Young Portobello market-trader Danny Lee meets briefly with his girlfriend, Rosa, in a pub at lunchtime. After she leaves, a plainclothed policeman arrives to warn some known criminals away from the area. He rounds on Danny, who is amused by the situation, and the hot-headed youth strikes him, knocking him out.

Sentenced to six months for assaulting a police officer, Danny is taken to Pentonville prison, along with habitual criminals and one of the great train robbers.

Meanwhile, his three brothers, all of them professional thieves, bring their loot back to their mother, 'Britty' Lee, a life-long petty criminal. She is disappointed that Danny pleaded guilt in court. Rosa visits and tells Britty that she is three months' pregnant with Danny's baby. Britty is outraged and offers to perform an abortion, but Rosa is determined to keep the baby. She plans to marry Danny. Britty doesn't think Danny's market earnings would be adequate to permit him to marry and keep a wife and baby, but she suddenly becomes more welcoming when Rosa reveals that her father has promised to give her a caravan and five hundred pounds if she marries a working man.

In prison, Danny makes an application to the governor, requesting permission for Rosa to visit him. When this is declined, Danny becomes irate and causes a disturbance. This is witnessed by two prisoners, who recognise Danny's potential usefulness in a scheme they are planning, and convince a warder to assign him to their cell. Rosa goes to the prison but is not allowed to visit Danny, who is now held in the punishment cell.

On expiry of his punishment period, Danny is duly put in the cell with the two prisoners who had witnessed his earlier outburst. They are underworld boss Johnny May and a member of his gang, Robbo Robertson. They take advantage of Danny's naïveté, talking him into taking part in a scheme to secure their early release. In return for a promised two thousand pounds upon his release, Danny must attack a warder. May and Robertson will subdue him, thus impressing the governor into granting remission of the ends of their sentences. May arranges for his underworld contacts to pass money to Rosa to secure Danny's involvement.

The plan is followed through, but just as the governor duly grants May and Robertson remission, news arrives that the attacked prison warder has died of his injuries. Danny is tried for murder and confusedly pleads guilty to the killing, maintaining that while he attacked the prison officer he had no intention to kill him. Against his plea, his mother's attempts to influence a juror are useless. He is convicted and sentenced to death.

Back in prison, Danny is held in the condemned cells while he awaits execution in three weeks' time. After making his confession to the prison chaplain, he receives a letter. It is a summons due to an obstruction caused by his market barrow. While dictating a sarcastic response, Danny realises the gravity of his predicament and weeps.

Danny is visited by Rosa and his mother, who report that they are petitioning for his reprieve. After Britty reveals that she is looking after Rosa's money for her and has sold her son's story to a newspaper, Danny becomes distraught, blaming his money-grubbing mother for his woes.

The night before the execution, the hangmen practice their method to ensure a quick and clean kill. The prison doctor informs them that the Home Office has refused Danny a reprieve. The senior hangman reports to his apprentice that the doctor is only able to attend each execution once he has fortified himself with a large amount of alcohol.

The next morning, a crowd gathers outside the prison, among them Rosa, Danny's mother and brothers. Rosa desperately prays.

The prison chaplain visits Danny in the condemned cell. When Danny finishes his prayers, he is given a drink. Then, within seconds, he is restrained by the hangmen, taken to the gallows and hanged.