Kyoko Saeki lives next door to Masahiko's family, sharing a long history with the household. After Masahiko's mother passes away, Kyoko steps into the void, routinely handling chores and cooking within his home. Her constant assistance makes her an ingrained part of Masahiko's everyday life.

The arrival of Shu Fong Lee, an international student training under Masahiko's father, sparks visible jealousy in Kyoko. Fong Lee taking on household tasks and growing close to Masahiko fuels Kyoko's resentment. This surfaces as clear discomfort and possessiveness over her established place in the home.

Beneath the surface, Kyoko harbors deep, long-held romantic feelings for Masahiko, feelings she largely keeps hidden. These suppressed emotions directly fuel her reactions to Fong Lee. The situation intensifies when Kyoko's younger sister, Asuka—who also develops feelings for Masahiko—discovers a pivotal memory: Kyoko is the unidentified woman who aided Masahiko during a childhood injury, the very figure central to his vow of celibacy until her rediscovery. Asuka deliberately sets aside her own romantic hopes to help resolve the tension between Kyoko and Masahiko.

The story culminates with Kyoko revealed as Masahiko's childhood savior. This acknowledgment allows their mutual feelings to finally surface, leading to an emotional and physical union. Kyoko's journey concludes with the establishment of her romantic relationship with Masahiko.