In this adaptation of the operetta by Rudolf Friml, secret agent Nina Maria Azara (Jeannette MacDonald) is working undercover for the King of Spain as a singer known as the "Mosca del Fuego" or "Firefly." Her mission is to uncover Napoleon's plot to invade Spain before it is too late. This film features a variety of songs including "Donkey Serenade," "Love Is Like a Firefly," " and "When a Maid Comes Knocking At Your Heart." ~ Iotis Erlewine, All Movie Guide