Late in 1939 Nazi Germany, with new military weapons and tactics, was poised to overrun Europe and impose Adolph Hitler's control over Western civilization. At that very time, two British physicists invented the cavity magnetron. About the size of a hockey puck, it unlocked the enormous potential of radar exclusively for the Allies. Since the discovery of radar early in the twentieth century, development across most of the world had progressed only so far. Germany and Japan had radar as well, but in just three years, the Allies' new radar incorporating the top-secret cavity magnetron turned the tide of war from a doubtful to a known conclusion before the enemy even figured out how. The tactical difference between the enemy's primitive radar and the Allies' new radar was akin to comparing the musket to the rifle. The cavity magnetron proved to be the single most influential new invention responsible for winning the war in Europe. Written for a non-technical reader, this historical narrative tells the relatively unknown story of radar's transformation from a technical curiosity to a previously unimaginable offensive weapon. We meet scientists and warriors critical to the story of radar and its pressure-filled development and implementation in just months. The story highlights two characters who are woven into the narrative as it unfolds: one a brilliant and opinionated scientist, the other an easy-going twenty-one-year-old caught up in the peacetime draft. This unlikely pair and a handful of their cohorts pioneer a revolution in warfare as they formulate new offensive tactics by trying, failing, and fixing, as well as overcoming the nay-sayers and obstructionists on their own side.