From Wikipedia: Kyōsuke Kōsaka, a normal seventeen-year-old high school student, hasn't gotten along with his younger sister, Kirino, in years. For longer than he can remember, Kirino has ignored his comings and goings and looked at him with spurning eyes. It seemed as if the relationship between Kyōsuke and his sister, now fourteen, would continue this way forever. One day, however, Kyōsuke finds a DVD case of a magical girl anime entitled Hoshikuzu Witch Merle (Stardust Witch Merle), which had fallen into the entranceway of his house. To Kyōsuke's surprise, inside the case is a hidden adult video game titled Imōto to Koishiyo! (Let's Fall in Love with Little Sister!)