Lady in the Iron Mask.

The King of France has twin daughters: Madeleine, well mannered and virginal; and Jaqueline, perverse and sexually obsessed. Before leaving on a sea voyage, he locks the wayward Jaqueline in the tower, her to wear an iron mask. Later, however, the royal ship is attacked by pirates and Francois, Madeleine's fiancee, is dispatched to search for the missing King. Meanwhile, Jaqueline uses her erotic charms on her guard, Maximillian, convincing him to help her escape. She takes the place of Madeleine and soon the future of France is in Jeopordy as she cuts a sexual swathe through the government. Eventually, Francois returns to save the day, and Jaqueline has to flee from the palace with Maximillian. Madeleine, who has been wrongfully imprisoned and constrained to wear the Iron mask, is now free to marry the man she loves.