Ryou Seibai!: Gakuen Bishoujo Seisai Hiroku asks the question: Would you, the male lead of an NTR-esque hentai, would really stand by and let your (sexy) sister be raped?
Kyoichiro answers that question with a HELL NO!! and goes out to seek revenge. After getting injured from a misunderstanding, he gets help from Yamiko, the school nurse. And so the plot unfolds.