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