Armor is the story of humanity's war against an alien race whose foot soldiers are three-meter-tall insects, referred to in the book as "ants".[1] It is also the story of a research colony on the fringes of human territory which is threatened by pirates. The two sub-plots intersect at the end, with each providing answers and insight into events of the other.

The title refers to the nuclear-powered exoskeletons worn by the soldiers, but also references the emotional armor the protagonists maintain to survive.