in 1996, 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. his attempt to arrest the suspect fails as ha commits suicide to prove his innocence. his death inspires his son moo-yeom to become a police officer to clear his father's name and restore his honor.