At the beginning of the game, you're told that Earth was overrun by monsters, and your character is part of a task force intended to take the planet back. Partway the game, it's revealed that in the past, there was a machine rebellion that got interrupted by the monsters appearing. The end of the game reveals that all the humans in the game (including your character) are actually robots, because after the machines wiped out the original humans the survivors turned themselves into monsters for revenge.