The saying "hindsight is 20/20" could not be more true for Lee Yuna, whose regrets from 13 years ago have irrevocably shaped her current situation. With little money to her name, she is prepared to marry a man she does not love. Yuna does not want anything special, just a comfortable life. But if she could change the past, there is one thing more than any other that she would do—avoid Ryu Jihoon at all costs.

Back in high school, she and Jihoon were the most lovey-dovey couple, well known for bickering one moment and flirting the next. How could a pair so in love break up so bitterly to the point that Yuna will not even mention Jihoon's name even in the present?

While visiting her alma mater one night, Yuna stumbles upon the urban legend "The Stairway of Time" and falls into the past before she ever met Jihoon. She resolves to fix the mistake of ever meeting him, but fate works in mysterious ways. As much as she wants to push him away, Yuna is drawn again to his bright personality. Can she change what has already come to pass?