H.G. Wells' the Invisible Man

A plane crash reveals that a canister of uranium 235, one-half of an atom bomb, was being smuggled into England. Peter Brady has a theory that this is part of a plan to launch another world war – that atom bombs are being placed under the capitol cities of the world during the peace years. Such bombs could be detonated by a radio signal, and in this way, London, Paris, New York, Moscow and other large cities could be blown up at the outbreak of war before a shot was fired. Peter believes the other uranium half is already in London somewhere, just waiting to be combined and detonated by a foreign power, and embarks on a search to find the source of the parts.