Mari and her husband Yoichi live in a quiet house, where Yoichi's younger brother, Shinji, has temporarily moved in to attend a prep school for the bar exam. One day, while Mari is cleaning, she enters Shinji's room and discovers her missing underwear tucked between the mattress and bed frame. In that moment, she is overwhelmed with confusion and uncertainty, struggling to make sense of the situation. This sudden, unexpected incident begins to shift the dynamics of her marriage. As Shinji's presence reshapes their daily routine, Mari starts to experience unfamiliar emotions stirring within her. Amid the small cracks in ordinary life, a complex web of adult desires and emotional tension begins to unfold.