Three years into her remarriage, Mami Shirai finds herself troubled by her relationship with her stepson. His suspicious behavior and the way he seems to view her as a woman hint at something hidden deep within his heart. Recently, her underwear has been going missing frequently, deepening her suspicions about his true intentions. One night, Mami discovers her lingerie in her stepson's room. Intending to confront him about it, she approaches him—but the encounter takes an unexpected turn, ultimately leading to a physical relationship between them.