Keiichi Nanase lives under the same roof with his five younger sisters, all of whom are now in their prime. Looking at these girls, Keiichi often felt unhealthy feelings. Then, during summer vacation, Keiichi's parents go on a trip overseas. Rina thinks that her sloppy and naughty brother has become more and more vulgar while her parents are away, so she takes a harsh attitude towards Keiichi. However, even though she says she doesn't like sex, she secretly masturbates. And Keiichi happened to know about it.