Kaname Okimura, a student of Umineko Shougyou High School, abreviated as "Umisho", joins the swimming club so he can learn to swim, but the club is full of bizarre people. Then, an extremely happy girl named Amuro Ninagawa, joins the club.

Her extreme velocity under water and her very strange style (with the habit of swimming naked) surprises all the members of the club, specially Okimura, because she reminds him of a mermaid he saw when he was young and that left him traumatized.