Nao and her husband have been married for a few years. Her husband has become increasingly indifferent and less caring towards her than when they first got married. Nao feels incredibly lonely as her husband seems to be drifting away from her. Her husband's birthday is approaching, and Nao hopes to use this occasion to improve their relationship. Wanting to look her best on that day, Nao has been attending swimming classes based on an advertisement. Nao gradually becomes attracted to the kindness and attentiveness of her handsome swimming coach. While her husband always avoids her, he even didn't come to the birthday dinner she prepared for him. Feeling too disappointed, Nao turns to her swimming coach, the only person who can warm her heart at this moment...