The film opens in London, England. Vietnam War vet Ngoc Minh Quan (Jackie Chan) picks up his daughter Fan (Katie Leung) from school. He takes her to a dress shop in Knightsbridge. As Quan tries to move up the street, a man in a motorcycle parks his bike in front of the shop. Quan hits another driver who backs up. The man gets out to confront Quan when a bomb goes off, destroying the dress shop and throwing Quan off his feet.
A news station receives a call from a group calling themselves the Authentic IRA, who are claiming responsibility for the bombing. The head of the station tells his employees to find out more information and to see if any other terrorist groups are behind the bombing. One photographer, Ian Wood (Rufus Jones), goes to the bombing site to take pictures. He stops when he sees Quan inside the store tearfully holding Fan's body.
Three IRA members are watching the news from their safe house, stating that they must stay put in order to avoid detection.
Irish Deputy Minister Liam Hennessy (Pierce Brosnan) is in bed with his mistress Maggie (Charlie Murphy). He gets a call from his wife Mary (Orla Brady) about the bombing. Hennessy then talks to a British politician, Katherine Davies (Lia Williams) about the same issue. Hennessy goes to his job and angrily confronts his subordinates over the bombing, demanding to find out who was responsible for killing civilians.
Quan is left inconsolable after the incident. He is visited at his home above the restaurant he owns by two detectives. Quan asks if they will find the bombers and punish them, and they assure him they will. He leaves his friend and co-worker Lam (Tao Liu) behind, despite her pleas to not pursue revenge.
Quan meets with Scotland Yard's Commander Bromley (Ray Fearon) in an attempt to bribe him with all the money he has in exchange for the names of the bombers. Bromley declines the bribe and tells Quan they don't have the information they need at the moment. During this meeting, it is learned that Quan had two other daughters who were killed when pirates attacked their ship on the way to London.
Quan goes to Belfast to personally meet with Hennessy. After being turned away by his secretary, two security guards pull Quan away after finding a Swiss army knife on his person. Hennessy tells them to let Quan go and he meets with him. Quan brings up Hennessy's history as a leader of the IRA, even showing a newspaper clipping stating Hennessy is proud of his IRA roots. Despite Quan's requests to find the bombers, Hennessy says he knows nothing about them or who they are.
Quan goes to the bathroom with matches, cigarettes, and two soda bottles. He creates a makeshift bomb that causes a minor explosion in the bathroom. Hennessy knows it was Quan, especially after Quan calls Hennessy himself.
Hennessy calls in his nephew Sean (Rory Fleck Bryne) to come help with the investigation, as well as trying to take care of Quan.
Four of Hennessy's IRA goons go after Quan in his apartment. They attack him, but Quan fights them off and escapes.
Quan continues to keep an eye on Hennessy, going as far as to take pictures of him while he is out with Maggie. Quan also blows up a stable outside Hennessy's country home while hiding out in the woods.
Sean visits Mary while she is alone. They engage in an affair until Sean receives a call from Bromley (whom he used to work for) over making a deal with Hennessy over finding the bombers' identities. Mary then tells Sean how she resents her husband as six years earlier, her brother was killed, and Hennessy never tried to bring justice to his killers.
Quan sets off another bomb outside the country house, and Hennessy's goons go after him in the woods. They fall to Quan's traps, but one of the men shoots Quan in the shoulder, forcing him to retreat. Quan later cauterizes the wound with a hot knife. As he passes out from the pain, he experiences a flashback to when the pirates attacked his family. He fought them off as well as he could.
We see Maggie out in town as she hops on the motorcycle of the same man that bombed Knightsbridge. A double decker bus explodes over a bridge, killing 16 civilians.
Hennessy meets with his enforcer, Hugh McGrath (Dermot Crowley) following the bus bombing. He angrily demands to know who is doing these terrorist acts against civilians. Hennessy shoots McGrath in the leg until he talks. McGrath tells Hennessy that Mary was behind the bombings, acting with the IRA and Maggie (real name Sara McKay) in retribution for what happened to her brother. Hennessy then shoots McGrath dead.
Sean tries to go after Quan himself, but Quan catches him in the woods. They fight but both settle their conflict. Sean tells Quan that the bomber that killed Fan is Patrick O'Reilly (Niall McNamee), who was identified by a burn mark on his hand. O'Reilly is apparently responsible for a bomb that killed Sean's family as well.
Maggie has just had sex with Ian after meeting him at a pub. While he's in the bathroom, Maggie plants a bomb in his laptop.
Quan finds the bombers' hideout and infiltrates it by posing as an electrician. He quickly begins to beat down the bombers despite them having guns. He manages to shoot dead two of the bombers while Maggie gets wounded by a bullet in the shoulder. He then fights O'Reilly and strangles him with a curtain.
Police arrive long after Quan has left. They interrogate Maggie over the whereabouts of the third bomb in Ian's laptop. Maggie taunts the police, but they smack her and press the bullet wound. They torture her with electricity to learn what they want to know. After learning that Ian is an unwitting mule set to board a flight at the airport, police go running to find Ian. They take his laptop and rush to find an isolated area to throw the bomb, where it explodes. The investigation is resolved to have no loose ends, so the police shoot Maggie dead. Meanwhile, Hennessy has Sean execute Mary as payback.
Hennessy receives a phone call from Davies, who was going to be on the flight Maggie was going to have bombed. Davies thanks Liam for helping save her life, but then reveals that Bromley has positively identified that he was involved with Maggie, and Davies vows to keep her eye on Liam.
Quan visits Hennessy for the last time. He shows him the picture he took of him with Maggie at the restaurant, which would implicate him in the terrorist attacks. Quan forces Hennessy to hit "send" so that the picture will go viral, and the world will know of his guilt.
Quan returns to his restaurant. An agent maintains surveillance on him under Bromley's orders. After asking if he should move in on Quan, Bromley says no and to let him be. Quan is then found by Lam, who hugs and kisses him.