In the coastal hamlet of Hinashi Town, Kai Ashimoto"s grandfather is among those who uphold the tale that mermaids haunt Merfolk Island and devour humans. Supposedly drawn to these sea‑spirits, his mother died, so music is rarely spoken about in the Ashimoto home. Yet Kai finds comfort crafting looping rhythms on his laptop and sharing them online.
Though he is quiet and somber, Kai catches the attention of Kunio and Yuuho Ebina, two upbeat classmates who stumbled upon one of his tracks. They invite him to join their rock group, but he initially shows little interest. Still, Kai agrees to travel with them for a rehearsal on Merfolk Island, where he soon encounters something inexplicable.
That night, while composing in his room, Kai summons a familiar presence—a mermaid. She spins and smiles to the music from his stereo, revealing herself as Lu. She can command water with ease; music causes her to sprout functional legs, and she is a genuinely kind creature who simply longs for acceptance. As their friendship deepens, Kai and Lu begin to reveal their true selves, challenging the old limits that have muted the vibrant harmony of Hinashi Town"s residents. Their bond inspires the townsfolk to listen more closely to one another.
