Erika Arimura spends three days having sex in her older brother's room before her younger sister, a sexual monster who is going to a boarding school in the spring, moves out.

On the day my parents were away from home for a memorial service, I was alone with my younger sister who was about to move in... Seeing the defenseless appearance of my younger sister, who the family is proud of because she's good at studying and cute... I couldn't resist my desire, so I forced myself. I end up raping my sister... Then, her sister tells her that she may actually have stronger sexual desires than other people. From then on, I had sex with her sister over and over again until the day I left home for her move... This is a record of that day.