While her husband is stationed overseas, stepmother Satomi lives with her college-student stepson, Yuichi. One night, Satomi accidentally discovers her stepson's erect penis as he sleeps. Long separated from her husband, she feels an intense attraction toward his firm, upright member and loses all self-control. She begins passionately fondling him, giving in to the illicit desire that consumes them both in the house they share alone. Until dawn breaks, their forbidden passion flows endlessly between them.