An outlaw, the Llano Kid is the scourge of south Texas and a Bible quoting lawman named Thacker. The kid joins a con man in a plot to deceive a wealthy old woman that he's her long-lost son and heir. Traveling to Mexico, move into the luxurious hacienda, but the Kid soon grows to love the woman and the beautiful neighbor Consuelo. He turns on his partner and the lawman from Texas is still on his trail.

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.