The Firefly is a 1937 American historical musical film directed by Robert Z. Leonard and starring Jeanette MacDonald, Allan Jones and Warren William.

[4] During the Peninsular War, secret agent Nina Maria Azara works undercover for Spanish King Ferdinand VII as a singer known as the "Firefly.

At the Vitoria inn, a contact instructs that a vendor in the Bayonne market will provide carrier pigeons upon hearing a password.

In Bayonne, Nina Maria entertains French troops ("He Who Loves and Runs Away") when she sees the persistent Andre and Colonel de Rouchemont arrive independently.

Her locket holds portraits of her parents, killed when the French invaded Spain 20 years before, but she replies it was long ago ("Sympathy").

Nina Maria goes to the chateau ("When a Maid Comes Knocking at Your Heart"), where she sees Rouchemont’s papers revealing that the Spanish king is being lured to France to be arrested.

While the French Intelligence Chief listens, concealed in a closet, Nina Maria asks "Don Diego" to take a message to Vitoria.

Rouchemont orders their entire formation changed, and the French send a deceptive message via Allied pigeons verifying the former positions.

As the Battle of Vitoria begins, she admits she was caught intentionally so the French would change their impregnable defensive positions, thus enabling Wellington and the Allies to win.