Postwar East London. Young Joe Corby and his friends find an issue of The Trump. Jokingly, Joe reads the comic aloud, until he finds a page missing. Later, he buys a copy, and becomes engrossed in the adventures of fictional detective Selwyn Pike.
As he walks along reading one section, about a gang of men disposing of dead bodies in delivery crates, Joe is forced to stop for two men carrying a crate. He is amazed to discover that the number plate on their truck, GZ 4216, exactly matches that of the men in the comic.
Leaving the crates at a fur shop owned by Mr Jago, the men depart. Joe, convinced the crates contain corpses, enlists a friend to distract Jago while he searches the crates. But Jago catches Joe, and calls the police Inspector Ford. Jago agrees not to press charges, but Inspector Ford warns Joe not to let his imagination run wild; his story is impossible, since there are no number plates beginning GZ. He suggests Joe find himself a job, and sends him to meet Mr Nightingale, a Covent Garden grocer, who is amused by Joe's stories.
Later, in a bombed out building used as a hideout, Joe meets friends. They tease him about the incident, until one of the younger boys, Arthur, says he saw a truck with the number GZ 4216 that morning. Joe muses that a criminal gang are using The Trump to plan their crimes, and the group ponder who might be the ringleader. They decide to pay a visit to Selwyn Pike's creator, Felix H. Wilkinson.
Joe and Alec track down the eccentric Wilkinson to his flat. They are suspicious at first, but when they all realise someone has been tampering with Wilkinson's stories, they tell him about the plot. Wilkinson realises the gang are using his own code to communicate their plans, but refuses to help the boys, afraid the gang might take revenge.
When Joe's attempts to convince Nightingale and Inspector Ford fall on deaf ears, he heads for the offices of The Trump, where he meets Norman, a young employee. The pair return to the hideout, where they tell the others they've cracked the code from a story in the next issue - 'Tattoo Jack' is planning to rob Rich's department store in Oxford Circus.
The kids lay in wait at Rich's. But after overpowering the 'thieves', they discover the men are really police - tipped off anonymously by Tattoo Joe. The kids escape down a manhole. They wander miserably in the sewers, until finally they find their way out.
Later, the kids squabble amongst themselves, until Norman arrives with new evidence, pointing to another Trump employee, Miss Davis. They follow Miss Davis to her home in Hampstead, where they bind her while Joe calls Nightingale, who tells him to wait at the phone box, promising to call Inspector Ford. While Joe is away, Nightingale rescues Miss Davis. One boy, Ricky, unnoticed, hitches a ride on the villain's car, and overhears Nightingale's plans - the stolen goods are being moved to Ballard's Wharf.
Joe persuades Wilkinson to write a new Trump story, luring all the crooks to Ballards Wharf. Next day, Joe tells Nightingale the whole plan. When he leaves, Joe notices the car number plate and realises Nightingale is the gang leader.
When the new Trump is printed, Nightingale and Miss Davis laugh at Joe's attempt to outwit them, until Nightingale realises Joe has second-guessed him - the crooks are being sent not to Ballard's Wharf, but to Nightingale's own warehouse.
At the warehouse, Joe finds the stolen furs, but is disturbed by Nightingale. But when the crooks arrive Nightingale, who didn't finish the Trump story, doesn't know the secret password, and they knock him out.
The crooks head for the ruined Ballard's Wharf, where the rest of Joe's gang are soon joined by hundreds of boys, who outnumber the villains and subdue them. In the confusion, Nightingale escapes in a van. Joe jumps on board, and as he struggles with Nightingale, the van crashes. Joe chases Nightingale to a bombed-out building. After a tussle, Nightingale falls from a ledge and is unconscious when the police arrive.
