(This book is also known by the title "Sixth Column")
An Asian totalitarian government using advanced technology conquers America practically overnight because America has basically disarmed itself. A supersecret laboratory becomes the last hope for America when a major breakthrough in technology is made. Unfortunately the breakthrough experiment also kills the lead researcher and most of the staff of the laboratory, leaving less than a dozen men alive, only one of them a scientist. A reserve intelligence Major, whose regular occupation is being an advertising executive, gets to the laboratory just after the experiment has killed most of the staff. His mission as the last free active military officer is to use the resources of the laboratory and the breakthrough (which looks like magic to the Major) to defend America and restore its freedom. The Major has to use his understanding of people in order to organize the resistance and understand the enemy in order to use the breakthrough effectively to win freedom. The difficulties of fighting a totalitarian regieme that has no respect for life are explored. The temptation of great power in the hands of a few is another thread. The battle for freedom must restore power to the people. An entertaining read. Typically Heinlein.