In 1672, two witches (Jennifer and her father Daniel) were burned by puritan Jonathan Wooley. In revenge, Jennifer cursed all future generations of the Wooley family, that the sons will always marry the wrong woman and be miserable. In the 20th century, a bolt of lightning frees Jennifer and her father from the tree that had kept their souls imprisoned. Jennifer assumes corporeal form and decides to make up-and-coming politician Wallace Wooley, then unhappily engaged, even more miserable by getting him to fall in love with her before his wedding. Wallace is a straight arrow, though, and Jennifer has to resort to a love potion. As we all know, love potions tend to backfire, with comedic results.
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.