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 present, and without they realize, they learn a lot about love again.

Cast
• Byun Baekhyun, a 28 years old man who works as a prosecutor. He's famous for being ambitious and ranked no. 1 ever since elementary school. He is rock-headed, always nagging around, but actually a warm person. As he gets older, Baekhyun turns into a workaholic and cold man, even forget his own birthday.
• Yoon Yoo-jin, a 28 years old woman who works as a lawyer in a famous law firm. A determined and somehow reckless person. As she gets older, she changed into a mature woman and learning to be a good wife for Baekhyun eventhough she realize he changed a lot.
• Lee Taeyong, one of Baekhyun's close friends. He is the son of Baekil Law Firm owner and going to be the next successor when actually, he doesn't want to. He's quietly in love with Yoo-jin. Later, he works in UNICEF to fullfill his dream.
• Eun So-yul, Yoo-jin's close friends. She is Yoo-jin's love life godmother although she's famous as two month's girlfriend because her relationship only last 2 months. Later, she married to Kim Minseok, a medical student that she truly likes since college days.
• Kim Jongin, another Baekhyun's close friends. He ranked last and always sleep during the class. He doesn't interested in study, but has a big interest to business. Later, he owns the biggest departement store in Seoul.
• Kim Minseok, a third-year medical student and Jongin's cousin. He works part time in the library and restaurant to help his family's life. Later, he become the famous neurosurgeon after saving the prime minister's life and married to Eun So-yul.