Jacques Lantier is a train engineer who is prone to violent seizures, a condition he attributes to his forefathers' habit of excessive drinking. Roubaud is a train conductor on the same railroad that Lantier works on, married to the much younger Séverine. When Roubaud catches wind of his wife's affair with her godfather, the wealthy M. Grandmorin, he kills him during a train journey in a fit of jealousy. He makes sure that Séverine is also present, making her an accomplice to murder. Lantier, despite having witnessed them quite clearly in the train corridor, hides the fact during the investigation as he is attracted to Séverine. They both begin an affair, all the while Roubaud becomes increasingly withdrawn and starts to gamble. Séverine urges Lantier to kill her husband so that they would be free but she is unaware of Lantier's unfortunate condition.