Jin Joo and Ban Do met when they were 20 and got married when they were 24. 18 years later, Jin Joo is exhausted from taking care of their child alone, and Ban Do is exhausted from work. They can't understand one another and decide to get a divorce. On the day that they decide to end their 18 years of marriage, something strange happens to them.

Ban Do and Jin Joo return to the year they were 20. Unfortunately, they face the worst day of both their lives. They are both excited to return to their college years, but that excitement turns out to be temporary. They meet at an unexpected place only to be reminded of the events that occurred that day.

Jin Joo and Ban Do thought their return to 1999 was a miracle that was to last one night. Contrary to their belief, they woke up the next day still stuck in 1999. Ban Do, who believed that he was the only one to travel back in time, finds out that Jin Joo had also come with him. The two agree not to get involved with one another and live new lives.

Jin Joo and Ban Do's argument is heard throughout the campus as it was broadcasted in the school radio show. The students of the school begin their search to find the married couple. Meanwhile, Nam Gil saves Jin Joo from getting hurt, and Ban Do goes on his first date with his first love, Seo Young.

Ban Do starts to worry about Jin Joo as she becomes involved with Hyun Suk. Nam Gil keeps calling Jin Joo out. He uses the leg he hurt while saving her as an excuse to see her. Jin Joo bumps into Ban Do and Seo Young at an unexpected place.

It's festival day at Hankook University. Ban Do remembers the humiliation he suffered on this day. He does whatever he can in order to prevent history from repeating itself. Meanwhile, Jin Joo, who is enjoying herself at the festival, ends up entering the May Queen contest.