Saori was living alone for her gravure model job, but she quit her job and moved back to the countryside. When she returned to her parents' house and met her younger brother Taichi for the first time in a long time, he had grown big, but he was still a virgin who was shy about receiving obligatory chocolate. In an attempt to tease the innocent Taichi, Saori shows off her cleavage to seduce him, but Taichi becomes more aroused than she expected and pushes her down. From that day on, Taichi transformed into a sex beast, and she was fucked over and over again in her own home with no way to escape...