Long-time friends; Bacon, Soap, Tom and Eddie decide to put together £100,000 to play 3 card brag against "Hatchet" Harry Lonsdale - a porn king of notorious disposition - in a high-stakes card game in the hopes of winning easy money. Aided by Barry "the Baptist", Harry's personal bodyguard and counselor, the game is fixed so that Eddie, the card shark representing the group, loses to Harry and is forced to pay a £500,000 debt within one week, citing the loss of fingers and Eddie's father's bar if he fails to pay. To Eddie's fortune, and the dismay of his friends, all four of the group are tasked with honoring this debt, as they were all responsible for fronting the stake money. Harrys loyal and violent debt collector, Big Chris - who often brings his son and apprentice, Little Chris, to his work - is assigned to collect the payment on the due date.
After several days and no ideas to come up with the money, Eddie returns home and overhears his neighbors, a gang of thieves known for robbing drug dealers, planning a heist on some marijuana growers supposedly loaded with cash and drugs. Eddie relays this information to the group, intending for all of them to rob the neighbors as they come back from their heist, therefore solving the debt. Tom uses his connection with an underground dealer, known as Nick "the Greek", to provide them with guns for the job, and to find someone to help them move the drugs. Nick then manages to acquire a pair of antique shotguns, and arrange a deal with Rory Breaker, a gangster and sociopath, to purchase the stolen weed.
Prior to the card game, a pair of lowlife criminals, Gary and Dean, were hired by Barry to rob a bankrupt millionaire for Harry, who wanted a specific pair of antique shotguns from the stolen pile for his personal collection. The two guns that Harry wanted, however, were the ones that Gary and Dean sold prematurely to Nick after the robbery. An enraged Barry then threatens the two into getting them back.
The neighbors' heist goes underway; Dog, the leader of the gang, learned of the weed chemists from one of the members, and uses his connections to the group to catch them off guard. Despite having a gang member killed by his own Bren Gun, and an incriminating encounter with a traffic warden, the job is otherwise a success. Unfortunately as they come back to the hideout, the four friends ambush the neighbors and take the loot, who later return that night to stash the goods next door, and then celebrate with a wild night of drinking.
The various characters finally collide when Rory discovers that the weed he was going to purchase was in fact his; the weed chemists that were robbed were under his employ. Rory interrogates Nick into revealing where the four friends live, and uses one of the chemists to identify the robbers. That same morning, Dog has become furious at having been cheated and, during a tirade, he launches one of his men into a wall, who discovers (through the hole he makes as a result) various sound recording equipment; Dog realizes that his neighbors were the ones who robbed him, and has the men prepare to ambush the friends in the flat as he takes the antique shotguns and counts the money. Gary and Dean call Nick, who (in frustration) directs them to the same address in their search for the antique shotguns, while Big Chris and his son depart to collect the debt, and the four friends drive home from the bar.
Rory and his gang assault the flat and enter a shootout with the neighbors, resulting in the deaths of all but Dog and the lone chemist to survive the slaughter, with the latter taking off with the marijuana. Dog is mugged by Big Chris of the shotguns and money during his escape, Gary and Dean stealthily follow Big Chris, and the four friends finally return to the flat, shocked by the carnage and the missing loot. Big Chris then gives the guns and cash to Harry, and as he returns to his car he encounters Dog threatening his son, who wants him to get the loot back from Harry. Desperate to get the guns, Gary and Dean attack Harry and Barry at their office, realizing who they were at the last minute before killing each other in another violent shootout. The four friends soon arrive to find another scene of carnage, and take the opportunity to re-steal the debt money, mystified by their strange fortune. Big Chris then crashes into their car to disable Dog, and after brutalizing him with his car door, he takes the debt money back from the unconscious friends, only to find his employer dead, and Tom just about to make off with the antique shotguns, which he'd briefly paused to examine.
The remaining friends were arrested, but were declared innocent after the traffic warden from earlier identified the neighbors bodies as the prime suspects. The four reunite at Eddies fathers bar, and decide to have Tom get rid of the only evidence linking them to all the bloodshed the shotguns. After Tom leaves, Big Chris arrives to bid them farewell, and gives them a catalog on antique guns. Big Chris then leaves, having kept the debt money for himself and his son. A quick perusal of the book reveals that the shotguns the four had bought for the job were each worth a fortune, and so they desperately try to call Tom. The film ends in a cliffhanger when Toms cell phone, stuffed in his mouth, starts ringing as he hangs over the side of a bridge, preparing to drop the shotguns into the River Thames.