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In 1808, King Ferdinand VII is summoned by Napoleon, who secretly wants the King to abdicate.  At a Madrid cafe, the popular singer Nina Maria, known as "The Firefly" sings and flirts with a member of the audience, Don Diego, because she would like to divert the attention of a jealous French officer named Etienne, while she meets secretly with the Marquis de Melito.

While Etienne challenges Diego to a duel, Nina meets the Marquis, who is a confidant of the king, and is assigned a spy mission to Bayonne to uncover Napoleon's plans for Spain.  The next day, while Nina travels to Bayonne, Diego meets her coach and promises to ride with it to protect her from bandits.  He also tells her that he has fallen in love with her.  Though Diego's pursuit prevents Nina from speaking with her contact, she soon begins to realize that she is in love with Diego.

In Bayonne, Nina meets Major de Rouchemont during a performance, when she playfully steals his hat.  After going backstage to retrieve it, the major, who has just been made an aide to Napoleon, invites Nina to his chateau.  There Nina discovers that the major is going to see Ferdinand and, searching through papers when the major leaves the room, she discovers a letter demanding that the king either abdicate or be arrested.  Realizing that she must send the information to the Marquis, she tries using carrier pigeons, but soon realizes that the birds she bought have been switched.

After the pigeon salesman is arrested, Nina becomes suspicious of Diego and goes to his room and begs him to take a message for her.  Members of the French secret service then break into the room and take the message.  The contents of the message are innocuous, but Nina now knows that Diego is a French spy.  He later confesses that his duty forced him to betray his love for her, but she will not listen.

Soon Ferdinand is forced to abdicate and Napoleon's brother Joseph is crowned King of Spain.  On the advice of the Marquis, "The Firefly" disappears and Nina works in secret while the people gather strength to overthrow the French.

Five years later, continuous war has ravaged Spain, but when the English come to her aide, the situation begins to change.  Now, dressed as a common gypsy, Nina goes into a French camp, where she again meets the Marquis de Melito.  Though he is surprised that she appears to be a camp follower, he offers to make Nina his mistress.

When he later finds a map of French military positions in her possession, however, he summons his aide, Captain Andre François, who is the man Nina she knew as Diego.  After Diego tells the major Nina's story, she is sentenced to death.  Diego changes the information on her papers and sends it by carrier pigeon back to the Spanish forces at Vitorio, then goes to Nina to again profess his love.  Just then a large battle begins at Vitorio and Nina reveals that her documents had merely been a ploy to force the French to change their strategy.  After the French are defeated, the Marquis finds Nina and tells her that she is now free.  She then goes looking for Andre, whom she finds lying wounded on the battlefield.  He recovers, and with their war now over, they then ride off together through the mountains.