Keita Suminoe is a third-year Japanese middle school student living with his father, stepmother, and older twin stepsisters, Ako and Riko. Since the remarriage of their parents at childhood, the siblings have always been affectionately close and supportive of one another. This prologue skips to the trio, now as teenagers, see their opposites in a noticeably less platonic fashion; Ako and Riko frequently flirt and lust after Keita while, much to his stress, he fights the urge to give in to them.
