Saeko Matsushita is a lady married with a husband much older than him. She is fully aware of the big age gap but believes on her own happiness. She also has an agreement with the husband for total loyalty and sincerity.
Little did she suspected that she would be the one to break her own rules and not follow her own preaching. But that was meant to be because the first lie was to herself, not acknowledging that she is not as much happy as she thought.
The third wheel here is Mister Hayato (Jyun Odagiri), an old friend of the couple who will awake Saeko from her mistakes. The first lie, but not the last.