After a robbery, a gang of criminals is followed to Maison Jules, a dressmakers run by Pearly Gates, by what appear to be three policemen. The policemen apparently arrest the thieves, but then disappear with the loot and do not return. Pearly, the leader of the gang, has been assisting a rich client in the shop under his alias of Monsieur Jules, but when he is told how police officers took the gang's money and ran, he realises they have been conned by an IPO (Impersonating Police Officers) mob.

Unbeknown to Pearly, the IPOs are Australian thieves, operating with the help of Pearly's girlfriend Valerie, who supplies inside information. After the IPOs have disrupted six of Pearly's enterprises in one week, he questions his gang, suspecting there might be an informer.

Valerie seduces Pearly to gain more information. Pearly tells her he is co-operating with Nervous O'Toole, another gang leader, so they can divide up jobs between them. When Nervous's men lose £50,000 worth of diamonds to the IPOs, Nervous suspects Pearly. Once it becomes clear that neither of the gang leaders are responsible for the impersonations, they decide to call an extraordinary general meeting of the crime syndicate.

Two of Pearly's men, Trainer King and Sid Cooper, put up a struggle when arrested robbing a jewellery store. After escorting them to the car, Inspector Parker returns for evidence, but finds it stolen. When King and Cooper explain about the IPOs, Parker realises they might be telling the truth, as he saw a police car driving away from the crime scene.

The crime syndicate meeting approves co-operation with the police in order to catch the IPOs. Pearly is elected as representative. With the help of Inspector Parker, he arranges a meeting at a funfair with Scotland Yard. Pearly offers a truce, guaranteeing no crime for twenty-four hours to enable the police to find the IPOs. The police locate the Australian mob, but are conned again, allowing them to escape in Parker's car.

A new meeting is arranged with Parker and the Assistant Police Commissioner in an aeroplane. Parker suggests a dummy job, with police co-operation, to lure the IPOs into being caught. Pearly's gang will plan to steal half a million pounds from a security van. Pearly agrees, but insists the job must seem genuine, with real money used. The Assistant Commissioner assigns Parker to work for Pearly as liaison officer. Pearly is also granted the help of an expert German criminal, Siggy Schmoltz. Valerie informs the IPOs of the job, believing it to be genuine.

Schmoltz arrives and the undercover operation begins, but Pearly, Parker and Schmoltz stop the wrong van. The furious driver later reports their activities to a police car. As the real van arrives the gang's vehicle breaks down, forcing them to ask the van's driver to crash into them. They finally escape with the loot, but are followed by a genuine police car, which they assume to be the ICOs. As they arrive at the rendezvous with members of Scotland Yard, chaos ensues. Detectives try to arrest the legitimate police officers who have followed the thieves. During the melee Pearly drives away with the money, still accompanied by Parker.

In the car, Parker handcuffs himself to the case of money, but Pearly successfully bribes him by saying they will share the loot. They are followed by the IPOs, but believe them to be real police. When Pearly and Parker arrive at the airfield, the IPOs, having taken a shortcut, are already waiting for them. Parker recognises the mob and they fight over the money. The actual police arrive, followed by Nervous's gang. In the resulting chaos Pearly and Parker escape in a plane with the money. When the plane is airborne, Valerie appears from behind the cockpit.

At Scotland Yard the Assistant Commissioner is pleased with the operation. They have captured the IPOs as well as some of Nervous's gang, and it is revealed the money Pearly and Parker escaped with was fake.

Pearly and his two companions are last seen setting up a new dressmaking business on a South Sea Island.