For the families living at Sky Castle, an exclusive residential community that's home to Korea's elite, their children’s success means everything.

Right after Ryu Sun-jae, a top star, ends his life, Im Sol, his top fan, somehow ends up at a time when they were in high school and tries to protect him. A fantasy romance unfolds where people who missed each other in time finally meet.

A fiercely competitive police officer and an idealistic prosecutor, former high school classmates, reunite while investigating a nationwide serial murder case, pulling them into both a dangerous pursuit and an unresolved personal past.