After her husband's death, Satoko raised her son, Kosuke, alone. As Kosuke grew up, he began to harbor sinful feelings towards his mother due to his awakening sexuality. Despite being aware of this, she chose to ignore it and carried on. On the eve of his wedding, unable to release his tension except through masturbation, Satoko said to Kosuke, "Tonight is the last night we'll spend as parent and child..." and decided to give him one final lesson in sexual education between mother and son. However, their overwhelming desires drove them into even more forbidden acts...