Mitsuki, a high school student, sneaks into the campus pool one night. Floating on the surface of the water, he thinks about his dead sister. Just then, his classmate Yutaka also sneaks in, and confesses to him that she released piranhas into the pool.

Naturally, pool lessons the next day are cancelled. Even though Mitsuki knows that Yutaka is the culprit, he tells no one. Finding a common ground in this secret they share, Yutaka begins to develop feelings for Mitsuki. She confesses these feelings to him...