Today, Yume's parents went on a trip, and her sister took the opportunity to invite her boyfriend over to play. Yume was working on her essay, so she asked her sister's boyfriend, Tsukasa, for help. Tsukasa had stolen Yume's underwear before and she found out. So if Tsukasa didn't want her to tell her sister about it, he had to agree to do it with her. And from then on, the two of them secretly made love to each other, in their own room, in the bathroom, in the kitchen,... even when her sister was nearby...