Byun Baekhyun and Yoon Yoo-jin, in their 9th years of relationship, finally going to get married although they already live together for 3 years. Right after their wedding dress fitting, Baekhyun ask Yoo-jin to break up. The two ended up break up, but when their car entering a tunnel, they suddenly entering a strange place and they figured out that they're going back to 10 years ago, when they first met in college. Baekhyun and Yoo-jin decided to work together so they could go back to the pre