As the threat of war hangs heavy in the air, Princess Serena of Atlantis knows she must take drastic action to protect her sisters from being hunted and killed. With a plan to marry the human prince, Edward, she takes him to the mystical underwater city of Atlantis and to the court of her stern father, Poseidon. But her plan goes awry and Edward is sentenced to death. With no other choice, Serena is forced to use a forbidden ritual to bind the prince to herself, surrendering her immortality in the process. But with the King of England ordering his men to slaughter all sirens on sight, how will the two be able to convince the royal court to give their blessing?