Hiroki (the son), a ronin (a student studying for university entrance exams), was neglecting his studies and spending his days masturbating. Yukie witnessed her son in this state and, unable to hide her shock at his powerful penis, decided to help him masturbate as a form of sex education, since he was a virgin and had never known a woman. Yukie initially ignored Hiroki's requests to go all the way, but seeing his disappointment, she reluctantly agreed. Even after ejaculating, Hiroki's erection didn't subside, and he tried to force himself into Yukie's vagina again, but he caught her as she tried to escape and forcibly raped her. From then on, Hiroki craved Yukie everywhere they went, and soon a twisted form of mother-son love began to develop...