In the 16th century, the Duchy of Doruzzo is under the tyrannical rule of Duke Zulian and his daughter, Isabella. When an unjustly accused sea captain, Mirko, and young and fiery Sandra, who believes Mirko to be her father, are brought before the duke, he decides to hang Mirko and sell Sandra into a Turkish harem. They are freed, however, by Cesare, Count of Santa Croce, a nobleman who the duke hopes will wed Isabella. Sandra and Mirko embark on a career of piracy, and Cesare sets out to capture them. Instead he falls in love with Sandra and joins her in planning an assault on the duke's palace. As the pirates battle their way ashore, they are joined by the oppressed peasants, and the attack is successful. The duke is fatally wounded by Cesare but before dying confesses that Sandra is the rightful heiress to the duchy. Many years ago, he reveals, he ordered Mirko to kill her, but the sea captain spared the young girl's life and reared her as his daughter. Isabella retires to a convent, and Sandra, now the Duchess of Doruzzo, prepares to marry Cesare and right the wrongs committed by Zulian.