Haunted by the lingering memory of Walter Holderlin, a soldier he killed during World War I, French musician Paul Renard (Phillips Holmes) seeks solace from a priest (Frank Sheridan) who grants him absolution. Utilizing the address found on a letter retrieved from Walter's body, Paul journeys to Germany to meet Walter's family. As anti-French sentiments prevail across Germany, Dr. Holderlin (Lionel Barrymore) initially hesitates to open his home to Paul. However, he changes his stance when his son’s fiancée, Elsa, identifies Paul as the individual who has been leaving flowers at Walter’s grave. Choosing not to disclose the true connection, Paul informs the Holderlin family that he was a friend of their son and had attended the same musical conservatory. Despite disapproval from the hostile townspeople and local gossip, the Holderlins grow fond of Paul, who finds himself enamored with Elsa (Nancy Carroll). During a visit to Walter’s former bedroom, Paul becomes overwhelmed and confesses the truth to Elsa. She persuades him not to disclose his secret to Walter's parents, who have welcomed him as their second son. Paul agrees to set aside his guilt and remains with his new family. Dr. Holderlin gifts Walter’s violin to Paul, who plays it while Elsa accompanies him on the piano. The original title of the film, "The Man I Killed," was altered to "The Fifth Commandment" to prevent the public from forming incorrect impressions about the storyline. It was eventually released under the title "Broken Lullaby."