Today Yume's parents went on a trip, and her sister took this opportunity to invite her boyfriend over to the house. Since Yume had to work on an essay, she asked her sister's boyfriend – Tsukasa for help. Previously, Tsukasa had stolen Yume's underwear, and she found out about it. So if Tsukasa didn't want her to tell her sister about this, he had to agree to help her. And from then on, the two secretly had sex with each other, in the private room, the bathroom, the kitchen,... even when her sister was nearby...