After being widowed by her husband, Rieko raised her son, Kosuke, with her own hands. As Kosuke grows up, he begins to have evil feelings for his mother from his sexual awakening. But even though she was aware of this, she pretended not to see it. And on the eve of his marriage, Kosuke, who can only vent by masturbating, says, "Tonight is the last night to spend with parents and children ....", Rieko finally gives sex education to mother and child. However, the uncontrollable desire drives the two to further forbidden acts ...