In 1986, the (fictional) city of Iltan in Gyeonggi Province: A young girl gets brutally murdered by an unknown person on a dark night.

As the city is gripped by a succession of serial killings, a total of nine murders within a twelve-kilometer radius since 1993, the police conclude that a man they've nicknamed "Gap-dong" is behind the crimes.

Detective Yang Cheol-gon is convinced that the actual killer is Ha Il-sik, a resident of the town who is intellectually challenged.

Seventeen years later, Moo-yeom is a police detective who spends his career chasing dead ends and helping juvenile delinquents.

Cheol-gon, now a well-decorated officer, has recently transferred back to Iltan, and to his dismay, Moo-yeom joins his investigation team to catch Gap-dong once and for all.