Reiko Kobayakawa lives with her daughter and son-in-law, and day by day, she grows increasingly restless. The cause? She can't help but overhear the couple's intimate moments. While it's good that her daughter and son-in-law have a strong relationship, for Reiko, who sleeps alone, it's a growing source of stress. Every time she wakes at night and catches sight of their lovemaking, she finds herself unable to resist masturbating. Though plagued by self-loathing, she manages to push through her daily life—until the day she discovers her son-in-law masturbating alone in the living room. Unable to restrain herself, she seduces him, and they begin a secret affair. But when her daughter starts suspecting her husband of infidelity, their illicit relationship comes to an end. Yet once ignited, Reiko's body can no longer find satisfaction through masturbation alone, and she inevitably craves the intense passion she shared with her son-in-law. This overwhelming desire pulls her back into the depths of forbidden lust once more.