Mari is a high school girl in the midst of puberty. Her parents are known in the neighborhood as a lovebird couple. They indulge in an intense, married life every night. Of course they have sex, but they also do blowjobs, fingering, and toy play... Their obscene sounds and voices are all heard by their daughter in the next room, separated by a single wall. Mari, who is at an age where she is very curious about sex, can't help but wonder what is going on next door. One day, Mari can't help but peek into her parents' bedroom...