A plain, gloomy little sister has transformed into an outgoing girl with a secret account. As an older brother, he needs to give her some educational guidance! But when he sees his cute, big-breasted sister in a defenseless state, he can't help but insert it! Then, he's surprised at how well their bodies match up, and the two of them end up in a relationship like lovers... The popular doujinshi series has finally been made into a live-action film!