The junior high school boy Hai, who lives in the quiet fishing port town of Nisumi, lives with his father and grandfather who is a parasol master. He originally lived in Tokyo, but after the divorce of his parents, he moved to his parents' hometown of Nishimura. Unable to tell the complicated thoughts about his parents, Hai was so depressed, he was also very negative at school. His only sustenance in life is to upload the music he composed to the Internet. One day, he was invited by his classmate Guo Nan and Youbu to join their band "Siren". He reluctantly went to the practice site Mermaid Island, where the mermaid girl appeared in front of the three people. Lu appeared to sing happily and dance innocently. With acting together with such Lu every day, Hai gradually became able to express his feelings. However, in Hino Town, the mermaid has brought disasters since ancient times. Due to accidental events, there was a huge gap between Lu and the townspeople. Immediately, the crisis visited the town. Can Hai’s heartfelt cry save the town?