"Introverted and shy, easily stressed," I was the worst kind of person, secretly having sex with her younger sister while harboring desire for her. (Mita Marin, SONE-626)

Marin is a timid and introverted girl. Though unconsciously attractive, her looks and mannerisms are captivating to men, always making her a popular choice. She has a tutor, Atsushi, who is her older sister's boyfriend. He has his own troubles. They've been dating for a while, but Marin's older sister is a serious person, leaving no opportunity for intimacy. Compared to her strong sister, the seemingly weak and vulnerable Marin... Atsushi, whose desire is about to erupt, can't resist making a move on his younger sister. However, Marin... accepts. Knowing her desire for the easily stressed Atsushi gradually intensifies...