Head railroad man Dan is as ugly as he is honorable. When he spots a drifter who'd hopped a freight held up by a landslide, Dan offers the man a job; then he finds the man was a railroader, too, and takes him under his wing. Engaged to Mary, Dan doesn't notice the growing attraction between his protégé and his intended but focuses instead on running the railroad.

Haunted by the memory of Walter Holderlin, a soldier he killed during World War I, French musician Paul Renard (Phillips Holmes) confesses to a priest (Frank Sheridan), who grants him absolution. Using the address on a letter he found on the dead man's body, Paul then travels to Germany to find his family.
As anti-French sentiment continues to permeate Germany, Dr. Holderlin (Lionel Barrymore) initially refuses to welcome Paul into his home, but changes his mind when his son's fiancée Elsa identifies him as the man who has been leaving flowers on Walter's grave. Rather than reveal the real connection between them, Paul tells the Holderlin family he was a friend of their son, who attended the same musical conservatory he did.
Although the hostile townspeople and local gossips disapprove, the Holderlins befriend Paul, who finds himself falling in love with Elsa (Nancy Carroll). When she shows Paul her former fiancé's bedroom, he becomes distraught and tells her the truth. She convinces him not to confess to Walter's parents, who have embraced him as their second son, and Paul agrees to forego easing his conscience and stays with his adopted family. Dr. Holderlin presents Walter's violin to Paul, who plays it while Elsa accompanies him on the piano.
The film's original title, The Man I Killed, was changed to The Fifth Commandment to avoid giving "wrong impressions in the minds of the public about the character of the story." It ultimately was released as Broken Lullaby.