Your younger sister, Riku-chan, is a bright, energetic, and honest high school girl. One day when your parents were out, the peace was shattered by her friends who came over. While being forced to help with their homework, you are made to do H things, and when you go to check the bathroom out of concern, you get dragged into a group of naked girls... Eventually, due to excessive provocation, you end up having sex with your real sister without even using a condom.
