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.

(Source: MangaHelpers)