Kasumi Kishimoto enrolls at Myōjō High School and moves into her aunt Chigusa Mizusawa's boarding house, unaware it already shelters four male tenants. This unexpected cohabitation sparks immediate tension, escalating after fellow resident Yūsaku Takasugi accidentally sees her bathing. Initially committed to Katsuhiko Muraki, her aunt's non-blood-related nephew living overseas, Kasumi navigates a long-distance relationship. Her loyalty to Katsuhiko faces challenges as daily interactions with Yūsaku foster growing feelings between them.

She takes on the role of manager for the school baseball team alongside Keiko Seki, forming a distinct duo where Kasumi's "Tomboy" nature contrasts Keiko's "Girly Girl" persona. Supporting the team, including novice player Yūsaku, deepens her connection to him. Kasumi's bond with Yūsaku ultimately overcomes her commitment to Katsuhiko, leading to their breakup. Her evolving relationship with Yūsaku culminates in a mutual declaration of love shortly before her departure from the boarding house.

Two years later, Katsuhiko returns to Japan as a professional racer intent on marrying Kasumi, reigniting past conflicts and forcing her to confront unresolved feelings. Yūsaku, now pursuing photography, remains a significant figure during this period. Throughout her experiences, Kasumi serves as a grounded counterbalance to the eccentric personalities inhabiting the boarding house, including her quirky aunt, embodying the "Only Sane Man" and "Wacky Parent, Serious Child" dynamics within their relationship.