Yuusei was kicked out by his wife after a brutal marital fight and was left utterly lost. Seeing his condition, Serino, his wife's older sister, steps in to mediate. Beautiful, kind, and deeply empathetic, Serino's support becomes a lifeline for Yuusei. However, she is not merely an impartial mediator—Serino has her own hidden agenda and deliberately draws Yuusei emotionally closer to achieve her goals. Gradually, Yuusei falls completely for Serino's tenderness, completely captivated. At that moment, Serino suggests he take a bath—and then begins undressing herself. Her deliberate actions deepen Yuusei's emotional pull, foreshadowing a new and intense development.