In October 1986, a grim discovery is made when two women are found brutally raped and murdered in a ditch near a local field. Facing the weight of this unprecedented case, local detective Park Doo-man feels overwhelmed. With the evidence collection being improperly conducted, the police's investigative methods in question, and a lack of forensic technology, the odds seem stacked against him. Park, claiming to rely on a unique ability to gauge suspects by eye contact, learns from his girlfriend about Baek Kwang-ho, a mentally handicapped boy with a noticeable scar, living in the town. Intrigued, he seeks out Baek and employs his eye contact strategy, erroneously suspecting him of the heinous crimes.
As pressure mounts, Park coerces his partner Cho into extracting confessions from Baek, even secretly recording him as he discusses one of the murders. Enter Seo Tae-yoon, a detective from Seoul, who volunteers his assistance but quickly finds himself at odds with Park's dubious methods, leading to a brawl at a social gathering. Seo examines Baek's hands, deeming them too frail to have executed such complex crimes, which subsequently clears the boy's name.
As the investigation progresses, another set of murders occurs under peculiar circumstances—the killer exclusively targets women dressed in red and seeks them out during rainy nights. Notably, a female officer, Kwon Kwi-ok, connects the dots when she observes a pattern: a particular song is played on a local radio station coinciding with the murders. Despite his relentless pursuit, Park turns up empty-handed.
During one tense investigation at a recent crime scene, the team—Park, Cho, and Seo—arrives with differing approaches. A local man disrupts the scene, brandishing women's undergarments and behaving indecently. A twig snaps underfoot, prompting the man to make a hasty escape into the crowd. Park identifies him after noticing the red lingerie peeking from beneath his clothing, all while pretending to use his eye contact method again, compelling Cho to apprehend the suspect.
In a series of shocking missteps, Park and Cho resort to brutal tactics against the man. Meanwhile, Seo uncovers a survivor's account detailing soft hands that could belong to the elusive killer. In a fit of desperation, Park and Seo must reconcile their differences, realizing that they need to pool their resources to catch the perpetrator.
An autopsy reveals unexpected evidence—a trace of peach in the latest victim's body. Through meticulous detective work, Seo follows clues back to Park Hyeon-gyu, a newcomer to the area. Tensions rise as Seo witnesses Hyeon-gyu's reaction to peaches, sparking suspicions that drive Seo closer to the truth. However, before he can delve deeper, Cho loses his composure and intervenes aggressively, resulting in their higher-ups restricting Cho's access to Hyeon-gyu for questioning.
As the investigation intensifies, Seo replays Baek’s recorded confession, realizing that he might have witnessed the crime rather than committed it. Confronting Baek's father in a restaurant leads to chaos, with a drunk Cho sparking a brawl. In the ensuing commotion, Baek tragically runs onto the tracks and is hit by an oncoming train, leaving Park burdened with guilt for dragging his friend into such a dangerous situation.
With the case spiraling out of control, a sample of semen is traced but must be sent abroad for analysis due to the lack of forensic technology. The lead detective's moment of failure manifests one fateful evening when Park's girlfriend walks through the darkened woods, unknowingly shadowed by a figure. In a sudden twist, another young girl falls prey, ultimately discovered the next morning—leaving Seo distraught as he recognizes her as the schoolgirl he had recently befriended.
Overwhelmed by grief, Seo lashes out at Hyeon-gyu, insisting that justice must be served, only to be interrupted by Park's arrival with crucial documents. Tragically, the results confirm that Hyeon-gyu's DNA does not match. Desperation turns into anger when Seo insists that the documents are fabrications, while Park struggles to maintain control of the situation. In the climax of frustration, Seo attempts to shoot Hyeon-gyu, only for Park to intervene and let the supposed killer go free.
As the closing credits roll, the haunting reality of an unsolved case settles heavy in the air. Years later, in 2003, Park Doo-man, now a businessman, revisits the crime scene, where a little girl mentions the recent arrival of a mysterious stranger with a 'normal' face. This revelation sends a chill down Park's spine as she recounts the man's vague reminiscence of something he once did, prompting Park to gaze into the camera, metaphorically employing his eye contact method to discern hints of the killer lurking within the audience.
