Roubaud and his wife, Séverine, are in Paris. Roubaud, an assistant station-master for a railway company, had made a political comment displeasing to his bosses, so the company had sent him to Paris, possibly to fire him. Because Séverine had grown up in the household of an influential judge, named President Grandmorin, Roubaud was able to keep his job.

While the couple has dinner at their lodgings in Paris, Séverine gives Roubaud a gift: a knife she has just purchased. However, Séverine accidentally reveals that the ring that she wears had been given to her by Grandmorin, and that she had long been his mistress. Roubaud goes into a fit of jealousy, beats Séverine, and forces her to help him murder Grandmorin. Roubaud makes her write a letter to the judge to lure him into taking the same train that they plan to take back to Le Havre.

Meanwhile, train driver Jacques Lantier is able to stop at his old home at La Croix-de-Maufras because of railway repair. His Aunt Phasie tells him that she suspects that her second husband, Misard, a seemingly timid man, has been poisoning her to inherit her one-thousand-franc legacy. She vows that he will never get his hands on the money and tells Jacques that she has hidden it so well that Misard will never find it.

While on a walk that night, Jacques finds his cousin Flore on the property of Grandmorin’s country house, which also happens to be at La Croix-de-Maufras. Flore has always been in love with Jacques, and the two kiss. Jacques pulls away, however, because he has long been suffering from a psychological impulse to kill the women who arouse him. Jacques is tortured by his desire to kill, and after leaving Flore, he walks along the railway tracks. While the express train from Paris goes by, he sees the shadow of a man with a knife raised ready to kill another man and another figure falling over the legs of the man about to be killed.

Jacques runs into Misard on the way home, and Misard tells Jacques that he has discovered the body of a man on the side of the railroad tracks. While Misard goes to contact the police, Flore arrives, turns the body over, and recognizes it to be that of Grandmorin. Seeing the body’s slit throat, Jacques’s desire to kill intensifies. He also realizes that he has witnessed the murder of the judge but cannot decide whether to reveal this information.

Monsieur Denizet is the ambitious magistrate in charge of investigating the murder. The Roubauds are suspects because they have inherited Grandmorin’s house at La Croix-de-Maufras. The stone carter, Cabuche, also is a suspect because of the rumor that his lover, Louisette, Flore’s sister, had been raped and left for dead by Grandmorin. During the investigation, Jacques reveals that he had seen the shadows of the murderers.

While in Paris, Séverine visits the secretary general of justice, Monsieur Camy-Lamotte, who has in his files Séverine’s incriminating letter to Grandmorin, the letter that lured the judge onto the train. Both because of her attractiveness and because of his own fear of exacerbating rumors about the judge’s evils, however, Camy-Lamotte conceals the letter from Denizet and tells Séverine that her husband is safe from prosecution and will retain his job. Desirous of promotion, Denizet accepts Camy-Lamotte at his word that the Roubauds are not responsible for the murder; also, not enough evidence exists to charge Cabuche.

During this trip, Séverine and Jacques fall in love, and Jacques tacitly agrees not to denounce the Roubauds, even though he...