From the back cover:
A Nightmare from Below. A Hero from Within.
The planet lies in ruin--cities crumbling, Man's greatest works fallen. Humanity is cornered, nowhere to run. The Locust Horde has risen, and they won't stop coming. They won't stop killing.
The Coalition is desperate for soldiers. The sick, the wounded, the imprisoned are all that remain. An inmate named Marcus Fenix, once left to die, is now charged with keeping humanity alive. He can take comfort in but one fact: The human race isn't extinct. Yet.