16 year olds Roy and Alfie mug an old lady in the post-war ruins of London. Alfie, who is not bright, is afraid of the nastier and cleverer Roy, who bullies him into doing the actual coshing and then takes most of the money afterwards. Moments later, the pair are caught by police.

Thanks to a lack of evidence, the pair escape with a year's probation, provided they attend the local youth centre. Roy feigns politeness; he is employed at the local garage and is not yet known to the police. His mother Elsie thinks he can do no wrong. Both boys' mothers are present but the fathers are absent, perhaps killed or disappeared during the war. Roy, Walshy to his gang friends, quickly realises that attendance of the gang at the youth centre will be the perfect alibi for the gang's future activities. The gang lies low for a while the men from the ministry observe.

At the youth centre dance, Roy meets Alfie's sister, the stunning Rene. Rene spent the war in the country and has only recently returned. She has a boyfriend, Brian, and rejects Roy's advances, to the amusement of the gang. Furious, Roy decides that the gang will cosh someone that night. The gang bicycle into the night and cosh Queenie, supposedly the 'loose woman' of the neighbourhood. Again, Roy organises but doesn't do the dirty work. The gang scatter; the police chase Roy and Alfie through dog kennels beneath a dog track, where Roy throws Queenie's handbag into a dog pound. The gang meet later and the money is split, highly in Roy's favour.

Roy tells Alfie to arrange a date for him with Rene. But on the appointed day, Rene goes out on the river with Brian instead. That night the gang beat up Brian, hospitalising him. Roy further menaces Rene, who finally submits, fearfully. She becomes besotted with Roy who, after the act, wants nothing to do with her.

Pete, a gang member, tells Roy of his idea to steal the box office takings of the Palidrome dance hall, where he works. Canadian Bob Stevens, the Palidrome's assistant manager, is having a relationship with Elsie, of which Roy is extremely jealous. When Roy steals his grandmother's life savings from under her mattress, the protective Bob waits up for Roy, accusing him, leading to a fight. Bob and Elsie finally marry, but Elsie is scared of how Roy will react.

At the youth centre, Roy plans the robbery with the gang. Rene arrives to tell him she is pregnant. He rejects her and she leaves, tearfully. Roy has bought a gun. He bullies Alfie into being the one to use it to threaten the box office staff during the robbery.

Later, as the gang prepare for the robbery, Rene's friend Marlene Smith arrives, telling Alfie that Rene has thrown herself in the river and is dead. Alfie has to leave. Roy retrieves the gun from himfirst, deciding to do that part of the robbery himself. Rene is in fact alive in hospital; the shock of the river has caused her to abort.

Wrestling night at the Palidrome: Roy is in the audience. He exits while the match is on, meeting the rest of the gang in the lobby. Bob is taking the box office takings away. The gang and Roy confront him. There is a struggle and an unforeseen member of staff appears. Roy threatens the man with the gun and accidentally fires, hitting him. The gang, including Roy, flee.

Roy goes to pieces. On seeing this Pete says he thinks Roy is 'yellow'. Roy flees home and begs Elsie to help him. The neighbourhood rise and come to get him, barricading him in the house. Bob, who is now Roy's legal father, arrives and disarms Roy, who threatens him with a razor. The police arrive but deliberately leave Roy to be thrashed by Bob for 10 minutes while they go to pick up Pete.