The year is 31XX. Humans are 50 years into living on Earth2 after the deer revolt. Overpopulation and science experiments caused the deer to take up arms and hunt the humans. They drove the humans into retreat and the humans had no choice but to leave earth. 

The time for revenge is nigh. Earth2 cannot sustain human life as we thought and there is no choice but to take the planet back by force. Use advanced weaponry to clear Earth1 of the invasive monsters and restore the land for humans to return.