Within the Kanagawa prefecture lies the seaside city of Umineko. There, Okiura Kaname, a 2nd year high school student, attends Umineko Shougyou High School (or Umisho for short). He is also the manager of Umisho's swim team though he himself does not know how to swim. In fact, he is afraid of the water stemming from an incident which occurred some years ago.

While he was at the beach, he had nearly drowned when a mermaid pulled him under the water. He joined the school's swimming club in order to learn how to swim, but the club is filled with weirdos, who don't teach him swimming.

One day, a floating house arrives at the shores of Umisho bearing two persons. One of these is a cheerful, sunny, happy-go-lucky girl named Ninagawa Amuro, a transfer student to Umisho, and the other is her father. Her incredible swimming speed makes her an instant hit with the swimming club, but Okiura is surprised, as she reminds him of the mermaid he saw only once in his early childhood. After her floating house is destroyed during a storm she moves to Kaname's house.