In a dystopian near-future Los Angeles , residents live under a regime of military occupation by an organization known as the Transitional Authority. The Authority serves an extraterrestrial group referred to as the "Hosts", about whom little is known until later in the series an alien robotic race finds itself hunted, who came to Earth to use humans as allies and labor in their own battle.
The symbol of the collaborating forces features stylized birds of prey, or raptors , which gives rise to their nickname, the "Raps". The Authority enforces Host policy via militarized police called Homeland Security and nicknamed the "Redhats" for the red helmet cover of their otherwise black tactical attire.
The Hosts took control on a day known simply as the "Arrival". That day began with a global blackout and mass technological failure, which came after a weeklong hunt for relevant key figures around the world. Late that day, massive rectangular blocks descended from the sky, linking together to build walls dividing every city and every nation.
One of these walls, 20 to 30 stories tall, many meters thick and many miles in length, surrounds the central part of Los Angeles, where the series is set. Other similar walls have been constructed around neighbouring urban areas, called "blocs", with the whole referred to as a "colony". Traffic passes through the walls at heavily secured checkpoints, called "gateways", which allow the Authority to strictly control the movement of people and the distribution of consumables, such as food and fuel, which are rationed.
The geographical extent of the alien invasion is unclear, but later scenes in the series shows Authority members from all over the world - hence making the invasion scale worldwide. A privileged class of elites, drawn by the Hosts from the local population, are denigrated by some bloc residents as collaborators. The ruling forces maintain control through the separation of family members, shoot-on-sight curfews , forced disappearances , random checkpoints , frequent electronic identity checks, limitation of motor vehicle usage most people walk or ride bicycles , pervasive visual propaganda , slave labor in a place called the "Factory" later revealed to be located on the Earth's moon to mine radioactive materials , and massive continuous electronic surveillance with both hidden cameras as well as Host-provided drone aircraft that launch from hangar bays inside the wall and capable of killing humans by extremely lethal high energy weaponry.
