One day, after coming home from shopping, mother Izumi hears moaning sounds coming from her son Mamoru's room. Hesitant at first, she opens the door and discovers Mamoru having sex with his girlfriend, Madoka Kokubo. As Izumi secretly watches the couple's intercourse, her body grows hot with arousal, and she can't resist starting to masturbate. Overcome by excitement, Izumi reaches her peak, drops her shopping bag, and quickly shuts the door—but Mamoru notices her. Later, Mamoru confronts her about spying and begins to tease and seduce her. Though Izumi struggles to maintain her composure, she ultimately gives in and loses control.