Journalism major Matt Buckner (Elijah Wood) is expelled from Harvard University after cocaine is discovered in his room. Though it belongs to his roommate, Matt is afraid to speak up because his roommate Jeremy comes from a wealthy and powerful family, and is offered $10,000 for taking the blame. Matt accepts the money and uses it to visit his sister Shannon (Claire Forlani), her husband Steve (Marc Warren) and their young son Ben in London. There, Matt meets Steve’s brother, Pete (Charlie Hunnam), a teacher and football coach who leads the local football hooligan firm – Green Street Elite (GSE). Steve asks Pete to take Matt to a football match. Though Pete is reluctant to take a “Yank” to a football match, he decides to take Matt to the game, thinking he might “learn something.”
Matt meets Pete’s friends and his firm in their local pub and they befriend Matt, with the exception of Pete’s stubborn right-hand man, Bovver who takes an immediate dislike to Matt. A few pints in and they head to the match. Afterward, Pete, Bovver, and the other firm members go off to fight some Birmingham City fans, but Matt decides that it is not for him and heads to the train home. On his way back, Matt is jumped by three Birmingham fans but is rescued by some GSE members. Though grossly outnumbered, the GSE manage to stand their ground until reinforcements chase off the Birmingham firm. Matt does well in his first true fight, is inducted into the GSE, and moves in with Pete.
The GSE firm then head to an away game against Manchester United. Matt ends up sneaking onto the train to help when they are warned that 40 Manchester United firm members are waiting for them at the station. They get off at an earlier stop and persuade a van driver to take them into Manchester, posing as a moving equipment van for a Hugh Grant film. When past them, the GSE charge out to attack the United firm members. They win the fight and leave, taunting the United firm.
Jealous of Matt’s rise in the ranks, Bovver talks to Tommy Hatcher (Geoff Bell), the head of GSE’s rival firm; the NTO. After one of the members of the GSE sees Matt meeting his father, a journalist for The Times, for lunch, they assume Matt is a “journo” as well. Bovver informs Pete of this, and Steve goes to the Abbey to warn Matt, discovering that Steve is the founder and former leader of the GSE; “The Major”, who retired from football hooliganism after witnessing the death of Tommy’s 12-year-old son in a fight. Bovver secretly informs Tommy and the Millwall firm of Steve’s presence.
Pete angrily confronts Matt in the bathroom over his identity as a “journo” right before the Millwall firm then crash the Abbey and petrol-bomb the bar. On arrival, Tommy confronts Steve and stabs him in the neck with a broken bottle, telling him that if he dies tonight then they are both even. Bovver, who had been knocked unconscious by Tommy’s right-hand man upon arriving, helps get Steve to the hospital where Pete slams Bovver for his betrayal. Shannon decides to return to the United States to ensure the safety of her family.
The two firms meet near the Millennium Dome the next day for a final brawl. Matt and Bovver show up to fight for the GSE, but Shannon turns up with Ben and is subsequently attacked in her car by Tommy’s right-hand man. Matt and Bovver come to their rescue. Pete notices that Tommy is approaching the car and goads him to “finish him off.” When Tommy says the NTO will end it, Pete retorts that Tommy is to blame for his son’s death, having failed to protect him. Tommy snaps and tackles Pete to the ground, eventually beating him to death. Everyone on both sides gathers around Pete’s dead body in shock.
Matt returns to the United States and confronts the now wealthy and successful Jeremy in a restaurant toilet; with Jeremy admitting to being the cocaine stash’s owner. Matt pulls out a tape recorder saying that it’s his “ticket back to Harvard.” Jeremy lunges at him to get the tape, but Matt fends him off with ease. Matt walks out with a smile down the street outside the restaurant singing “I’m Forever Blowing Bubbles”.