In October 1986, two women are found raped and murdered in a ditch near a field. Local detective Park Doo-man, not having dealt with such a serious case before, is overwhelmed — key evidence is improperly collected, the police’s investigative methods are suspect, and their forensic technology is near non-existent. Park claims he has a way of determining suspects by eye contact. He learns from his girlfriend that a scarred mentally handicapped boy, Baek Kwang-ho, resides in the town and decides to question him first. He finds him and uses his eye contact method, instinctively thinking Baek is responsible. Park has his partner Cho beat confessions out of Baek and secretly record him talking about one of the murders.
Seo Tae-yoon, a detective from Seoul, volunteers to assist them. However he and Park’s methods clash and they get into a fight during a party. Seo looks at Baek’s hands and deems them too weak and scarred to be able to commit such an elaborate crime, clearing his name. After more murders are committed they realize that the killer waits until a rainy night and only kills women wearing red. A female police officer, Kwon Kwi-ok, realizes that a local radio station is always requested to play a particular song during the nights the murders are committed. Park continues to search for the killer, but finds nothing.
At the latest crime scene, Park, Cho and Seo all arrive to investigate in different ways, but then a local man arrives, pulls out undergarments, and masturbates in a woman’s red lingerie. Cho accidentally steps on a twig, prompting the man to run. They chase him into a crowd. Seo attempts to find a way to get everyone’s attention, but Park identifies the man first after seeing him bend over and partially exposing the red lingerie underneath his pants. Despite already having identified him, he acts as if he is using his eye contact method once more and has Cho apprehend him.
Park and Cho continue their tactic and brutally beat the man. Seo finds a survivor of the killer after tracing rumors around the local school with the help of Kwon. Upon learning from the survivor that the killer’s hands were noticeably soft, Seo manages to clear the man as his hands are very rough. Infuriated that they lost their suspect, Park scuffles with Seo, until Kwon alerts them that the same song the murderer requested is playing. They realize it’s raining and arrive too late to find another woman murdered. Park and Seo and Cho decide to all work together.
Upon doing an autopsy of the latest victim, they discover pieces of what looks like a peach in the body. Seo talks to a schoolgirl about a possible suspect and help her put a band-aid on an injury on her waist. They follow a trail of clues to a factory worker, Park Hyeon-gyu, who had only moved to the area a short time before the first murder. They track the song using documents (a source that Seo always trusts, saying, “The documents never lie.”) from the radio station to his address. The detectives are initially unable to pin anything on him. However, Seo notes that his hands are soft like the survivor he questioned earlier had described. Hyeon-gyu begins to show discomfort when Seo presents the peaches and he thinks he’s found the killer. Before he can question more, Cho loses control and beats Hyeon-gyu, prompting their superior to ban him from the interrogation room.
Park and Seo listen to Baek’s earlier confession to a previous murder. Seo points out that he talks as if someone else did it and they realize he knew details of the murder because he witnessed it. They go to Baek’s father’s restaurant, only to discover a drunken Cho there. As people make fun of Cho on the TV, he beats everyone in the restaurant. Baek arrives partway through the brawl and joins the fray, swinging a wooden board at Cho’s leg and accidentally piercing it with a rusty nail. Park and Seo chase Baek and question him, but he gets frightened and runs into the path of an oncoming train where he is hit and killed.
Park learns that Cho’s leg will have to be amputated because the rusty nail caused tetanus, leaving Park feeling guilty about what he had gotten his partner and best friend into.
They discover a trace of semen on one of the bodies, but because of lack of forensic technology the sample has to be sent to the United States to confirm if suspect Hyeon-gyu is the killer with DNA evidence.
Seo tries to follow Hyeon-gyu, but he dozes off and misses his chance. That night Park’s girlfriend walks through the forest watched by an unknown figure in the shadows, but a younger girl passes by and she is captured and killed instead. The next morning her body is discovered. Seo sees a band-aid on her waist and recognizes her as the same schoolgirl that he befriended while investigating the murders. Enraged at the loss, Seo’s frustrations finally flow over and he snaps. He attacks Hyeon-gyu until he is interrupted by Park bringing documents from America regarding the semen sample. However, the papers state the sample does not match Hyeon-gyu’s DNA. Seo shouts that the documents are lying and that he knows Hyeon-gyu is the murderer. Park uses his eye contact method on the suspect, but realizes he cannot draw any conclusions unlike before. Seo attempts to shoot the suspect, but Park stops him and lets Hyeon-gyu go.
In the end, the crimes remain unsolved. While visiting the crime scene years later in 2003, Park Doo-man, now a businessman, learns from a little girl that the scene had recently been visited by another, unknown, man with a ‘normal’ face. The little girl had asked the man why he was looking at the ditch and was told that he was reminiscing about something he had done there a long time ago. The film ends as Park, realizing this, looks straight at the camera, seemingly using his eye-contact method to spot the killer among the audience.