Postwar East London. Freddie Haywood, the adolescent son of a local policeman, has a teenage crush on twentysomething kiosk attendant Molly Wilson. Molly is flattered by Freddie's attention but doesn't take it seriously, her affections centred on her fiancé, Gerry Carter.

Molly's brother Mike is a mechanic at a local car dealership, the back of which doubles as a hideout for a criminal gang - led by Gerry and his friend Steve - who are planning a diamond robbery. The gang's getaway driver pulls out of the robbery, concerned at the gang's lack of experience. The situation deteriorates as they realise a further contact has failed to provide them with a vital disguise - a policeman's uniform. Mike impetuously suggests that he can get a uniform from Freddie, and Gerry persuades Molly to help. She asks Freddie to borrow the uniform, claiming it is for a practical joke. Cautious, but eager to please, Freddie agrees to steal a uniform from his father. On the eve of the robbery, Gerry takes Molly to a new housing development to see some modern flats being built to replace the bombed out East End. He uses the moment to propose marriage.

On the morning of the robbery, the gang's plans fall into place. Freddie takes his father's uniform and Gerry recruits newcomer Paddy as a replacement getaway driver. Steve and Gerry disguise themselves as an inspector and police officer. The plan works and the gang make off with £60,000 worth of diamonds. However, once back at the garage, their fence refuses to take the diamonds uncut and tells them they will have to wait until tomorrow before they can find a cutter. Agreeing that the diamonds shouldn't be left at the garage or their own places, the gang force Mike to take them.

At Mike's house, Gerry hides the diamonds inside Molly's record player. When she arrives home, she gives the player to Freddie as a thank you gift for the loan of the uniform. It is only after he leaves that Molly realises where the diamonds have been hidden. She visits him straightaway, but Freddie becomes suspicious and, seeing Molly hide an incriminating newspaper report, realises the police uniform has been used for the crime. Upset and hurt, he smashes the player in anger and discovers the diamonds.

The following day, Gerry announces they have a diamond cutter. Assuming Freddie doesn't know about the diamonds, he visits him under the pretence that Molly needs the player back. Freddie returns the smashed player and Gerry realises the diamonds are gone. He threatens the boy with blackmail, claiming his father will be arrested as an accessory to the robbery through the use of his uniform. A stalemate develops as Gerry, Paddy and Mike surround Freddie's house waiting for his mother to go out. Ashamed of the bullying tactics involved, Steve quits the gang.

Molly tries to convince Freddie that she never knew about the diamonds being hidden in the record player but he still refuses to help her. Instead, he writes an anonymous letter to the police and slips out of the house when the gang are distracted. He hides the diamonds in a his secret place located in a row of disused blitzed terraces but is spotted by Paddy who, together with Gerry, chases him through London before bringing him back to return the diamonds.

Freddie hands over the diamonds to Paddy and Gerry, but when Molly suddenly appears, he seizes the chance to snatch them back and escape. Enraged, Gerry chases the boy to a nearby building site, pursued by a distressed Molly. Despite being well hidden from a furious Gerry, Freddie reveals his hiding place when he alerts Molly to stop her falling down a lift shaft. He runs outside to climb down some scaffolding but struggles and drops the diamonds to Molly's feet. Forced to choose between her distressed fiancé and the vulnerable boy, she decides to help Freddie. The chase continues but Freddie manages to find his way to the police when they arrive. He is reunited with his father and the diamonds are returned.