After her husband died, Rieko raised her son Kosuke on her own. As Kosuke grew up, he began to have sexual awakenings and harbored evil feelings towards his mother. However, even though she noticed, she pretended not to notice. Then, on the night before his wedding, Rieko gave Kosuke, who could only release his frustrations through masturbation, a final mother-son sex education lesson, telling him, "Tonight is the last night we'll spend together as a mother and son..." However, their uncontrollable desires drove them to further forbidden acts...