"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 begin