Abiding by his father's will, Shougo was transferred into Miryuuin Academy where many young girls attend. It appears as though his younger sister, separated from him at birth, also attends this academy, and that she is trying to get closer to him without letting him know who she actually is. At the same time, Shougo becomes popular with the other girls as well.

Kono Naka ni Hitori, Imouto ga Iru! (この中に１人、妹がいる！, My Little Sister is among them!?) is a Japanese light novel series by Hajime Taguchi. It was adapted into an anime television series by Studio Gokumi.[1] The anime was aired between July 6 and September 28, 2012 and was simulcast in North America by Crunchyroll and the Anime Network as Nakaimo - My Sister is Among Them!.