Base on webtoon TOMORROW'S BEST.
The series follows the journey of Im Sol (Kim Hye-yoon), an ardent fan of idol Ryu Sun-jae (Byeon Woo-seok) who died in a tragic accident. One fateful night, as Im Sol mourns Sun-jae's untimely demise, she stumbles upon a mysterious pocket watch that inexplicably transports her fifteen years into the past when both of them are still young and just 19 years old. Realizing the opportunity she has been given, she embarks on a mission to change the course of events and save Sun-jae's life.