Maximilian, Duke of Bavaria, and the Emperor Franz Josef of Austria's uncle, has six daughters, including Helena and Elizabeth, in a household run by his wife Sofia.  Sofia has planned for Helena to marry the emperor, not the man she loves, the officer Palfi.  Despite Elizabeth's attempted intercession on Helena's behalf, their father remains too intimidated to defy his wife, who leaves with Helena for Austria.

Elizabeth manages to convince her father to join her in a plan to invade the palace and retrieve Helena, and the father and daughter arrive together in Hellbrun and check into the Golden Ox Inn.  Elizabeth gains access to the palace and attracts the eye of Franz Josef, who has yet to meet Helena.  Elizabeth hides when Maximilian ineffectually visits his nephew, and her father conceals himself rather than face Sofia and his sister the Empress, who has commanded her son's proposed marriage.

That evening at the inn, Franz Josef calls on Elizabeth, who has only identified herself as a dressmaker, to attend incognito the birthday fair given in his honor.  To his mother's consternation, Franz Josef fails to return to the palace until the next morning.  Elizabeth, now in love with Franz Josef, tells Helena she can safely end the royal engagement, but her sister merely believes that Elizabeth is jealous.

The Empress has Elizabeth jailed, but Franz Josef rescues her after a conversation with Palfi.  Maximilian finally stands up to the empress and rejects the arranged marriage, then tells Franz Josef Elizabeth's true identity.  She had kept her royal blood a secret from him because he proclaimed himself unenthusiastic over marrying nobility—when it concerned Helena.  Now the couple appear together on the balcony of the Golden Ox, to the cheers of a crowd.