In London, a gang of criminals from Australia led by Jack Coombes (Bill Kerr) impersonate policemen to carry out robberies. Local gang leader "Pearly" Gates (Sellers), who operates from the cover of a French couturier, finds his takings cut severely, and blames rival crook "Nervous" O'Toole (Bernard Cribbins). When it emerges that they are both being scammed by the same gang, they join forces, along with Lionel Jeffries' Police Inspector "Nosey" Parker, to bring the so-called IPO mob to justice.