The stepmother Shiho, who remarried the father, has a plump, alluring big ass that stirs her stepson's heart. As a young man coming of age, he finds himself increasingly drawn to her charm, gradually beginning to see her as a sexual object. One day, while watching Shiho bend over to clean the floor, he is overwhelmed by uncontrollable desire. From that moment on, a new emotion takes root in his heart and becomes part of his everyday life. As her husband continues coming home late, Shiho too grows more eager to touch her stepson. Between the two, complex feelings slowly begin to deepen.