In 2021, aliens called "Heavies" invade Earth and make quick gains against a disorganised response from various nations. When the United Nations is disbanded and a united military front, the USDF (United Space Defense Force), replaces it, the aliens are driven off Earth, though thousands of their troops are left behind. A series of defensive satellites is thought to protect Earth from a second invasion. In 2033, two embedded journalists accompany reinforcements to a demilitarised zone between Pakistan and Afghanistan where Outpost 37, one of the last USDF outposts, is located. After a period of hazing, the reinforcements settle into life at the outpost.