After her divorce, Mari returns to her parents' home. Coming from a wealthy family, she has a younger brother with a significant age gap between them. Wanting to cheer up his older sister, who is emotionally and physically drained after the divorce, the brother invites her on a long-overdue private date. However, this outing is driven not by mere sibling affection, but by the brother's hidden sexual desires masked beneath the guise of brotherly love. Mari is already aware of the romantic feelings her brother has harbored for her since before her marriage. Precisely because of this, she realizes this date isn't just a sweet gesture—it's a taboo act. As her brother's passionate advances shake her emotionally, loneliness and lust intertwine, and Mari begins walking down a path from which there's no return. Her mature, voluptuous body in her mid-thirties becomes exposed to her younger brother's lewd desires, blurring the line between affection and lust. This series of events reshapes the bond between brother and sister, becoming the catalyst for an entirely new relationship.