The first full history of the highly trained and ruthless civilian volunteers secretly trained across Britain to be deployed in the case of a German invasion.The narrative surrounding Britain's anti-invasion forces has often centered on 'Dad's Army'-like characters running around with pitchforks, on unpreparedness and sense of inevitability of invasion and defeat. The truth, however, is very different. Top-secret, highly trained and ruthless civilian volunteers were being recruited as early as the summer of 1940. Had the Germans attempted an invasion they would have been countered by saboteurs and guerrilla fighters emerging from secret bunkers, and monitored by swathes of spies and observers who would have passed details on via runners, wireless operators and ATS women in disguised bunkers. Alongside these secret forces, the Home Guard were also setting up their own 'guerrilla groups', and SIS (MI6) were setting up post-occupation groups of civilians - including teenagers - to act as sabotage cells, wireless operators, and assassins had the Nazis taken control of the country. The civilians involved in these groups understood the need for absolute secrecy and their commitment to keeping quiet meant that most went to their grave without ever telling anyone of their role, not even their closest family members. There has been no official and little public recognition of what these dedicated men and women were willing to do for their country in its hour of need, and after over 80 years of silence the time has come to highlight their remarkable role.