After stealing an 86-carat (17.2Â 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 jeweler "Cousin Avi". One of the other robbers advises Franky to obtain a gun from ex-KGB agent Boris "The Blade". Unbeknownst to Franky, Boris and the robber are brothers and plan to steal the diamond from him before he can turn it over to Doug.
Meanwhile, boxing promoter and slot machine shop owner Turkish convinces gangster "Brick Top" to put boxer "Gorgeous George" in a matchup against one of Brick Top's boxers. However, when Turkish sends his partner Tommy and Gorgeous 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 convinces 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. Avi, knowing Franky has a gambling problem, flies to London with his bodyguard "Rosebud" to claim 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 and Sol, Vinny, and their driver Tyrone are caught on-camera, but manage to kidnap Franky.
Instead of throwing the fight, Mickey knocks his opponent out with a single punch. Infuriated, Brick Top robs Turkish of his savings and demands that Mickey fight again, and lose this time. Meanwhile, Boris retrieves the diamond and murders Franky with a pistol. Brick Top tracks down Sol, Vinny, Tyrone, and their friend, Yardie "Bad Boy" Lincoln and plans on killing them for robbing his bookies. 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 and intimidates them into leaving. The wounded Boris arrives with an assault rifle with a grenade launcher looking for the diamond back but is shot and killed by Tony, wounding 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 travelers. Avi fires at the fleeing dog, accidentally killing Tony. He gives up 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 travelers. 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 travelers. Mickey has bet on himself to win, and waited until the fourth round to allow the travelers time to ambush and kill Brick Top's men at the campsite.
The next morning, Turkish and Tommy find the travelers 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. They consult Doug about selling the diamond and he calls Avi, who returns to London.

Question: Why does Avi fly to London?
Answer: To get the diamond from Franky personally.
Alternative answer: Because Avi accidentally killed Tony