Mariko, a schoolgirl entering puberty, finds her emotions stirred by her parents' illicit relationship. Known throughout the neighborhood as the perfect couple, they spend every night immersed in intense intimacy. Their bedroom is filled with various sexual acts—not just intercourse, but blowjobs, handjobs, and toy play. The lewd sounds and moans easily reach Mariko in the next room. At an age when her sexuality is awakening, she can't help but be drawn to what's happening just beyond the wall. One day, unable to resist any longer, she peeks into her parents' bedroom...