Natsuki, who was supposed to be a plain, introverted girl, transformed into someone cute and irresistibly sexy. Shouta, unable to help himself, started seeing her as more than just a sister, while Natsuki took every opportunity to tease and provoke him. After crossing the line, the two went through a lot but eventually promised to become an official couple, and Natsuki left her secret party-girl life behind. Even so, Shouta couldn't shake his worries about Natsuki cheating, so to ease his fears, the two kept indulging in passionate sex...