms he shared with Odile. Mère Colline comforts him. The verdict is returned and Odile is acquitted. She returns to Pierre and the couple finally declare their love for each other.