Jean Raymond appears in court and is sentenced to serve 12 months in prison. She is taken down to the cells and allowed a brief visit from her boyfriend, Michael, before transportation to Blackdown women's prison. She is later joined in the cell by Betty Brown, a glamorous blonde who has got into trouble covering up for her boyfriend Norman's crime. Jean confides in Betty what got her a jail sentence: a gambling habit which led her to bet more than she could afford at the casino, incurring the proprietor's wrath. In revenge, he framed her in an insurance scam involving a cigarette case, leading to her arrest on a fraud charge.

Arriving at Blackdown, Jean and Betty are instructed to undress, bathe and exchange their own clothes for prison uniform. After medical examination, they are taken to their cells. As the cell door slams behind her, Jean feels the full impact of her wretched situation.

The next morning, while waiting to meet the prison governor, Jean and Betty meet Nellie and Winnie Baden, a mother and daughter shoplifting duo who tell the women about their family's criminal career, including incidents such as Nellie's husband stealing an entire dress suit, and Nellie getting caught when a stolen radio started blasting out football commentary from under her coat.

Jean becomes accustomed to the grim prison routine, and is comforted by the prison chaplain, who assures her that Michael will still want to marry her when she's released. But this routine is disturbed when she is stabbed in the arm with a pair of scissors while trying to intervene in a sewing room altercation between a French prisoner and a brutal warder. Jean recuperates in the prison hospital, where she meets some expectant mothers and witnesses the cruel spectacle of a nine-month old baby taken from its mother to be adopted, in line with prison regulations. Jean also meets Babs, who tells Jean about her crime. Persuaded by her boyfriend to leave her two children at home for the night while she went to the dance hall, she drank too much and stayed out for longer than she had intended. When she returned home the next morning, she found the youngest child had died in the night. Jean reassures a distraught Babs that if she's repentant the authorities will let her have custody of her older daughter upon release.

Jean recovers from her injuries, leaves the hospital, and is selected for transfer to The Grange, an experimental 'prison without bars'. Betty is also transferred. Just before she leaves Blackdown, Jean receives a visit from Michael, who tells her he's accepted a job in Africa.

Life at The Grange is very different from life at Blackdown, centred on hard work rather than confinement in cells, with classes to provide practical training for life outside. Jean and Betty meet an elderly inmate, Millie, and listen to her story. Her life-long friendship with Mabel ceased when Mabel got married to Harry. Upon marriage, Harry revealed his previously hidden irascible side, claiming ill health, taking to his bed and treating Mabel like a slave. Seeing her friend worn out, Millie suggested poisoning Harry by putting weedkiller in his tea, after which the two of them could live happily on Mabel's inheritance. But before they could go ahead with this plan, and unbeknownst to Millie, Harry died of natural causes, and Mabel went off to London without Millie. Peeved, Millie wrote a letter threatening to reveal their plot; Mabel responded by reporting Millie to the police, safe in the knowledge that Millie's accusations couldn't be proved. Millie was given three years for blackmail.

As Jean gets nearer to her release date, the governor suggests she and Betty spend a day in town to re-acclimatise to life outside prison, but only if Jean is sure that Betty won't abscond. Betty is tempted to run away to make contact with her estranged boyfriend Norman, but in the end she remains loyal to Jean and decides to come back to The Grange. Jean finally reaches the end of her sentence and says a tearful goodbye to Betty. Leaving The Grange, she is surprised to find Michael waiting outside for her.