During a snowy night, the car of Joo-yun (Oh San-ha) gets a flat tire and Kyung-chul (Choi Min-sik) drives up and offers to help fix her tire. Joo-yun turns down the request. However, she does not know that Kyung-chul is a sadistic murderer, and he ends up kidnapping her and brutally kills her. Afterwards, Kyung-chul cuts Joo-yun into pieces and scatters her body parts. The next day, a boy encounters one of Joo-yun’s severed ears, and the police are called in to find the rest of the body, under the command of Section Chief Oh (Cheon Ho-jin) and Squad Chief Jang (Jeon Kuk-hwan), who is the father of Joo-yun. After it is confirmed that Joo-yun is the victim, her fiancée, Soo-hyun (Lee Byung-hun), who is a secret agent, becomes determined to track down and take vengeance against Joo-yun’s murderer, even if he is forced to become a monster himself.
After receiving a list of potential killers from Jang, Soo-hyun receives Kyung-chul’s location from his son, whom he has abandoned and sent to live with his grandparents. Upon finding his fiancée’s wedding ring in Kyung-chul’s home, Soo-hyun attacks him in his greenhouse, while Kyung-chul is raping one of his would-be victims. After suffocating him until he passes out and breaking his wrist, Soo-hyun places a transmitter inside Kyung-chul, so he can find his exact location and listen to his conversations. Waking up, Kyung-chul goes to a hospital where he sexually assaults a nurse, only to be again attacked by Soo-hyun, who severs his achilles tendon.
Kyung-chul goes to the home of his friend Tae-joo (Choi Moo-sung), a cannibalistic murderer, for a place to stay. After explaining his situation, Tae-joo remarks that whoever is after him must have some relation towards one of his victims. Through the transmitter, Soo-hyun arrives at the house, where he proceeds to incapacitate both murderers and Tae-joo’s girlfriend Se-jung (Kim In-seo). The next day, both Tae-joo and Se-jung are found by the police and sent to a hospital. Meanwhile, Kyung-chul is sent to a private medical area where he and Soo-hyun are treated for the their wounds by a trusted subordinate of Soo-hyun’s. However, a barely conscious Kyung-chul manages to learn about the transmitter inside him.
While Soo-hyun is buying painkillers, Kyung-chul taunts him over the transmitter, now knowing who he is and how he’s being tracked. To make Soo-hyun lose track of him, Kyung-chul injures a clerk, defecates out the transmitter, and places it inside a taxi driver he assaults, and then steals his car. Determined to find out where Kyung-chul is going, Soo-hyun interrogates Tae-joo and learns that Kyung-chul is going after Joo-yun’s family; Jang and Joo-yun’s sister Se-yun (Kim Yun-seo). Afterward, Kyung-chul intends to turn himself over to the police.
Soo-hyun arrives too late to stop Kyung-chul, who blinds Jang with a dumbbell and murders Se-yun. However, he manages to prevent the police from capturing Kyung-chul by abducting him before Oh and his men can reach him. Soo-hyun brings Kyung-chul to his old home, where he states his intent to only kill Kyung-chul when he is filled with fear and pain. Eventually, Soo-hyun sets up a guillotine above Kyung-chul’s head that will be activated when his parents or son open the door to the room he is in. Placing a new transmitter nearby, Soo-hyun’s hears the death of Kyung-chul and his family’s reaction to his decapitated corpse. He attempts to walk away from the house, but breaks down crying on the street.