Lance Corporal Hondo McKeever is a peacetime Marine. Humanity is involved in a limited war with the alien Klethos, where gladitorial combat decides the fate of worlds, but that doesn't affect the common Marine or sailor. His career has been training hard but never seeing actual combat.
All of that changes when a new enemy invades the galaxy, one that threatens both humanity and Klethos.
Mankind hasn't fought an all-out war in centuries, but they're about to. McKeever is assigned to a special task force with the mission to find out what makes this new enemy tick, and, if possible, stop them in their tracks before humanity is eliminated like so much insignificant vermin.