Kasumi Kishimoto begins attending Myōjō High School and moves into her aunt Chigusa Mizusawa's boarding house, unaware it houses four male students from her new school. This situation ignites instant friction, especially after tenant Yūsaku Takasugi accidentally intrudes on her bath shortly after her arrival.

Kasumi carries a longstanding commitment to Katsuhiko Muraki, her aunt's nephew studying abroad. Yet, her daily proximity to Yūsaku sparks a growing inner conflict. She initially resolves to stay true to Katsuhiko but finds herself increasingly distracted by Yūsaku, a tension central to her journey.

Shared experiences at the boarding house and school shape Kasumi's evolution. Events like a power outage trapping her alone with Yūsaku, or a cave encounter during a winter trip, heighten her emotional turmoil. Her loyalty to Katsuhiko weakens as her feelings for Yūsaku intensify, particularly when Katsuhiko returns temporarily and proposes marriage. Observing Kasumi's bond with Yūsaku, Katsuhiko retracts his proposal and initiates a symbolic contest for her affection.

External pressures further complicate her emotions. The arrival of new tenant Maria Ōta triggers Kasumi's jealousy due to Maria's interest in Yūsaku. Later, Katsuhiko returns to Japan permanently as a professional racer, renewing his intent to marry her, forcing Kasumi to confront her past commitment directly against her unresolved feelings for Yūsaku.

Her defining relationships remain: Katsuhiko embodies a stable yet distant past; Yūsaku represents a complex, evolving attachment forged through shared moments; and her aunt Chigusa provides the domestic setting where these dynamics play out.