A family-friendly take on the classic fairy tale about a little mermaid who comes ashore to join a middle-school rock band and propel them to fame. Kai is talented but adrift, spending his days sulking in a small fishing village after his family moves from Tokyo. His only joy is uploading songs he writes to the internet. When his classmates invite him to play keyboard in their band, their practice sessions bring an unexpected guest: Lu, a young mermaid whose fins turn to feet when she hears the beats, and 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. But since ancient times, the people in the village have believed that mermaids bring disaster and soon there is trouble between Lu and the townspeople, putting the town in grave danger.