A man's car is hit by a stranger who kills him when he gets down to take pictures of the damage for claiming car insurance. Jung Tae-suk is an honest cop who goes through the crime scene and suspects it to be the work of a serial killer, but his senior does not believe him.Gangster Jang Dong-soo is on his way home after a meeting when the same killer hits his car and eventually tries to kill him. A brutal fight ensues, leaving both of them injured. Finally, the killer escapes after knocking Dong-soo with his car. Dong-soo is admitted to the hospital, and his henchmen assume that the attack was committed by a rival gang, so they counter-attacked the latter. However, Dong-soo believes the killer was not a rival gang member as his moves seemed to be random and purposeless. Tae-suk visits Dong-soo at the hospital and asks him to cooperate so that he can catch the killer for him, but Dong-soo does not comply. Instead, Dong-soo orders his men to find the killer through a sketch, and they manage to find his car and a knife. Dong-soo eventually informs Tae-suk of the findings, and they decide to team up to catch the mysterious killer. Dong-soo agrees to provide manpower and cover expenses-but on the condition that the killer belongs to the one who 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.Meanwhile, Dong-soo orders his right-hand man, Kwon Oh-sung, to kill his rival Heo Sang-do 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 informing Sang-do's second-in-command man that it was Dong-soo who actually killed Sang-do with the killer's knife.When Tae-suk and Dong-soo check the killer's car for any evidence, they are attacked by Sang-do's 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 to investigate a kidnapping case, and, while solving it, he spots the killer. A chase ensues, but the killer flees. Through the forensic tests, Tae-suk discovers the killer to be a missing person. He informs Dong-soo about this and lets him hear a voice clip to confirm the missing person to be the killer. Later, Dong-soo helps a school girl by giving her his umbrella and soon learns that she has been killed and his umbrella was found at the crime scene. They start looking for the killer and eventually find him in a car. In an ensuing chase, the killer manages to kill Oh-sung but is finally captured and incapacitated by Dong-soo. The gangster takes the killer away to torture and kill him, but Tae-suk tracks them and crashes his car into the hideout, hitting Dong-soo unconscious and arresting the killer.With no conclusive evidence against the killer whose name is revealed to be Kang Kyung-ho, 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 demonstrated on the Kyung-ho's body. The court sentences Kyung-ho to death; however, Dong-soo is also arrested due to his criminal activities. Tae-suk is finally promoted while Dong-soo gets transferred to the prison where Kyung-ho is also kept-the condition that Dong-soo had requested Tae-suk to fulfill 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.