Rumiko Kudou discovers an adult magazine and her son's underwear in his room, and secretly begins reading the magazine while sniffing the underwear and masturbating. Seeing her son's pitiful state, she grows concerned and consults her husband, who is away on a business trip. Her husband suggests that showing her naked body to their son could be part of sex education, and reluctantly gives her permission to let the son peek while she masturbates. As Rumiko hears her son getting excited from watching her, she becomes immersed in a forbidden thrill. Unable to resist any longer, her son thrusts out his erect cock, giving in to overwhelming lust. When their lips meet and she feels the heat of his cock, Rumiko crosses the boundary of mother and son, stepping into the realm of incestuous desire.