In Belfast, Gerry Conlon is mistaken as an IRA sniper by British security forces and pursued until a riot breaks out. Gerry is sent to London by his father Giuseppe to dissuade an IRA reprisal against him.
One evening, Gerry burgles a prostitute's flat and steals £700. While he is taking drugs in a park with his friend Paul Hill, alongside homeless Irishman Charlie Burke, an explosion in Guildford occurs, killing four off-duty soldiers plus a civilian as well as injuring many others. Returning to Belfast sometime later, Gerry is captured by the British Army and Royal Ulster Constabulary and arrested on terrorism charges.
Gerry is flown to England, where he and his friend Paul together with two others are dubbed the Guildford Four and are subjected to police torture as part of their interrogation.
Gerry maintains his innocence, but signs a confession after the police threaten to kill his father, who is later arrested along with other members of the Conlon family, later dubbed the Maguire Seven. At his trial, although Gerry's defence points out numerous inconsistencies in the police investigation, he, along with the rest of the Guildford Four, is sentenced to life imprisonment.
During their time in prison Gerry and Giuseppe are approached by new inmate Joe McAndrew, who informs them that he was the real perpetrator of the bombing and had confessed this to the police. The police, in order to save face, withheld this new information.
Though Gerry warms to Joe, his opinion changes when Joe sets a hated prison guard on fire during a riot. Giuseppe later dies in custody, leaving Gerry to take over his father's campaign for justice.
Giuseppe's lawyer Gareth Peirce, who had been investigating the case on Giuseppe's behalf, discovers vital evidence related to Gerry's original alibi with a note attached that reads: "Not to be shown to the defence." Through a statement made by Charlie Burke, at a court appeal, it totally exonerates Gerry and the rest.
The film ends with the current activities of the wrongly accused being given, and also by stating that the police who investigated the case were never prosecuted for any wrongdoing. The real perpetrators of the Guildford Bombing have not been charged with the crime.
