Tough, ruthless gangsters from Chicago descend on London and for two weeks their violent campaign of murder and intimidation holds the city in a crushing grip of fear. Scotland Yard has never seen such an onslaught. When a lull ensues, Captain Jiggs Allermain of the Chicago Detective Bureau suspects the rival gangs of forming an uneasy alliance. Suddenly a shot rings through the House of Commons - unleashing an outburst of terror even more bloody.