Umise is a private school on the coast. In its pool, a swimming team trains—12 girls and 5 boys. Kaname Okura is their coach; essentially, this is the only role in which he can be a member of the team, because he cannot swim. As a child, Kaname‑kun experienced a shock: he almost drowned when a mermaid attacked him, and now he never goes into the pool. One fine (though excessively hot) May day, a new student arrives at their school—literally "sails in"—Amuro Ninagawa. She is very cute, excessively straightforward, swims amazingly… and for some reason seems familiar to Kaname.