After her husband's relatives die in an accident, Toko Namiki and her family take in their young son, Yuta. Initially indifferent toward him, Toko gradually finds herself drawn to his unusual emotions and behavior. To Yuta, Toko feels like a real older sister, and as his wish to live with her comes true, he begins to grow increasingly dependent on her. During bath time, he reaches toward her lower body, leaving her confused. This act triggers a rising desire within him, and he starts pressing his crotch against her. Left alone without her husband, Toko eventually succumbs to his advances and finds herself enjoying sex with him. Though she thought she was only letting her guard down, she soon realizes she's riding on top of Yuta, passionately grinding her hips. Responding to her intense reactions, Yuta grows even more aroused, and a new relationship begins between them.