One snowy night, school-bus driver Jang Kyung-chul runs into Jang Joo-yun and offers to fix her flat tire. After beating Joo-yun unconscious, Kyung-chul kills her with a makeshift guillotine at his home and scatters her body parts into a local stream. 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 (Joo-yun’s father). 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 suspects, including Kyung-chul. Searching Kyung-chul’s home, Soo-hyun finds jewelry and underwear taken from numerous victims. The discovery of Joo-yun’s engagement ring proves that Kyung-chul is the killer. Soo-hyun places a tracking device on the school bus, following and 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 a 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 sexually assaults a nurse. 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 after a fierce fight. 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 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 pharmacist while searching for laxatives, 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 in a public restroom. Soo-hyun interrogates Tae-joo and learns that Kyung-chul is going after Joo-yun’s father Jang and sister Jang Se-yun, and that he plans to be arrested by the police before Soo-hyun can reach him.
Soo-hyun is too late to stop Kyung-chul, who brutally beats Jang with a dumbbell and mutilates Se-yun. He narrowly abducts Kyung-chul before he can give up himself to the police. Soo-hyun tortures him before setting Kyung-chul’s guillotine above Kyung-chul’s head, which is triggered when his parents and son arrive to visit him and open the door to the room. Placing a transmitter nearby, Soo-hyun hears the death of Kyung-chul and his family’s reaction to his decapitated corpse. After the transmission concludes, Soo-hyun begins crying uncontrollably and suffers a nervous breakdown while walking away from the house.