London. In the underpass beneath the capital's Centre Point building, two teenage muggers, Sam and Danny, attack a couple of passers-by and steal their mobile phones. In Thamesmead, South London, schoolboy Dean goes to meet his mum after her stint on the night shift at a biscuit factory. As they walk home, Dean's mum tells him she's saving money so they can have a holiday. That night, she says how proud she is that Dean has begun to apply himself at school. She urges him to stay away from the criminal life pursued by many of the boys on their housing estate.

In an estate in Camden Town, North London, Danny's mum also warns her son away from drugs. Playing football in the park that afternoon, Danny meets a girl and they sleep together later that day. Back in Thamesmead, Danny gets a visit from his old friend Charlie Boy, who has recently been released from a young offenders' institution. Dean's mum warns Charlie Boy away from her family, but he lies in wait for Dean and confronts him later that day. Dean is soon won over by his old friend and they become reacquainted. Dean returns home to find his mum devastated that he has rekindled his friendship with Charlie Boy.

When Dean's mum is out, Charlie Boy turns up in a stolen BMW and invites Dean for a ride. A patrolling police car spots the boys with the missing vehicle and a chase ensues. Charlie Boy crashes the BMW and the pair flee, but Dean is caught. Meanwhile, Sam and Danny rob a shop; Sam pistol-whips the shopkeeper while Danny empties the till. They return home full of bravado but the police raid their homes and they are led away.

Danny, Sam and Dean are all sentenced to a term in the same young offenders' institution. Danny and Sam are given two years for their crimes; Dean receives a four-month custodial sentence. All three boys meet an earnest and concerned officer, Mike, whose 'Florence Nightingale' act is lampooned by his colleague Jim. On visiting day, Danny finds out that the girl he met in the park is now pregnant by him. He resolves to clean up his act inside. Meanwhile, Sam begins to torment Dean mercilessly. Sam also clashes with Li, the 'top dog' among the inmates. He and Li fight and are separated by officers, including Jim, who punches Sam repeatedly while supposedly restraining him.

Sam continues to taunt Dean, who occupies the cell next to his, and subjects him to a barrage of abuse throughout the night. One night, his relentless demands that Dean sing a nursery rhyme for him are echoed by other inmates around the wing. Verbal abuse soon turns to physical assault. Sam attacks Dean while the other boys play football outside. Seeing Dean's bruised face, Mike says he will take the boy responsible off the wing but Dean refuses to name names. Mike gives Sam a warning anyway, but the taunts continue. Speaking to her son on the phone, Dean's mum is worried about his wellbeing and contacts his care worker, who organises for Dean to be placed on suicide watch.

That night, Sam continues to taunt Dean through the wall dividing their cell and strikes up a chant demanding that Dean kill himself. The officer responsible for night watch is Jim, who looks into Dean's cell in the morning and finds he has hung himself. Dean's mum is called to the institution and picks up his belongings. Danny leaves the institution and begins a new life with his family. Dean's mum goes on holiday with her young daughter. Sam is also released from the institution but remains unreformed.