A bookish history professor from Cambridge, Massachusetts, Tom Mason probably never imagined he would one day be putting his extensive knowledge of military tactics to use in an actual war, much less one being waged against an occupying force from another world. His wife now dead and his son Ben captured by the alien invaders, Tom does everything he can to protect his oldest son, Hal, and his youngest son, Matt. Despite having only an academic understanding of military history, Tom is named second in command of the 2nd Massachusetts, a makeshift regiment assigned to protect several hundred survivors, with their military commander, Weaver. Tom quickly proves himself a strong leader. Most of all, though, Tom is determined to find and rescue Ben from captivity, no matter what it takes.