An attempted robbery collapses when Jackie Knowles fails to start the getaway car. He runs home and confesses to his girlfriend Kitty, who reminds him that he's already on probation. She agrees to cover for him when the police arrive, but he is captured trying to sneak out round the back.
Jackie is sentenced to three years in borstal. The governor addresses the new arrivals, telling them that his job is to mould them into decent citizens. They are kitted out in standard borstal uniform, and complain about knee-length brown shorts. Jackie meets Alfie Rawlins, who tells him about the relentless routines and loneliness. Jackie rebuffs his attempts at friendship.
Fellow inmate Casey is brought up before the governor, and complains that the work he does is poorly-paid and pointless. Meanwhile, Jackie has struck up a friendship with Bill Foster, and they talk about girls and their life outside.
Bill is about to be released, and the governor talks to him about his family life, recommending that he live at a YMCA because of his adopted mother's alcoholism. He asks Bill if he'd like his real mother tracked down, but Bill decides against it. Jackie asks Bill to call in on his mother and Kitty to see how they are.
'Sparrow' Thompson plans an escape bid, but they need someone's mother to provide a cover story. He and his collaborators think Mrs Knowles might be ideal, but Jackie will have nothing to do with it. Alfie says that he'll wear him down.
Bill goes to see Mrs Knowles, and meets Kitty when she drops round. Despite initial awkwardness, Bill and Kitty hit it off and go out together. He tells her about borstal life and she tells him about her relationship with Jackie.
The governor hears that a despairing Bill has lost his factory job, and goes to see Bill's real mother. Mrs Smith refuses to have anything to do with him, as her husband doesn't know that she had an illegitimate child. Meanwhile, Bill admits to Kitty that he's fallen in love with her.
Six of the boys (Alfie, Sparrow, Casey, Walker, Barker and Bates) plan the breakout. Jackie reaffirms that he wants no part of it. A scuffle breaks out, interrupted by Bill's return: he has been readmitted to borstal, and tells them about his failures outside, fired under pressure from colleagues after they discovered his background. An escape plan is devised that requires the theft of a suit from one of the officers. They draw lots, and Alfie fiddles the draw so that Jackie is singled out.
Jackie tells Alfie in private that he still doesn't want to participate. Alfie shows him a signed picture of Kitty that he found concealed in Bill's things. In despair, Jackie changes his mind.
During a concert revue in which six would-be escapees play a scene from Julius Caesar, Jackie attempts to slip away, but encounters Kitty backstage, and is cold and distant with her. He finally reaches officer Knight's room, but is interrupted both by a telephone and by Knight's arrival. Jackie hides behind the cupboard but is discovered, and hits Knight over the head with a table lamp before running off.
The boys climb over the wall. Sparrow dons Knight's suit and goes to the nearby village. While the others wait, Bill hands Jackie the picture of Kitty: it had been intended for him all along as a birthday present.
Back at the borstal, the alarm is sounded and the governor is told that Knight's skull has been fractured. The boys are recaptured and face a possible murder charge. The governor interrogates them, and is given conflicting confessions by Alfie and Jackie.
Knight makes a full recovery. Kitty visits the governor and asks to see Jackie, assuring him that he wasn't like that before his incarceration. The governor asks her to make sure he has something to look forward to and (bending the rules) allows her to see him. Jackie explains everything to her, and she says she'll wait for him.
The governor reflects on his job: not just to separate the wheat from the chaff, but also to work out what caused the chaff to get that way.
