London. PC George Dixon, on his beat, gives directions to Paddington station, before meeting new recruit PC Andy Mitchell. Later, in the canteen, Andy chats to CID officer Campbell, who reminds him not to acknowledge plainclothes officers on his beat, in case he blows their cover. Dixon and Mitchell are sent to investigate a domestic. Mrs Lewis reports her daugher, Diana, has gone missing.
Tom Riley and his cohort Spud are examples of a new breed of young criminal, whose recklessness sets them apart from the older, more disciplined, criminal fraternity. After attempting, unsuccessfully, to interest local crime boss Roberts in their robbery plans, Tom and Spud meet Tom's girlfriend, Diana.
George tries to persuade Ma Dixon to let Andy lodge with them, taking the room of their son, Bert, who was killed in the war. She is reluctant, but soon warms to Andy and relents.
That night, Andy is on his beat when he hears a scream and rushes to investigate. A jeweller and his mistress have been robbed by two youths, who took the keys to the jewellers. Tom and Spud rob the shop, knocking out PC Taff Hughes before making their escape. The only evidence is a belt which Taff pulled from Tom's coat.
Next day, Tom meets Diana outside the Coliseum cinema, where she has taken a job under a false name. Diana is spotted by Andy, who recognises her from her missing person's report. Despite her denials, he brings her in. Diana becomes hysterical, and is allowed to leave. As she goes, an officer spots her compact, which resembles one missing from the jewellers' job.
Tom and Spud have obtained guns and, with Diana's inside information, begin planning to hold up the Coliseum. That night, the pair make an appearance at the nearby music hall to establish an alibi, before heading for the Coliseum. But while they hold up the box office, they are disturbed by a couple of customers, who fetch the nearby Dixon. George tries to reason with Tom, but he panics and shoots George. Desperate, Tom and Spud make their getaway, but a passer-by gets their number-plate.
Ma and Andy visit George in hospital; his condition is serious, but he has a fighting chance. Meanwhile the investigation is underway: the car has been found. Andy comes upon a group of children playing with a pistol. Andy returns to the station with the young girl who found it, Queenie. He learns that George is dead; the investigation has become a murder case. Andy and the Chief Inspector break the bad news to Ma Dixon.
Queenie leads officers to where she found the gun. A hunt turns up Tom's coat, which matches the belt found earlier. They trace the coat to Diana's father, who tells them Diana lent it to a boyfriend, but doesn't know the name.
Tom, increasingly desperate, learns that police are looking for Diana. He decides to go to the police to try to establish his innocence. But he is caught unawares when questioned about the coat. Inspector Cherry becomes suspicious and organises an identity parade, but the witness, the cinema manageress, fails to recognise Tom. He is released, but tailed.
He heads home, to find Diana gone. He finds her at Spud's and, when she refuses to go with him, he tries to strangle her, but Inspector Campbell arrives and wrestles him. Campbell sends Andy for help, but Spud returns and he and Tom fight off Campbell, steal a car and get away. A chase ensues, until Tom and Spud crash, killing Spud. With Andy close behind, Tom runs away, losing his pursuer in a dog stadium.
Inspector Cherry gets the stadium management to lock the doors, trapping Tom inside while police reinforcements are called. Inside, officers run into Roberts. When he learns Riley is in the stadium, he initiates his own manhunt with his criminal colleagues.
Roberts learns of Tom's whereabouts and alerts the police, who corner Tom near the exits. Tom points his gun, but is trapped by the departing crowd, and the officers disarm him. Inspector Cherry congratulates Andy for his work. Later, on his beat, Andy gives directions to Paddington station.
