Sixteen-year-old Natsume Aihara has never been on a date, let alone been in a relationship. She blames her lack of experience on one person: her next-door neighbor and handsome older brother figure, Sousuke Mikami. Despite their nine-year age difference, the two are seldom apart. Sousuke helped raise Natsume since childhood, but even now remains completely overbearing. He personally wakes Natsume up every morning and takes a lower-paying job at her high school to keep a close eye on her.

Natsume used to adore Sousuke, but realizes that his doting has become excessive. Having seen his true perverted nature and suffering as the object of his obsession for too long, she finally decides that enough is enough, and tries to find a boyfriend her own age. However, can she ignore the strange beating of her heart whenever she is near the person who already knows her better than anyone else?