A single father and son household. A father who is a guardian has requested private tutoring for his son who is preparing for entrance exams. The tutor, Rie-sensei, is a beautiful and intelligent married woman who runs a private cram school in the same town. She has been taking care of her son since he was still in elementary school, calling him "Yuri-sensei." One day, her son asks her to show him her butt as a reward for improving his grades. Unable to refuse, Yuri-sensei tells him to keep it a secret and shows him a little of her perky peach butt. From there, a forbidden chain of events begins...