Due to his father's remarriage, Shohei Shindo—an otaku—finds himself living under the same roof as his classmate Rino Mizusawa, a popular "gal." He is initially flustered by the situation, but a shared interest in otaku culture leads to them having their first sexual encounter and developing a relationship that goes beyond that of mere siblings. One day, Shohei quietly approaches Rino while she sleeps; when he lifts the towel blanket covering her, he discovers she is sleeping completely naked. His hands revel in the softness of her breasts and stomach before wandering down to her buttocks. He is captivated by the sight of her anus, revealed as he parts her firm, rounded cheeks. Although he and Rino go on to have sex many times, the image of that alluring anus never leaves his mind.