Gerry Conlon is shown in Belfast stripping lead from roofs of houses when security forces home in on the district with armoured cars, and a riot breaks out. Gerry’s father, Giuseppe Conlon, later saves him from IRA punishment, and he is sent off to London to stay with his aunt, Anne Maguire, for his own good. Instead, he finds a squat, to explore, as he puts it, “free love and dope”. In October 1974, Gerry happens to gain entry to a prostitute’s flat and he steals the £700 he finds there and chats briefly with a man sitting in a park. On that evening in Guildford, southwest of London, there is an explosion at a pub that kills four off-duty soldiers and a civilian, and wounds sixty-five others.
While Gerry has returned to Belfast to show off his stolen money, one of the squat residents talks to the authorities and the Conlon home is raided by the British Army and Royal Ulster Constabulary, who arrest Gerry as part of the Guildford Four and immediately place him on a military flight to the UK mainland. Gerry and his friend, Paul Hill, are interrogated by police who torture and threaten them until both finally agree to sign a confession after being held for up to seven days under the Prevention of Terrorism Act. Giuseppe, who goes to England to help his son, is staying with his Maguire family relatives in London when they are all arrested in December 1974. In the subsequent trial, the family (known as the Maguire Seven, including Conlon’s father) are convicted of supporting the Guildford bombing on the basis of nitroglycerin traces, receiving long terms of imprisonment. The confessed Guildford Four, including Gerry, are sentenced to life imprisonment.
Gerry’s time in prison shows a progression from a bitter son who rails at his father until an awakening when he is approached by new inmate Joe McAndrew, the real perpetrator of the bombing. McAndrew tells the Conlons that he had confessed the truth to the police, who then withheld the information to avoid official embarrassment. When McAndrew leads a prison protest and then sets a hated prison guard on fire, Gerry is the one who saves the guard with a blanket. Gerry takes over the fight for justice himself when his father dies in custody. His case becomes public, gaining support from Dublin, Belfast and London. A common slogan used by his supporters is “Free The Four”.
Gareth Peirce, a campaigning lawyer who has been investigating the case on behalf of Giuseppe, has a breakthrough when she tries to access Giuseppe’s file and is able to look instead at Gerry’s. She finds vital police documents in the file that are marked “Not to be shown to the Defence”. During the course of their appeal, the production of these documents leads to a triumphant scene in court when Peirce produces the evidence that the police have been lying throughout about the existence of a witness who had provided Conlon with an alibi during their initial investigation. This leads to the overturning of the verdict and immediate release of the Guildford Four.
The film ends with a triumphant Gerry revealing his story to the media and proclaiming his father’s innocence. Title cards reveal the current activities of the Four, the exoneration of the Maguire Seven, that the police were acquitted of any wrongdoing, and that the real perpetrators of the Guildford Bombing have not been charged with the crime.