When the sea witch summons Ariel, she hardly believes what comes next; Siena, the beautiful, charming, most favored princess has traded in her tongue and tail for a pair of legs and a chance to live a human life. The sea witch fears that if the king finds out about Siena's life on land, he'll flood the earth with the oceans. All of Merdom has shunned the princess, but Ariel, who has never been more than unremarkable, is deemed the right candidate to go after her.
Ariel sets out after Siena, determined to bring her home. However, on land, Ariel's regular mermaid magic makes her a human with unique abilities, ones that get her noticed by the royal family. Ariel tries to remain focused, but, in time, she realizes that instead of saving Siena from life on land, she may be taking the one she gave up everything to have.