After his family moves from Tokyo to a small fishing village, teenager Kai spends his days sulking and adrift. When his classmates invite him to play keyboard in their band, their jam 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. But when an ancient prophecy threatens Lu and the village, Kai and his new friends must save the day in this toe-tapping adventure for the entire family.