Gerry Conlon is a petty criminal. As the film opens he is busy stealing the lead from the roofs of houses in the middle of Belfast. His endeavors are interrupted by the arrival of security forces who have got the area surrounded to prevent his fleeing the scene. A riot breaks out and this puts Gerry offside with the high up members of the IRA who don't like anyone connected to them opening themselves up to capture and investigation. Luckily for Gerry, his father Giuseppe is well ingratiated with the IRA and manages to save him from likely punishment. He packs Gerry off to London where he has an aunt he can stay with, but Gerry wants no part of this plan and instead finds a squat in an abandoned house where it will be far easier to enjoy the free ice of the early 1970s and to smoke as much dope as possible.
On the evening that changes his life, two things happen. Gerry procures a prostitute and manipulates an invitation back to her flat, stealing the £700 he finds there and then leaving. He stops briefly to chat with a man who is sitting in the park. The second hung that happens is an explosion caused by a bombing in a pub in the upmarket city of Guildford, about an hour south of London. Four off-duty soldiers are killed by the explosion, and one civilian member of the public is murdered too. Sixty five people are seriously wounded and the psyche of the city is irreparably damaged.
With the stolen cash in his pocket, Gerry goes home to Belfast to show off his riches. In his absence one of the other people living in the London squat reports him to authorities as a likely pub bomber; the Conlon home in Belfast is raided by the Royal Ulster Constabulary and the British Army. Gerry is arrested and flown back to England along with his friend Paul Hill. They are interrogated for days under the Prevention of Terrorism Act. The police bully and threaten and eventually both men break and confess. Hearing about the confession, Giuseppe Conlon travels to London to try to help his son and is arrested at his sister's house. She is also arrested and both are tried and subsequently convicted of supporting the bombing, traces of nitroglycerin that were found in the home used as evidence. They are known in the press as the Maguire Seven. Gerry, Paul Hill and the other two members of the Guildford Four are also convicted of the bombing and given long sentences.
Bitter and angry, blaming his father for his situation, Gerry has an epiphany one day when he learns that the man who really masterminded the bombing is also imprisoned in the same jail. The man starts a prison riot and sets a guard on fire. Gerry saves the guard's life by covering him with a blanket and putting out the flames.
After his father dies in jail, Gerry becomes the leader of the fight for justice. He gains publicity and support and the Free The Four movement gains enough momentum to grab the attention of a lawyer called Gareth Peirce. She is campaigning to clear the Maguire Seven but when she gains access to Gerry's file she discovers a document that is marked "Not to be shown to the Defence". She triumphantly produces the annotated file during the appeal and states that as the police have lied and withheld evidence the convictions should be overturned. The convictions are quashed and the Guildford Four are released. In talking to the media after his release, Gerry proclaims his father's innocence as well as his own - perhaps even more than his own. The Maguire Seven were subsequently exonerated but the Guildford Four were not; the police were acquitted of any wrongdoing during the investigation.
