The central twist of "Memories of Murder" is that there is no resolution. The investigation, which the audience and the detectives believe is building towards a final confrontation and reveal, ultimately collapses into failure. After pursuing multiple dead ends, the detectives finally corner their most promising suspect, Park Hyeon-gyu. Seo Tae-yoon, having abandoned his logical principles in a fit of rage and grief over the murder of a schoolgirl he knew, is about to execute Hyeon-gyu in a tunnel.
At that moment, Park Doo-man arrives with the long-awaited DNA results from the United States, which they believe will definitively prove Hyeon-gyu's guilt. In a devastating reversal, the document states the DNA is not a match; the results are inconclusive. This revelation shatters the detectives. Hyeon-gyu walks free, disappearing into the darkness of the tunnel, his guilt or innocence forever an open question within the film's narrative.
The film then jumps forward to 2003. Park Doo-man is no longer a cop but a salesman with a family. He happens to visit the location of the first murder and is told by a small girl that another man had recently been there, reminiscing about something he had done there a long time ago. Park, realizing it was the killer, asks what he looked like. The girl replies, "Kind of plain. Just ordinary." This is the film's final, chilling revelation: the monster they were hunting was not a discernible fiend, but an average person hiding in plain sight. Park then looks directly into the camera, a haunting acknowledgment that the killer was never caught and could be anyone—even a member of the audience watching the film.