After his father and mother divorced, the young boy Kai (voiced by Shimoda Shodai) followed his father back to his hometown surrounded by mountains and sea, and started a new life with his grandfather who made umbrellas. The family changes made Kai depressed, and only when he was composing music could he find a little bit of long-lost happiness. Youbu (voiced by Minako Kotobuki) and Kunio (voiced by Souma Saito) discovered Kai's musical talent and enthusiastically invited him to join their band "Siren". One night, the music played by Kai attracted a mermaid named Lu (voiced by Tani Kanon), and soon, Lu and Kai became friends. However, in the village, mermaids are an ominous symbol, and their appearance often foreshadows disaster and death. Finally, Lu's identity was exposed, and the residents of the town soon divided into two factions. One faction was moved by Lu's innocent dance, while the other faction was determined to expel the mermaids from the human world.