Detective Jung Tae-suk (Kim Mu-yeol) is a Cowboy Cop. He is dissatisfied with the rampant corruption on the force that allows organized crime to operate almost in the open, and the lack of respect officers get. This is made only worse when he realizes a serial killer is operating in his region and the surroundings, but lacking firm evidence and fearing Jurisdiction Friction, Da Chief refuses to treat the murders as anything but isolated incidents.
Jang Dong-soo (Ma Dong-seok) is a brutal mob boss with a penchant for brawling, who recently had to put a weaker rival back in his place. As he drives home alone one evening, he's attacked by the serial killer, but manages to fight him off. Eager for revenge, and wanting to remedy the loss of face his hospitalization caused, he sends his men to find the killer. While he has the manpower, he lacks the scientific and crime expertise the cops have, and soon makes an offer to Jung Tae-suk: He'll give Jung the manpower he lacks to pursue the killer, while Jung provides the expertise, and they will share information. If Jung finds the killer first, Jung can arrest him. If Jang gets the killer first, he'll gruesomely kill him in revenge to show what happens to those who cross him. Sensing there's no other way to stop the killer who has so far operated in the shadows, the detective accepts.
Little do both parties know that the killer is not done, that he has plans of his own, and Jang Dong-soo's own criminal dealings are about to make the situation way more complicated when his henchmen begin believing that the attack on their boss happening so close to him dealing with a rival can't be a coincidence...
On 5 May 2019, it was announced that Sylvester Stallone will produce an American remake of The Gangster, The Cop, The Devil, with Ma Dong-seok reprising his role as Jang Dong-soo.
The Gangster, The Cop, The Devil contains examples of:
- Almost Dead Guy: Dong-soo is chasing Kyung-ho when he stumbles on his own friend Oh-seong, just stabbed by Kyung-ho several times. A dying Oh-seong is still able to point Dong-soo which way to go.
- Ax-Crazy: Kang Kyung-ho, a serial killer who knifes people at random.
- Bad Boss: Jan Dong-soo batters even his own underlings bloody if he feels his authority is being threatened or even mocked.
- Boring, but Practical: Unlike most movie serial killers, Kyung-ho's M.O. is nothing special. He just drives around until he catches someone alone, rear-ends them, and then stabs them to death when they get out to check the damage.
- Bullying a Dragon: A driving question for most of the movie - who would attack the boss of a gang? Besides the resources Jang has at his command, he's also a ruthless gang boss and a brutally efficient fighter, one would have to be insane to stab him. Turns out that the killer didn't know who he was attacking, he picked Jang because he was a target of opportunity. Somewhat subverted that even when the killer realizes who he attacked, he can't help himself but antagonize Jang further by killing a young girl he helped.
- The Cassandra: Jung Tae-suk correctly guesses that the murders are related, and correctly warns that the killer isn't done. He's ignored by his boss who refuses to risk the negative publicity that they've had a serial killer in their midst and done nothing about him.
- Cowboy Cop: Jung Tae-suk, who raids criminal organizations against his superior's orders. Has no patience or care for procedure.
- Enemy Mine: Jung Tae-suk and Jang Dong-soo. The former has gone out of his way to annoy the later and hinder his criminal activities. But as neither can see a way to stop the killer without the other's help, they are forced to cooperate.
- Evil vs. Evil: Jang Dong-soo is not a nice man. He's introduced having just unleashed a No-Holds-Barred Beatdown on someone as a work out session. He brutalizes his allies, underlings, and enemies without a thought to maintain his image and his position. He'll have a man killed in his bed for getting a leg up on him. Despite all this, he's still the lesser of the two evils compared to Kang Kyung-ho.
- For the Evulz: The motive behind Kyung-ho's murders. When Dong-soo attempts to kill him after a climactic car chase, Kyung-ho asks him if he too enjoys the feeling of power over life and death.
- Framing the Guilty Party: The Seoul police, and specifically Tae-suk's chief, are refusing to admit that Tae-suk is right and there is a Serial Killer on the loose. So Dong-soo forces the issue by having one of his minions murder Dong-soo's gangster rival...with the same knife that Kyung-ho used for two murders, one that the gangsters recovered before the lazy Seoul police. Finding a bloody knife with DNA on it from three different victims forces the authorities to admit there's a serial killer. Kyung-ho, still at large, is very upset to have a murder he didn't do attributed to him.
- Freudian Excuse: Tae-suk's investigations reveal that Kyung-ho was abused by his father, and spent time in an orphanage.
- Freudian Excuse Is No Excuse: Despite this, nobody in the film, not even the man himself, really considers it an excuse for Kyung-ho's murderous behavior.
- Go-Karting with Bowser: The cops and gangsters actually start hanging out as they work the case, having lunch and drinking together.
- Hospital Gurney Scene: Dong-soo being whisked into the emergency room after being stabbed by the killer and then struck by his car.
- Hot-Blooded: Tae-suk is noticeably more temperamental and impatient than either Dong-soo or Kyung-ho.
- A Lighter Shade of Black: Jang Dong-soo is a ruthless and violent gangster who has brutally beaten enemies and allies alike. However, he has a friendlier side to him and doesn't just kill people for kicks like the serial killer does.
- Made of Iron: Jang Dong-soo. The man takes a massive beating throughout the movie, including being stabbed and hit by a car, and shrugs it off. By his own account, most people wouldn't have survived the knife wounds he received from Kyung-ho.
- "Not So Different" Remark: Kang Kyung-ho tries to give one to Jang Dong-soo after the later has captured, tied him up and are preparing to kill him. Jang Dong-soo punches him in response.
- Pet the Dog: Jang Dong-soo gives a schoolgirl waiting for the bus his umbrella, encouraging her to keep studying. He's enraged when the killer murders her that same day.
- One-Man Army: Jang Dong-soo again. The man basically takes out two-thirds of Hur's gang solo, receiving barely a scratch.
- Oh, Crap!: Kyung-ho has this reaction twice at the end of the movie. First when Jang arrives at the same prison and looks at him, and lastly when Jang approaches him with a rope in his hands. Both times Jang has a Cheshire Cat Grin on his face.
- Serial Killer: Kang Kyung-ho. Unlike most examples, he's not at all picky with his choice of victims. He kills grown men, young girls, hardened gangsters, and even a wealthy CEO before he's done.
- Sissy Villain: Kyung-ho is a very slight and effeminate man, which contrasts him even more with the athletic Tae-suk and the brawny Dong-soo.
- Slasher Smile: Kyung-ho's default expression. He even flashes one just as Dong-soo prepares to kill him in prison.
- Smug Snake: Kyung-ho oozes calm and arrogance during his trial.
- Taking You with Me: Dong-soo is willing to go to prison if it means that Kyung-ho won't get away with his crimes.
- Villain Protagonist: Jang Dong-soo is arguably focused on as much as Jung Tae-suk, enough to be considered a Deuteragonist, and he is an unrepentant and violent criminal who happens to be hunting down an even worse man.
