The one thing not ordinary about high school student Ryouta is that he has an aspiring manga artist for an older sister. She is so dedicated to her drawing that, despite being 21, she's never really been in a romantic situation. Unfortunately, when she gets her first real job, it's to write an erotic manga. Desperately, she enlists the reluctant Ryouta's help, as a 'model', to figure out how exactly to draw this sort of manga. The girl, Mana, has no problem with seeing the scenes she plays with her brother as nothing but a hollow reference for her works.... But she fails to realize that her brother seems to become more and more flustered with each 'live performance'.