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I Saw the Devil (Angmareul boatda)
Jang Kyung-chul, a school bus driver, encounters a woman named Jang Joo-yun and offers to fix her flat tire. After beating her unconscious and bringing her to his house, Kyung-chul methodically dismembers her, unaware of her ring falling into a floor drain. Kyung-chul scatters the body parts into a local river, where it is discovered, prompting the police to conduct a search led by Squad Chief Jang, Joo-yun's father, and Squad Chief Oh. Joo-yun's fiancée, Kim Soo-hyun, an NIS agent, vows to exact revenge on the perpetrator.
After learning of the four suspects from Jang (Joo-yun's father), Soo-hyun privately interrogates two of them. The violent encounter (part torture) of the first two suspects absolves them as his wife's killer. But they are not without fault, one suspect turns himself in for killing multiple schoolgirls in fear of Soo-hyun's return and seeks help from the police in the hospital. Upon searching the home of Kyung-chul (the third suspect), Soo-hyun finds Joo-yun's ring which confirms Jang Kyung-chul to be his fiancé's killer.
A short time later, Kyung-chul brings a schoolgirl home and begins to rape her. Soo-hyun interrupts and knocks him unconscious. Instead of killing Kyung-chul, Soo-hyun forces him to swallow a GPS tracking transmitter, allowing him to track Kyung-chul's movements and listen to his conversations while also fracturing his left arm.
Waking up, Kyung-chul is offered a ride by a taxi, which already carries a passenger. The taxi had a license with a profile picture of the taxi's owner, which Kyung-chul notice is not the one driving this taxi. He has stepped into a hijacked vehicle and the occupants are acting strange. Kyung-chul attacks them seconds before they attack and murders them. Kyung-chul checks the trunk of the taxi and finds a murder victim (presumably the taxi driver) and a bag of fresh clothes. He leaves their bodies on the side of the road, and cleans up in a nearby river.
Kyung-chul drives to a clinic to have his wounds looked at. After being treated, Kyung-chul proceeds to rape a nurse. Soo-hyun arrives, subdues him, and slashes his achilles tendon before leaving. At this point, Soo-hyun's intention becomes clear: he wants to torture Kyung-chul as long as possible. Kyung-chul visits the home of his friend Tae-joo, a cannibalistic murderer and meets his girlfriend Se-jung who seems nonchalant about the meeting.
After learning of the situation, Tae-joo remarks that his tormentor must be related to one of his victims. Kyung-chul deduces Soo-hyun's identity after recalling that his wedding ring matched that of Joo-yun. Soo-hyun arrives and incapacitates Kyung-chul, Tae-joo, and his girlfriend Se-jung. The next day, Tae-joo and Se-jung, still unconscious, are arrested by the police and sent to the hospital. Soo-hyun and Kyung-chul receive treatment for their wounds, aided by Soo-hyun's trusted subordinate, who helps them evade the cops. Kyung-chul wakes up and overhears Soo-hyun and the subordinate talking about the transmitter.
After being released, Kyung-chul steals and uses laxatives to excrete the transmitter, where he plants it on a taxi driver at a truck stop. Soo-hyun enters Tae-joo's hospital room to question him and learns that Kyung-chul is going after Jang (Joo-yun's father) and his other daughter, Jang Se-yun. Enraged by Tae-joo telling him the details of Joo-yun's murder, Soo-hyun rips his mouth open. Kyung-chul arrives at Jang's house, brutally assaults him, and kills Se-yun. Kyung-chul attempts to avoid Soo-hyun's revenge by surrendering himself to the police, but Soo-hyun abducts Kyung-chul in a car before the police can apprehend him.
Soo-hyun drives to Kyung-chul's house, where he tortures him, places him under Kyung-chul's makeshift guillotine and leaves him holding a rope between his teeth to keep the blade from falling. Though he mocks Soo-hyun, Kyung-chul begins to panic when he learns that his son and elderly parents, whom he had abandoned some time ago, have arrived and are trying to visit him. As his family opens the door, the guillotine blade is Booby-trapped to fall and beheads Kyung-chul. Listening to the family's anguish through the transmitter which he placed next to Kyung-chul's head, Soo-hyun breaks down and begins crying hysterically as he walks away from the scene.