Kai, a young middle schooler, lives with his father and grandpa, a sun-umbrella manufacturer, in Hinashi Town, a lonely fishing hamlet. He used to live in Tokyo, but after his parents separated, he returned to the hometown of his parents. Kai feels lonely and gloomy about his academic life, and he has problems informing his parents about his mixed feelings for them. Uploading music he's written on the internet is one of his favorite pastimes. Kunio and Yuuho, his classmates, persuade him to join their band, "SEIRN," and he grudgingly accepts. They meet Lu, the mermaid girl, on Merfolk Island, where they practice. Lu sings cheerfully and dances naively. Kai learns to be able to communicate with her as he spends more time with her.