The book comprises of a series of brief tales, among which the title story narrates the account of Youzuke. The tale takes us back to the summer of Youzuke's childhood when he lived with his uncle. His uncle's house had a pool, and during nights, Youzuke could see a mermaid visiting his uncle and swimming in the pool. After several years, Youzuke's uncle vanishes, and surprisingly his estate is bestowed upon a stranger named Ayumi Kenjou. When Youzuke encounters Ayumi, he feels as if he has already met him during that summer from the past.
