Natsuki, the beautiful madonna of the university photography club, is famous for having relieved countless men of their virginity. However, the girl that Hirata the freshman has his eyes on is not Natsuki, but her big sister Fuyuki. Fuyuki is a 30-year-old bookstore employee who's never had a boyfriend in her life, and she even looks younger than her little sister. How can Natsuki help Hirata in wooing this late bloomer and introducing her to the pleasures of sex?
