At a class reunion, Mio ends up in a one-night stand with a former classmate, only to be consumed by guilt toward her husband and fear of pregnancy. During this turbulent time, her brother-in-law, Takashi, coincidentally visits and stays over. Noticing her unusual behavior, he grows concerned yet increasingly intrigued. Hearing from his brother that their marriage is practically devoid of intimacy, Takashi becomes suspicious—why is his sister-in-law worried about pregnancy? Convinced she’s having an affair, he begins to investigate. Already drawn to Mio since their first meeting, Takashi uses his suspicions as an excuse to get closer, gradually deepening their forbidden relationship.