Natsu-chan is now at an age where she is more interested in sex than studying, and has a brother who failed the entrance exams three times who stays at home all the time and peeks at her masturbating or having sex with her boyfriend. One day, the brother sneaks into Natsu-chan's room and fails in his usual nighttime act, and from that day on, the balance of power is reversed and the brother becomes her servant, and his sister treats him like a butter dog on a daily basis...They say blood is thicker than water, but this brother and this sister are like a brother!