Night. A burning children's boxcar runs down a hill. Two policemen extinguish the flames before apprehending a group of children playing in a warehouse up the hill. Teenager 'Spider' is among them. One of the children is found to have a packet of cigarettes. He says he was given it by a man called Stanley. Spider identifies him as Stanley Wood, a man known to the police. The child says they had all been around Stanley's house earlier. The senior policeman takes him aside to find out exactly what they were doing with Stanley.

Meanwhile, the Mitchell family struggle to get their excitable young children to bed. Shelley Mitchell tells her husband John that she is worried about Stanley, who has not phoned as he does every Saturday. John knows nothing about Stanley. Shelley recalls how she came to be involved with him.

The prison governor asks if Shelley, a Voluntary Associate, can take on another prisoner. He points out an old man in the exercise yard, segregated from the others for his own protection. He is there for indecently assaulting young boys. He has been in and out of prison since the age of 30.

The police call at Stanley's house and arrest him.

Shelley recalls meeting Stanley in prison. He is a quiet, pathetic man. She explains what a Voluntary Associate is, that she is to help him get along outside of prison. She arranges a day's parole and takes Stanley to the park and then her home. Shelley's children arrive home and Stanley shows them a card trick. When one of the children appears in his underwear, Shelley is suddenly struck by how provocative the situation could be to Stanley. When returning to the prison, Stanley says he would never touch her children. He is ashamed of his offences and asks if he could be helped with treatment. Shelley thinks he could.

Shelley calls at Stanley's house, but finds only Spider, who has been waiting for Stanley. Returning later, Shelley finds the police there. They tell her that Stanley has been arrested and will be in court the next day.

Shelley misses the hearing, at which Stanley was remanded in custody, but meets the probation officer. He wishes Stanley would get treatment, to help him break the cycle of offence and imprisonment. Shelley explains that he did try. She recalls Stanley telling her about his parents, and that the prison governor had told him that it was too late for treatment.

Shelley confronts the governor. He explains that he meant treatment couldn't be arranged within the prison. He suggests she arrange treatment for him now that he is on release. They attempt this, but the fifteen minutes' psychotherapy per month, with different doctors each time, proves inadequate.

Shelley explains this to the probation officer, and that meanwhile boys were turning up at Stanley's house, tempting him.

Shelley visits Stanley's legal aid solicitor, offering to speak in his defence. He is unenthusiastic. She visits the police officer in charge of the investigation but, when he realises that she is a friend of Stanley, he becomes hostile.

Shelley recalls Stanley telling her of his first offence, when two boys asked to be taken for a ride in his small boat. He takes Shelley to his house and explains how his mother died of cancer, and how afterwards kids wanting sixpence would go to him.

Shelley visits Stanley in prison and tells him that she wants to fight his case, rather than just accept another custodial sentence.

Shelley's mother is disgusted by Shelley's involvement with Stanley. Shelley confronts her about her ignorance and bigotry and insists on explaining exactly what indecent assault means: merely fondling of small boys.

In the schoolyard, the boys talk about Stanley being sent up to assizes for sentencing, and it is revealed that Spider offered to set him up with young boys for money.

John wants to buy a new car with the extra money he will receive from a promotion. Shelley asks if they can instead use some of the money to get Stanley good legal representation. John refuses.

Stanley sits silently in his police cell. The children of the local school sing 'Jerusalem'.