A man's car is hit by a stranger who kills him when he gets down to take pictures of the damage for claiming of car insurance. Jung Tae-Suk (Kim Mu-Yeol) is an honest cop who goes through the crime scene and suspects it to be the work of a serial killer, but his senior doesn't believe him. Tae-Suk is also investigating gangster Dong-Soo, who has bribed Tae-Suk's superiors.On the other hand, gangster Jang Dong-Soo (Ma Dong-Seok) is on his way home after a meeting with his rival gang's head (where they have a disagreement over how to split territories) when the killer hits his car and eventually tries to kill him. A fight ensues, and both of them end up injured. The killer escapes after knocking Dong-Soo with his car. Dong-Soo is admitted to the hospital, while his henchmen assumed that the crimes were committed by a rival gang and attacked the latter. Dong-Soo believes it wasn't a rival gang member as his moves seemed purposeless. Tae-Suk visits Dong-Soo at the hospital and asks him to cooperate so that he can catch the killer for him but he doesn't comply.Dong-Soo orders his men to find the killer through a sketch, and they manage to find his car and a knife (with which the killer killed another man in the vicinity). Dong-Soo eventually informs the findings to Tae-Suk and they decide to team up to catch the killer (Dong-Soo needs to "take care" of the killer as this random incident has made a dent in his personal reputation that is now hurting his business). Dong-Soo agrees to provide manpower, but on the condition that the killer belongs to whoever that finds him first. Dong-Soo wants to kill him for revenge, while Tae-Suk wants to arrest him in order to solve five connected murder cases and then eventually get promoted.Dong-Soo orders his right hand man Kwon Oh-Sung (Choi Min-Chul) to kill his rival Heo Sang-Do (Yoo Jae-Myung) with the killer's knife that already has the blood imprints of the killer's previous victims. An enraged Tae-Suk gets into a fight with Dong-Soo upon learning what he has done, as the killing is confirmed to be the serial killer's work and the case would get transferred to the Major Case Squad. Dong-Soo visits Sang-do's funeral where the killer also turns up and informs Sang-do's right hand man that Dong-Soo killed Sang-do with his knife. When Tae-Suk and Dong-Soo check the killer's car for any evidence, they are attacked by Sang-do's right hand and his henchmen. A fight ensues, resulting in the death of Sang-do's right hand man. Dong-Soo gets him buried and clears the scene.Tae-Suk is now assigned a kidnapping case (The serial killer had killed another man, but now pretended to kidnap him to extort money from his wife. So Tae Suk asks the wife to take the money to the designated drop in the hope of finding the kidnapper there), and while solving it, he sees the killer. A chase ensues, but the killer flees. Through the forensic tests, he discovers the killer to be a missing person. He informs Dong-Soo about this, and a voice clip he received confirms the missing person to be the killer. Later, Dong-Soo helps a girl by giving her his umbrella and soon learns she has been killed when his umbrella is found at the crime scene. They start looking for the killer and eventually find him in his car. A car chase ensues, and the killer flees again but is then captured and beaten up by Dong-Soo. However, the killer kills Oh-sung and Dong-Soo takes the killer away to kill him, but Tae-Suk crashes his car and hits Dong-Soo, then arrests the killer.With no conclusive evidence against the killer whose name is revealed to be Kang Kyung-Ho (Kim Sung-Kyu), Dong-Soo, being the sole survivor of his attack, testifies against him and proves him guilty when scars caused by Dong-Soo's stabbing due to self-defense are discovered on Kyung-ho's body. The court sentences Kyung-Ho to death while Dong-Soo is also arrested due to his criminal activities. Tae-Suk is finally promoted, while Dong-Soo gets transferred to the same prison as Kyung-ho: something Dong-Soo had requested Tae-Suk to do in exchange for the testimony followed by his arrest. While Kyung-ho is having a shower, Dong-Soo closes the tap and approaches him with a smile and a rope in his hands.