It's about a boy named Nishizawa Kazuya who is in 2nd year of middle school. Unlike his younger sister, Rika, he is good at a lot of thing and is the fastest runner in the school. Parents and others think he is responsible and hard working child. It seems that he is perfect good child but he has a secret that he can't tell anyone. Every day he enjoys taking a peek at his sister, undressing herself.