Britain stands alone against the might of the advancing German armed forces. But before Hitler can put his planned invasion into effect, he needs to destroy Britain's Royal Air Force. The Germans believe they are invincible. For four long months in the summer of 1940, the RAF and the German Luftwaffe fought an epic battle in the blue skies over the green fields of southeast England. For more than 60 years, the story of the battle has been the story of an unprepared nation winning against overwhelming odds–a tale of heroism, of a handful of plucky pilots, of the battle-winning Spitfire aircraft. In our investigation, scientists, historians, and veterans reveal that in fact Britain was far from unprepared. What were the secret systems and tactics that forced the Germans to withdraw from battle–and that led them to postpone, and then cancel, their plans for invasion?