The world is covered in wastelands that are dotted with numbered "Towns." All Towns are supervised by computers called "Little Mamas" (nodes conected to the world governing super-computer called "Big Mama"); the underprivileged live in the slums and are referred to as "People." One can only become one of the privileged "Citizens" by joining the army and fighting the other Towns' forces, as the Towns are continually at war with one another. Combatants earn 50 credits per mission, plus 30 credits for every enemy killed. Once the total reaches 1000, the fighter's class is increased by one category (starting at F, then going to E, D, C, B and making one's way up to A). The A class guarantees a transfer to the idyllic, legendary "City."

Citizens have numerous advantages over People: in addition to getting out of the ghetto, they do not worry about being beaten up, raped or otherwise victimized. It's this advantage that leads Grey's lover, a girl known as "Lips," to become a Citizen. Her death leads Grey to join the army as well and follow her dream of getting to the City by sheer force of will. Grey soon earns the nickname "Grey Death" for his tenacity on the battlefield and ability to survive when the rest of his comrades are killed. He only accepts orders that keep him alive, and will not kill the resistance fighters that fight against the Town system because they are not worth any points. Sullen and cocky, Grey exists only to survive and kill, in that order.

Grey's sense of loyalty gets him into trouble when he finds out that his field commander, a man that saved his life many times, is lost in the African sector. He abandons Town 303 to go look for him. Along the way, he discovers who and what the resistance fighters are, why the Towns are all different in both military might and how they are run, and what the true secret of "Big Mama" is.
source: wikipedia