After several years of marriage, married woman Yū struggles with growing emotional distance from her husband, who is constantly busy with night shift work. The demanding schedule has drained his energy, and for the past three years, their intimate life has nearly disappeared. At the suggestion of a part-time coworker, Yū decides to try a swimming class to refresh her mood. When she tells her husband, he shows no interest—deepening her sense of loneliness. Carrying this quiet sorrow, Yū begins attending the swimming lessons. The instructor, a young man named Hosoda, is honest, kind, and possesses a muscular, well-built physique. Gradually, Yū finds herself drawn to him, and new emotions begin to awaken within her.