"Operation Proposal" tells the story of a Kang Bae-Ho (Yoo Seung-Ho) who travels back in time to protect his first love.   Baek-Ho (Yoo Seung-Ho) and Yi-Seul (Park Eun-Bin) have been best friends since they met first time at the age of 8. They attended the same elementary school, middle school and high school. After they graduated from college and went on to work in different fields, they have remained best friends. On February 14, 2012, Yi-Seul gets married to Jin-Won (Lee Hyeon-Jin), a high school baseball coach Baek-Ho played for. During the wedding, Baek-Ho is overcome with regret for never expressing his true feelings to Yi-Seul, even though he has had 20 years. Yi-Seul's mother then asks Baek-Ho to carry Yi-Seul's stuff to Jin-Won's car. While carrying the boxes, Baek-Ho drops a box and discovers a letter. The envelope is addressed to Baek-Ho from Yi-Seul. Baek-Ho slips the envelope into his jacket. After the wedding, Baek-Ho walks over to a nearby playground. Baek-Ho sits on a swing and begins to read the letter. The letter was written after they graduated from middle school. Yi-Seul writes that she felt Baek-Ho was a special person from the first time they met and she has liked him since. Baek-Ho feels is overcome with his emotions and wails uncontrollably.  A man (Kim Tae-Hoon) walks over and hands him a handkerchief. The man introduces himself as a "time coordinator". He states that he can control time and space. Baek-Ho, at first, thinks the guy is nuts, but with just a hand gesture the man makes everyone on the playground freeze. Baek-Ho asks the man what he wants from him and the man turns his questions around to ask, "more importantly, what do you want from me?" Baek-Ho tells the man that he wants to go back in time to change his present situation. The next moment, Baek-Ho finds himself in his room with the time conductor. The man gives him a small bottle of liquid and then he disappears. Baek-Ho looks at some old pictures taken with Ye-Seul and notices how sad she looks. He decides that he wants to make her happy. Baek-Ho then drinks the liquid ...