One day, Reiko, who lives with her daughter and son-in-law, accidentally catches her young stepson Hikaru deeply engrossed in masturbation due to sexual frustration. Mesmerized by his enormous, thick cock, she gathers her courage and begins approaching him. Learning that her daughter's sexless marriage is the cause, Reiko offers herself in her daughter's place and persistently seduces Hikaru. Since Hikaru constantly builds up sperm, he cannot resist his stepmother's advances and soon gives in completely. Thereafter, the two repeatedly meet in secret, evading their family's watchful eyes, deepening their illicit relationship.