In 1999, Yong (Yeo Jin-Goo) studies mechanical engineering at a university. He first entered the school in 1995, but took time off to perform his mandatory military service. After finishing his military service, he came back to school as a student. His first love is Han-Sol (Kim Hye-Yoon). Yong then somehow communicates with Mu-Nee (Cho Yi-Hyun). Mu-Nee lives in the present day. She enrolled at her university in 2021 and majors in sociology. Yong and Mu-Nee, who are from different time periods, communicate and their story unfolds.