From Wikipedia: Kyōsuke Kōsaka, an ordinary seventeen-year-old high school student, did not get along with his younger sister Kirino for many years. Longer than he could remember, Kirino ignored his arrival and care and looked at him with contemptuous eyes. It seemed that the relationship between Kesuk and his sister, which is now fourteen years old, will continue so forever. Once, however, Kyōsuke finds a DVD Case Magic Anime Girl called Hoshikuzu Witch Merle (Stardust Witch Merle), which fell in the entrance of his house. To the surprise of Kyōsuke, inside the case is a hidden video game for adults called Imōto to Koishiyo. (Let's fall in love with younger sister!)