Your younger sister, Riku-chan, is a bright, lively, and obedient high school girl. One day, while your parents are out and you are relaxing, your tranquility is instantly destroyed by her friend who comes over to visit you. While forcing you to help her with her homework, she sneaks in and makes you do naughty things, and when you go to check on her in the bathroom out of concern, you are dragged into a group of naked girls...Finally, due to excessive provocation, you end up having sex with your own sister without a condom.