After parting ways with her troubled mermaid tribe, Luce just wants to live peacefully on her own. But her tranquility doesn't last long: She receives news that the tribe is on the verge of collapse and desperately needs her leadership. The tribe's cruel queen wants Luce dead. Dorian, the boy Luce broke mermaid law to save, is determined to make her pay for her part in the murder of his family.