Haruka Narumi is Sayuri's younger sister, living close to her sister's home and always quick to rush over whenever something happens. Sayuri divorced twenty years ago and has been raising her only son, Kenichi, alone. Today, after discovering her husband's infidelity and a huge fight erupting, Haruka shows up unannounced. Left alone at home is Kenichi, and upon learning of her sister's affair, Haruka becomes consumed by the urge to seek her own revenge affair, giving in to temptation and suddenly making advances on Kenichi. Tension within the family home intertwines with the theme of infidelity, leading to a dramatic and intense storyline.