The Gangster, the Cop, the Devil

After a man's car is hit by a stranger, the stranger kills him as he takes pictures of the damage to submit to his car insurance. Jung Tae-suk is a cop who goes through the crime scene and suspects it to be the work of a serial killer, but his commanding officer does not believe him. Jang Dong-soo is a crime boss who, while on his way home after a meeting, is hit by the same killer, who tries to kill him as well. A brutal fight ensues, leaving both of them injured. Finally, the killer escapes after hitting 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, whom they counter-attack.

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 the police can catch the killer for him, but Dong-soo refuses. Instead, Dong-soo orders his men to find the killer. Using a sketch and Dong-soo's memory of the killer's license plate, 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 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 traces of blood from 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 work of a serial killer, causing the case to be transferred to the major case squad. Dong-soo attends Sang-do's funeral; the killer also turns up and informs Sang-do's second-in-command that Sang-do was killed with the killer's knife, but by someone else. 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 while in a fight with Tae-suk.

Dong-soo gets him buried and clears the scene. Tae-suk gets assigned to investigate a kidnapping case and, while solving it, spots the killer. A chase ensues, but the killer is able to escape. Through forensic test, 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 is the killer. Later, Dong-soo helps a high school girl by giving her his umbrella but soon learns that the girl has been murdered and his umbrella was found at the crime scene. Dong-soo and Tae-suk and their men start looking for the killer in the area where the girl was murdered and eventually find him in a car. In the ensuing chase, the killer manages to kill Oh-sung but is finally captured and incapacitated by Dong-soo. Dong-soo takes the killer away to torture and kill him, but Tae-suk tracks them down and crashes his car into the hideout just as Dong-soo is about to kill the killer. Tae-suk hits Dong-soo with his car and arrests the killer.

The police have no conclusive evidence against the killer, Kang Kyung-ho. As a last resort, Tae-suk asks Dong-soo, the sole survivor of the attacks, to testify. Simultaneously, Tae-suk threatens to release a tape of Dong-soo's confession of having Sang-do killed. During Kyung-ho's trial, Dong-soo arrives to testify and provides decisive testimony. Dong-soo successfully predicts the existence of a stab wound on Kyung-ho's body, previously inflicted by Dong-soo during their first encounter. Dong-soo also provides misleading evidence framing Kyung-ho for Sang-do's murder. The court sentences Kyung-ho to death; however, Dong-soo is also arrested due to his illegal activities.

Tae-suk is finally promoted while Dong-soo is sent to the prison where Kyung-ho is kept: the condition which Dong-soo had requested Tae-suk to fulfill in exchange for the testimony followed by his arrest. Spotting Kyung-ho, Dong-soo gleefully enters prison. While Kyung-ho is showering, he is confronted by Dong-soo with a rope in his hand and a smile on his face.