Mio, a member of the swimming club, is dating Takumi, a boy from the cultural club at the same school. Just sitting next to him makes her heart race so fast her chest feels like it's going to explode. Holding hands, kissing… it all feels like a page from a sweet, innocent teenage romance. Mio, who was living such a happy life every day, is sexually harassed by the swimming club's advisor, Mr. Oki, during club activities. He gropes her breasts, touches her vulva, grabs her penis, and even forces him to penetrate her. However, she becomes obsessed with the skilled, married teacher's penis and sex—engaging in an illicit sexual relationship at the pool every week after school.