Legend says that eating mermaid flesh grants immortality. Yuta and Mana, living proof of that legend, travel together in search of meaning for their existence, or perhaps a cure for their condition. One day they encounter Masato, a boy who seems terrified of his mother Misa, whose relationship with her appears highly unusual. Curious, Yuta and Mana investigate the dark secret Masato and Misa are keeping, while also confronting the monster‑like creatures that have begun to appear.