Chitose, who works at an apparel shop in Tokyo, went back to her parents' home during the long weekend. However, she was worried about her younger brother Yota, who hadn't come over, so she asked her parents about it. They told her that he had been dumped by a girl and wasn't going to school. So Chitose tried to cheer up Yota by showing off her huge breasts without a bra in order to get girls interested in her again, but this awakened Yota's dormant abnormal sexual desire, and she was chased and screwed by her super-horny brother who wanted her like a monkey...