The drama will tell the story of a man named Sung Hae Sung (played by Yeo Jin Goo) whose birth record states that he's 31 years old, but both his inner and outer appearances seem like that of a 19 year old.

Jung Chae Yeon will be playing the 19 year old Jung Jung Won, a bright high schooler. Jung Jung Won's 31 year old character will be played by Lee Yeon Hee. In the drama, Yeo Jin Goo and Jung Chae Yeon's characters have grown up together since they were young. The two actors are also the same age, 1997-born, in real life, and got along well on set, according to the staff.