Kaname Okiura enrolls in Prefectoral Umineko Shougyou High's swimming club with the goal of learning to swim. However, he finds himself amidst a quirky group of individuals, none of whom seem capable of teaching him. Things take an interesting turn when a cheerful and unconventional girl named Amuro Ninagawa, hailing from Okinawa, transfers to the club. Her remarkable speed underwater, unique swimming technique, and penchant for nude swimming captivate the attention of every member, particularly Okiura. Strangely, she bears a striking resemblance to a mermaid-like figure from his distant childhood memories.