With a string of murders staged as suicides beginning to occur in the metropolitan city of Seoul, Kim Myungsoo, a well-respected detective in an elite police unit, is determined to capture the killer. As the number of deaths increase, Myungsoo and his team gather enough evidence to track down the serial killer's next victim — third-year university student and Kim Myungsoo's first love from high school, Lee Sungjong. However, Sungjong, oblivious to the fact that he's being targeted, seemingly has no ties to the murders to which Myungsoo has no choice but to spiral backwards into his past and hope that he would find the clue he desperately needs. As the detective unit hurries to close this case by finding the criminal behind these mass murders, Kim Myungsoo unknowingly finds himself running in a pursuit of longing where he rediscovers the feelings he once abandoned and understands what it means to protect the one he loves.