Today Yume's parents went on a trip, and her sister took the opportunity to invite her boyfriend to her house. Because Yume had to do an essay, she asked her boyfriend, Tsukasa, for help. Tsukasa once stole Yume's underwear before and she found out about it. So if Tsukasa didn't want her to tell her sister about this, he had to agree to do it with her. And from then on, the two secretly made love to each other, in private rooms, bathrooms, kitchens,... even when their sister was nearby...