The film opens with a short prologue wherein unlicensed boxing promoter/small casino operator Turkish and his business partner Tommy sit in a darkened office before an unidentified, powerful looking man at a desk. Turkish finds his thoughts turn to musings about diamonds, which acts as a segue into the main plot...
After stealing an 86-carat (17.2 g) diamond in a heist in Antwerp, Franky "Four-Fingers" goes to London to see diamond dealer Doug "The Head" on behalf of New York jeweller "Cousin Avi". His contact that helps him escape Antwerp and reach London advises Franky to obtain a gun from ex-KGB agent Boris "The Blade", then later calls Boris and encourages him to steal the diamond from Franky before it is turned over to Doug.
Meanwhile, Turkish has reluctantly agreed to allow his fighter, "Gorgeous George", to compete in a fight organised by "Brick Top", a local gangster and underground boxing promoter with a reputation for fight-fixing. However, when Turkish sends Tommy and George to purchase a caravan from a group of Irish Travellers, George gets into a fight with Mickey O'Neil, a bare-knuckle boxing champion who badly 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 agrees to the change on the condition that Mickey throws 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. At the same time Boris hires Vinny and Sol, two small-time crooks and the owners of a pawn shop, to rob Franky while he is placing the bet at the bookies. Knowing that Franky is keeping the diamond on him at all times, as long as the robbery is successful, they should be able to take the case containing the diamond from Franky. Of course the robbery goes awry and Sol, Vinny, and their driver Tyrone are caught on camera, but they do manage to kidnap Franky. Upon learning of Franky's disappearance, Avi, knowing Franky has a gambling problem and fearing that Franky might use the diamond in a bet, flies to London with his bodyguard "Rosebud" to find Franky and claim the diamond personally.
Instead of throwing the fight, Mickey knocks his opponent out with a single punch in the first round. Infuriated, Brick Top robs Turkish of his savings and demands that Mickey fight again, and lose this time. With that settled, Brick Top now sets out to find and punish Sol, Vinny, and Tyrone for robbing his bookie. Boris arrives at the pawn shop first, where he retrieves the diamond and murders Franky. The next day Brick Top tracks down Sol, Vinny, Tyrone, and their friend, Yardie "Bad Boy" Lincoln with the intent of killing them all. Sol bargains for their lives by promising 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, closely pursued by Sol, Vinny, and Tyrone. Coincidentally Turkish and Tommy 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 and killing 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 back 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 a dog that Vinny got from the travellers. Avi wildly fires at the fleeing dog, accidentally killing Tony. He flees London and returns to New York.
Mickey refuses to fight again unless Turkish buys a better caravan for his mother, but Turkish has no money left since Brick Top stole his savings. Furious, Brick Top has his men vandalize Turkish's gambling arcade and burn down Mickey's mother's caravan while she is asleep inside. 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's men will execute Turkish, Tommy, Mickey, and the entire campsite of travellers. Mickey makes it to the fourth round, when he suddenly knocks out his opponent. Outside the arena, Brick Top and his men are killed by the travellers. Mickey has bet on himself to win, and waited until the fourth round to allow the travellers time to ambush and kill Brick Top's men at the campsite.
The next morning, Turkish and Tommy find the travellers' campsite deserted. 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. This reveals where the two were during the prologue — consulting Doug about selling the diamond. Doug calls Avi, who returns to London.
