In London, a gang of three Australian criminals impersonates police officers to pull off robberies by seizing cash and jewels from crooks who had already moved the loot. The local gang leader Pearly Gates (Peter Sellers) operates under the front of an elegant French couturier named Charles Jules in a London fashion house, Maison Jules, but his criminal takings are starting to thin out. Pearly doesn’t realize that his girlfriend and model Valerie (Nanette Newman) is secretly in league with the Australians, feeding them details of the crimes being arranged by Pearly and his associates.
London’s criminal underworld has recently set up a closed-shop syndicate to carve up territories and protect each other’s interests. Initially, Pearly accuses rival nervy O'Toole of breaking their agreement, but it soon turns out they are both being targeted by the same interloping gang. Realizing they share a common foe, they join forces, along with the police, with Inspector Fred "Nosy" Parker (Lionel Jeffries) serving as their liaison to flush out the so-called I.P.O. Mob (Impersonating a Police Officer) from the shadows.
After the underworld calls a 24-hour no-crime truce to allow a full search, the IPO Mob slips through Parker’s fingers. Parker then proposes a bold ruse: stage a large crime and present it to Scotland Yard’s assistant commissioner (John Le Mesurier) to trap the IPO Mob once and for all. To safeguard the cash and bullion, the assistant commissioner insists that Parker join Pearly’s gang, weaving him deeper into the scheme. Yet, once the bullion robbery goes down, Pearly proves reluctant to relinquish the proceeds, signaling a clash of loyalties that tests everyone involved.
The plan hinges on tight coordination between the police and the criminal insiders, a precarious balance that threatens to topple at the slightest misstep. As the trap tightens, alliances fray: Valerie’s double-dealing, Pearly’s stubborn grip on the take, and Parker’s stubborn resolve to deliver justice all collide in a tense cat-and-mouse game through London’s night-lit streets. The story unfolds with wry wit and brisk plotting, revealing how easily trust can be weaponized in a city where everyone wears a mask and every ally could be a potential informant.
Ultimately, the investigation exposes a web of deception that blurs the line between law enforcers and those they pursue, leaving characters to navigate a murky moral landscape. In this caper, the thrill comes not just from the heists themselves but from the shifting loyalties, the clever schemes, and the quiet desperation of those who chase a payday while trying to stay one step ahead of both the criminals and the system they rely on.
