Yüya Kanzaki finds himself having a stepsister named Mitsuki. At first she is cold to him, addressing her stepbrother as "you" or "him," but suddenly one day Mitsuki collapses, which causes Yüya to panic. When Mitsuki awakens in the hospital she passionately embraces Yüya calling him her "big brother" before changing moods again, literally kicking him out of her hospital room. Later on after a rest at home, she awakens to find a chastity belt on her...