Ayum, a new student at Tokyo's prestigious school, finds living in the dormitory difficult due to her dislike of communal life. So she stays with her older sister, a promiscuous gal who constantly brings different men back to their room for nonstop sex. As Ayum, now entering puberty, becomes increasingly curious about these erotic encounters, he struggles to focus on his studies because of them. Note that recording contents may vary depending on the distribution method used.