Kai is talented but adrift, spending his days sulking in a small fishing village after his family moves from Tokyo. When his classmates invite him to play the keyboard in their band, their practice sessions bring an unexpected guest: Lu, a young mermaid whose singing causes humans to compulsively dance - whether they want to or not. As Kai spends more time with Lu, he finds he is able to tell her what he is really thinking, and a bond begins to form.