Kaname Okiura is admitted to Prefectoral Umineko Shougyou High, commonly known as "Umishou," and joins the school's swimming club hoping to learn how to swim. However, he quickly discovers that the club members are eccentric and provide little useful instruction. Soon after, a spirited and carefree girl named Amuro Ninagawa transfers from Okinawa and joins the club. Her astonishing speed underwater and unorthodox swimming technique—along with her habit of swimming without clothes, which draws the attention of the male students—stuns everyone. For Okiura, Amuro's presence brings back memories of a creature resembling a mermaid he glimpsed as a child.