From the back of the book:
It's the end of the world
In the aftermath of a nuclear holocaust, alien invasion, supernatural Armageddon, or some other terrible cataclysm, life as we know it has ended. Doomsday has left in its wake a world filled with mutants, scavengers, rogue technology, environmental hazards, or other perils. In d20 Apocalypse, heroes must salvage the remnants of civilisation and survive the dangers that await in a grim future.