Cockney cat burglar and jewel thief Jethro is as smart as he is streetwise, but even he has to be a bit nimble to prosper in the austerity-ridden world of postwar England. Having barely survived the war, Jethro tells everyone he's gone straight and takes up as a stagehand around London's many theatres. But the truth is that hiding in plain sight around London's West End is the perfect cover for his diamond capers in wealthy Mayfair and Belgravia. Jethro steals into the embassy of an Iron Curtain power to pinch some fabled Czarist jewelry, but his daring caper brings him to the attention of His Majesty's Secret Service and they blackmail him into going back to retrieve a secret code-book and help a beautiful cipher clerk defect. It's all a setup for a thief to catch a thief that pits Jethro against London's top villains, as well as one of the Soviet's topmost secret agents and assassins.