Seven years old, Hee Joo, loses her detective, single father to Ki Beom, a notorious serial killer who is caught after murdering his cheating girlfriend and sentenced to only 15 years in prison because the court failed to find evidence linking him to the other killings. Detective Dae Yeong takes in Hee Joo and raises her. Fifteen years later, Hee Joo is the mascot of the police precinct, where she volunteers. When Ki Beom is released from prison, he gets complicatedly involved with the girl who has been waiting 15 years to find out who murdered her father.