Husband cold, wife has an affair with swimming coach

Nao and her husband have been married for a few years. Her husband has become cold and towards her compared to when they first got married. Nao feels extremely lonely as her husband seems to be growing further apart from her. It is her husband's birthday, and Nao wants to use this occasion to improve their as a couple. Wanting to look her best on that day, Nao went to a swimming class on a leaflet.

Nao gradually becomes attracted to the handsome and caring swimming coach. When her husband continues to distance himself from her, even skipping the birthday dinner she prepared for him, Nao is extremely. She seeks solace in her swimming coach, the only person who can warm her heart at this moment...