I Saw the Devil – Academy-bus driver Jang Kyung-chul happens upon Jang Joo-yun one snowy night and offers to help fix her flat tire. Kyung-chul kills her and scatters her body parts. When a boy discovers one of Joo-yun’s ears, the police are called in under the command of Section Chief Oh and Squad Chief Jang, the latter of whom is the father of Joo-yun. Kim Soo-hyun, a secret service agent of the National Intelligence Service (NIS) and Joo-yun’s fiancé, vows to track down and take vengeance on Joo-yun’s murderer.
Jang supplies Soo-hyun with a list of four suspects, including Kyung-chul. Searching Kyung-chul’s home, Soo-hyun finds jewelry and underwear taken from (apparently) numerous victims. The discovery of Joo-yun’s engagement ring proves that Kyung-chul is the killer. Soo-hyun puts a tracking device on the academy bus, following then attacking Kyung-chul while he is sexually assaulting his latest victim—one of the schoolgirls he was transporting home. Beating him unconscious, Soo-hyun places an NIS transmitter inside Kyung-chul, allowing him to track him on radar and listen to his conversations. Waking up, Kyung-chul flags down a taxi. During the ride, Kyung-chul realizes that the two men in the cab are thugs looking to rob and possibly kill him; he stabs both men to death. After finding the real taxi driver in the trunk, Kyung-chul disposes of all three bodies before going to a medical center, where he attempts to sexually assault a nurse Han Song-yi. Soo-hyun intervenes and slashes Kyung-chul’s Achilles tendon before letting him go again.
Kyung-chul goes to the home of his friend Tae-joo, a cannibalistic murderer. After explaining his situation, Tae-joo remarks that whoever is after him must have some relation to one of his victims. Soo-hyun arrives, proceeding to incapacitate both murderers along with Tae-joo’s girlfriend Se-jung. The next day, both Tae-joo and Se-jung are found by the police and sent to a hospital. A trusted subordinate of Soo-hyun’s ensures he and Kyung-chul are sent to a private medical area away from the police. The barely conscious Kyung-chul hears them talk about the transmitter inside him.
Soo-hyun dumps Kyung-chul, intending to continue stalking him. Kyung-chul taunts him over the transmitter, now knowing who he is. Kyung-chul brutally assaults a store owner, forcing Soo-hyun to rush to the aid of the victim. Kyung-chul uses this time to defecate out the transmitter and place it inside a taxi driver he viciously assaults. Soo-hyun interrogates Tae-joo and learns that Kyung-chul is going after Joo-yun’s father Jang and sister Jang Se-yun. I Saw the Devil
Soo-hyun arrives too late to stop Kyung-chul, who blinds Jang with a dumbbell and mutilates Se-yun. He abducts Kyung-chul before he can turn himself over to the police. Soo-hyun tortures him physically and mentally before setting up a guillotine above Kyung-chul’s head that is soon activated when his parents and son arrive to visit and open the door to the room he is in. Placing a transmitter nearby, Soo-hyun’s hears the death of Kyung-chul and his family’s reaction to his decapitated corpse. Alternating between sobbing and fits of laughter, Soo-hyun suffers a mental breakdown while walking away from the house.