Yuya Kanzaki finds himself having a step sister named Mitsuki. At first she is cold to him, addressing her step brother as "you" or him" but suddenly one day Mitsuki collapses which cases Yuya to panic. When Mitsuki awakens in the hospital she passionately embraces Yuya calling him her "big brother" before changing moods again and literally kicking him out of her hospital room. Later on after a rest at home, she awakens to find a chastity belt on her. Mitsuki then meets Hiyori who calls herself an angel, before she can ask more she leaps into Mitsuki's body possessing her. The possession causes the real Mitsuki who is now transparent to float outside of her body, Hiyori tells Mitsuki not to worry though as she is not dead. She then proceeds to masturbate while in Mitsuki's body causing the real Mitsuki to jump back into her own body driving Hiyori out. Dismayed, Hiyori then pleasures her while talking about her big brother (Yuya) resulting in Mitsuki having an orgasm that causes a heart located on the chastity belt to fill up a little. Upset that someone might see her chastity belt she demands Hiyori tell her how to take it off. Hiyori tells of a button that just releases the crotch bit which Mitsuki proceeds to press. While explaining about the belt, she is interrupted by Mitsuki who has to use the bathroom. While there the chastity belt clamps back on, Hiyori explains that it only stays off for three minutes before going back on and she must wait an hour before pressing it again. Hiyori explains that she tried to finish what she was going to say but got interrupted. Mitsuki suffers for an hour in the bathroom worrying both her aunt and Yuya before having the opportunity to go again with great relief.