On 2 November 1952, two youths are surrounded by police on the roof of a Croydon warehouse after their attempted break-in. Unarmed 19-year-old Derek Bentley is arrested by Sergeant Fairfax, but 16-year-old Christopher Craig shoots at the police. Although Fairfax has been wounded by Craig, Bentley makes no attempt to escape. As more policemen climb the stairs, Craig and police marksmen all take aim, and in the confusion PC Sidney Miles is shot dead. Bentley shouts angrily at Craig, who taunts the police. After running out of bullets, Craig jumps off the roof, breaking his back, wrists and ribs.

The police burst into the Bentley home and take a knife, which Bentley's mother thinks they will pretend to have found on Derek. Bentley's defence counsel, Frank Cassels, tells Craig's defence counsel, John Parris, that both youths ought to hang. After being appointed to the case very late, Parris requests an adjournment for preparation, but has to explain his defence to the judge, Lord Chief Justice Goddard, and to Christmas Humphries, the prosecution counsel.

At the Old Bailey, Craig and Bentley are both accused of murder. The Bentley family wait outside with Derek's coat. Tests on the coat disprove Fairfax's claim that he wrestled Bentley to the ground, but this evidence will never be produced in court.

In court, Fairfax claims that he fought Bentley, found a knife on him, and that after Bentley called out "let him have it, Chris", Craig shot him in the shoulder from six feet away. Parris notes contradictions from Fairfax's previous statement, and that his wound and the bullet-hole in his jacket are consistent with Craig's story that the bullet ricocheted after he fired into the ground to scare Fairfax, 37 feet away. Fairfax made no notes, dictating a statement after other policemen visited him in hospital.

Goddard dismisses witness Dr Jazwon when Parris questions him on Fairfax's injury. When an expert testifies to the wild inaccuracy of Craig's gun, particularly when used with his smaller calibre bullets, Goddard vows to tell the jury to dismiss Parris's defence of accident because circumstances change when applied to what he describes as the murder of a policeman. This leading phrase is just one of Goddard's 250 interruptions.

Craig denies that Bentley incited him, and did not hear the words "let him have it". Goddard dwells on the unpleasantness of Craig's knuckleduster. Craig denies taunting officers in hospital because he was unconscious, which Goddard describes as nonsense. Bentley denies police testimony that he said "let him have it", or "he's got a Colt .45", because he knew nothing about guns.

Goddard interrupts Parris's final statement to dismiss the defence of accidental shooting, over a disagreement on legal precedent. Cassels wonders whether "let him have it" could mean 'hand over the gun' rather than 'shoot him'. In his summing-up, Goddard advises against a manslaughter verdict. When a juror asks to see Fairfax's jacket, Goddard refuses, saying that the murder of Miles is their concern.

Documentary voice-over outlines information denied to the jury, points out contradictions in police statements, queries the absence of testimony from other officers at the scene and queries Craig and Bentley's supposed confessions, which conveniently satisfy the essential criteria for a conviction. Furthermore, a medical report which documented Bentley's unfitness to plead - he was an epileptic with a mental age of ten - was never presented.

Craig and Bentley are found guilty, but only Bentley is old enough to receive the death penalty. His family campaign for a pardon, supported by public demonstrations, although they also receive hate mail, including a noose sent for Derek. However, an appeal is dismissed and, after complaints from MPs, the Home Secretary insists he cannot intervene. The Bentleys are informed by a letter, temporarily lost in their mass of mail. In his death cell, the illiterate Derek dictates a final letter.

When the noose is placed around Bentley's neck, he insists he never told Craig to shoot Miles. The trapdoor opens and he is hanged.