Kaname Okiura enrolled in Prefectoral Umineko Shougyou High to learn how to swim but found himself amidst a strange swimming club lacking in proper instruction. His fortunes change when Amuro Ninagawa, a cheerful and lively girl from Okinawa, transfers to the school and joins the club. She shocks everyone with her incredible speed underwater and peculiar swimming approach, all while adding to her allure by frequently swimming in the nude, something that inevitably captivates teenage boys. Okiura, in particular, recognizes her striking resemblance to a mermaid he encountered as a child.