Married woman Nao has been with her husband for several years, but the warmth they once shared has faded, and the distance between them keeps growing. Feeling lonely yet still hoping to repair their relationship, she plans a special dinner for her husband's birthday. Wanting to look her best for the occasion, she decides to join a swimming class after seeing a flyer by chance. There, she meets her instructor, Naoki Ariki, whose kindness contrasts sharply with her husband's demeanor, and she gradually finds herself emotionally drawn to him. However, the swimming class Nao started for her husband's sake ends up creating a new rift in their marriage.