Seseri, Futaba, Chiko and Yuu are four very close childhood friends who share everything amongst themselves. Kazuki has been with them since they were young and loved them all like they were his younger sisters (including his real younger sister Seseri). One day, they wondered who’s big brother (Onii-chan) he is. Of course, everyone’s big brother! As such, he’s free to return to any of their homes and spend the night under the same roof as them. Wouldn’t you like to experience all these various daily lives with your younger sisters?