Matt Buckner (Elijah Wood) is thrown off his journalism course at Harvard University after cocaine is discovered in his room. However, the cocaine belongs to Jeremy Van Holden (Terence Jay), his roommate. Buckner is afraid to speak up because the Van Holdens are a powerful family, and Jeremy pays him $10,000 for taking the fall. Matt moves to the United Kingdom to live with his sister Shannon (Claire Forlani), her husband Steve Dunham (Marc Warren) and their young son, Ben (James Allison). There, Matt meets Steve's brother, Pete (Charlie Hunnam), a loud and thuggish Cockney who runs a local football hooligan firm - a group of football supporters that arranges fights after matches - and teaches at a local school. Steve asks Pete to take Matt to a football match between West Ham United and Birmingham City, though Pete reluctant to take a "Yank" to a football match, because of the xenophobic nature of his friends. He is persuaded only because Steve will give Matt money. After defeating Matt in a fight, Pete decides to take Matt to the football match, thinking the yank might learn a thing or two.
Matt meets Pete's friends and his firm in the Abbey, their local pub. His friends all befriend with Matt, with the exception of his right-hand man, Bovver (Leo Gregory). After a few pints of lager, they head to Upton Park for the match. After the match, Pete, Bovver and the other firm members agree to go and fight some Birmingham fans, Matt decides that it is not for him and tells Pete he is going to take the train home. On his way back to the underground, Matt is jumped by three Birmingham fans, who nearly give him a 'Chelsea Grin', but is rescued by some GSE members, who are on their way to a larger fight. Though grossly outnumbered, the GSE manage to hold their ground until reinforcements chase off the Birmingham firm. Matt does well in his first true fight and is inducted into the GSE. After a row with Steve, Matt moves in with Pete, and the two exchange stories. The GSE firm then head to an away game against Manchester United. Matt was not meant to come but ends up sneaking onto the train. Whilst on the train they are pre-warned that 40 ManU firm members are waiting for them at the station. Bovver hits the emergency stop button which allows the GSE to get off at an earlier stop (Macclesfield). Having failed to find a taxi they persuade a van driver to take them into Manchester. Matt sits in the front of the van with the driver; the rest of the GSE are in the rear. As the van approaches the ManU fans, Matt tells them that they are moving equipment for a Hugh Grant film, so the fans let them through. When past them, he stops the van, opens up the back, and the GSE charge out to attack the ManU firm members. They win the fight and run away singing "There's your famous GSE". It is soon revealed to Matt that the GSE's sworn enemy is Millwall's firm, led by Tommy Hatcher (Geoff Bell), whom Bovver makes negotiations with after getting jealous of Matt.
After one of the members of the firm see Matt meeting his father, a renowned journalist for The Times, for lunch, they assume Matt is a "Journo" as well. Bovver informs Pete about this, and, when Steve finds out, he goes to the Abbey to warn Matt. Matt finds out that Steve used to be "The Major," of the GSE. When Steve was The Major, the last game he went to was against Millwall, to which Tommy Hatcher brought along his 12-year-old son, whom Tommy had raised to be hard. The son was killed in the ensuing fight. Since then Tommy Hatcher "lost it," and blamed Steve and the GSE for his son's death. After seeing this happening, Steve left football hooliganism for good.
At that moment, Bovver arrives, and there is a big argument in the Abbey, in which Bovver comes out humiliated. Infuriated, he goes to Millwall's local and asks Tommy Hatcher to attack The Abbey. Initially reluctant, Tommy Hatcher agrees upon learning that Steve is there. The Millwall firm crash the Abbey, and petrol bomb the bar. Tommy Hatcher confronts Steve Dunham. Steve's attempt to convince Tommy Hatcher he is no longer involved in the GSE only reminds Tommy Hatcher of his son, and he stabs Steve in the neck with a broken bottle, telling him "If you die tonight, you and me are even." Bovver, who had been knocked out by Tommy Hatcher, comes round just in time to help Steve. At the hospital, Pete blasts Bovver for his betrayal.
In the aftermath, the two firms meet near the Millennium Dome for an all-out brawl. Matt and Bovver show up to fight for the GSE, but during the fight, Matt's sister, Shannon, turns up with their son, and are attacked by a Milwall hooligan. Matt and Bovver come to their rescue. Pete notices that Tommy Hatcher is approaching the car, and distracts Tommy by taunting him to "finish him off." When Tommy Hatcher declares to have finished with him, Pete then declares that Tommy Hatcher was to blame for his son's death, having failed to protect him ("He was your SON!"). Tommy Hatcher, driven to insanity, attacks and kills Pete, all the while shouting out a variation of the words to the chant 'Only a poor little Hammer,' using it as an analogy for Pete's condition. The fight completely halts at this point, and Tommy Hatcher is eventually pulled off Pete by some of his friends as he breaks down in tears. Everyone on both sides gathers around Pete's dead body in shock, with Bovver sobbing at his side.
Matt returns to the United States and confronts Jeremy Van Holden in a restaurant toilet, where Jeremy is snorting cocaine. Jeremy arrogantly tells Matt to leave. Matt then pulls out a tape recorder and plays back what Jeremy just said, including their discussion about how the cocaine in Matt's room was Jeremy's, saying that it is his "ticket back to Harvard." Jeremy lunges at him to try and get the tape, but Matt casually reverses the attack and raises his fist as if to punch Jeremy. He does not do so, but instead walks out with a smile as Jeremy collapses to the floor, defeated. The film ends with Matt walking down the street outside the restaurant singing "I'm Forever Blowing Bubbles."