After stealing an 86-carat (17.2 g) diamond while dressed as an ultra-Orthodox Jew during a heist in Antwerp, Franky "Four-Fingers" goes to London to see diamond dealer Doug "The Head" on behalf of New York jeweler and Jewish-American organized crime figure "Cousin Avi". One of the other robbers advises Franky to obtain a gun from his brother, arms dealer and ex-KGB agent Boris "The Blade", then later calls Boris and encourages him to steal the diamond from Franky before he can turn it over to Doug.
Meanwhile, Cockney boxing promoter and slot machine shop owner Turkish is persuaded by crime boss "Brick Top" to put his boxer "Gorgeous George" in a match against one of Brick Top's boxers. However, when Turkish sends his partner Tommy and Gorgeous George to purchase a caravan from a clan of Irish Travellers, George gets challenged to a fistfight against Traveller bare-knuckle boxing champion Mickey O'Neil, who beats up and severely injures George. Turkish persuades Mickey to replace George in his upcoming match by agreeing to purchase a new caravan for Mickey's mother. Brick Top grudgingly agrees, but only on the condition that Mickey will throw the fight in the fourth round.
Boris gives Franky a revolver in exchange for a favour: Franky is to place a bet on Boris' behalf at Brick Top's bookies. Avi, knowing Franky has gambling addiction, flies to London with his bodyguard "Rosebud" to pick up the diamond personally. Boris hires Vinny and Sol, two small-time crooks, to rob Franky while he is at the bookies. The robbery goes awry when they crash their car into Franky's van while trying to park, trapping Franky inside. Sol, Vinny, and their driver Tyrone are caught on camera and find no money at the bookies due to the bets being cancelled since Gorgeous George had to drop out, but manage to kidnap Franky in their escape. Sol and Vinny confront Boris afterward and demand he gives them half of the cash when he sells the diamond. Boris ends up killing Franky after Vinny says his name, accidentally revealing to Franky that Boris betrayed him, and leaves with the diamond.
Instead of throwing the fight, Mickey accidentally knocks his opponent out with a single punch due to his overwhelming power. Infuriated, Brick Top robs Turkish of his life savings and demands that Mickey fight again, and lose since the majority of the gamblers will now bet on him. Mickey refuses to fight again unless Turkish buys an even better caravan for his mother, but Turkish has no money left since Brick Top stole his savings. Furious, Brick Top's men vandalize Turkish's gambling arcade and burn down Mickey's mother's caravan while she is asleep inside. Meanwhile, Boris retrieves the diamond and murders Franky with a pistol. Brick Top tracks down Sol, Vinny, Tyrone, and their friend, Jamaican Yardie "Bad Boy" Lincoln and plans on killing them for robbing his bookies. Sol bargains for their lives by offering Brick Top the stolen diamond, and is given 48 hours to retrieve it.
Avi and Doug hire "Bullet-Tooth" Tony to help them find Franky. When the trail leads to Boris, they kidnap him and retrieve the diamond, while being closely pursued by Sol, Vinny, and Tyrone. Turkish and Tommy, who are on their way to purchase a gun from Boris, are driving on the same stretch of road at the time. When Tommy throws Turkish's carton of milk out of their car window, it splashes over Tony's windscreen, causing him to crash which accidentally kills Rosebud in the process. Boris escapes from the wreck only to be hit by Tyrone's car. Tony and Avi are confronted by Sol, Vinny, and Tyrone at a pub where Tony realizes that the trio's pistols are replicas, which he contrasts with his real handgun, intimidating them into leaving. The wounded Boris arrives with an assault rifle and a grenade launcher looking for the diamond, but is shot and killed by Tony, who wounds Tyrone at the same time. Sol and Vinny leave a wounded Tyrone and escape with the diamond, which Vinny hides in his pants. When Tony catches up to them, they tell him that the diamond is back at their pawn shop. Once there, they produce the diamond, but it is promptly swallowed by the dog that Vinny got from the Irish Traveller clan. Avi wildly fires at the fleeing dog, accidentally killing Tony. He gives up and returns to New York City.
Mickey agrees to fight to avoid more carnage, but gets so drunk after his mother's wake that Turkish fears he will not make it to the fourth round. If he fails to go down as agreed, Brick Top vows that his men will murder Turkish, Tommy, Mickey, and his entire clan of Travellers.
After arriving to the fight, Mickey makes it to the fourth round, when he suddenly knocks out his opponent. Outside the arena, as Tommy, Turkish, and Mickey try to run for their lives, Brick Top and his men are all killed by the Travellers. Mickey secretly bet on himself to win, and waited until the fourth round to allow his clansmen time to avenge his mother by ambushing and killing Brick Top and his men.
The next morning, Turkish and Tommy find the Traveller campsite deserted as Mickey and "the pikeys" have escaped with their winnings. When confronted by the police, they cannot explain why they are there, until Vinny's dog suddenly arrives and they claim to be walking it. Sol and Vinny are arrested when the police find Franky and Tony's bodies in their car. Turkish and Tommy take the dog to a veterinarian to extract a squeaky toy that it had swallowed, and discover the diamond in its stomach, as well. They consult Doug about selling the diamond and he calls Avi, who returns to London to purchase it.