The story centers on a train engineer Jacques Lantier (Jean Gabin) who lusts after Séverine Roubaud (Simone Simon), the wife of his co-worker Roubaud (Fernand Ledoux).
Roubaud discovers that his young wife, Séverine, has been seduced by her godfather, the wealthy Grandmorin. Jealous, Roubaud forces Séverine to assist in the murder of Grandmorin during a train journey. The murder is witnessed by a railway worker, Jacques Lantier, but he keeps quiet because he is in love with Séverine. Disgusted by what her husband has done, Séverine has an affair with Lantier and pleads with him to kill her cruel husband. Little does she know that Lantier also has a dark secret.
Source : Wikipedia