On the eve of her son's marriage, her mother became a woman.

After her husband and her husband, Rieko raises her son, Kosuke, on her own. As Kosuke grows up, he begins to develop evil feelings towards his mother from his sexual awakening. But even though she realized this, she kept pretending not to see her. Then, on the eve of his marriage, Rieko finally gives her mother and child sex education to Kosuke, who can only be masturbated by masturbation, saying, "Tonight is the last night we're spending together with our parents...." However, uncontrollable desires drive the two of them into even more forbidden acts...