‘Cœur fidèle‘ tells the melodramatic love story of Marie, who having lost her parents, is ruthlessly exploited by her foster parents. While Marie is courted by Petit Paul, she is herself secretly in love with the dock-worker Jean. When the two of them get into a row a policeman is stabbed to death. Petit Paul manages to escape but Jean ends up in prison. Having completed his prison sentence, he once more seeks out Marie who, in the meantime, has been caring for her sick baby and is living unhappily with the alcoholic Petit Paul. Another confrontation between Jean and Petit Paul ensues, but this time with a different outcome …