Thera is a planet introduced in SAS: Zombie Assault 4. It was a desert planet in the milky way galaxy, have been mostly a testing planet where the economy was based on scientific discoveries. With a population of 14,989,674 (200,000,000 according to a intro cutscene on mobile) to people, Thera was once a successful place.
But all that ended when an alien virus broke out among the population, mutating people into walking corpses and undead monstrosities. The raging virus infected the entire population of Thera, making it a planet-wide zombie infestation. On the surface of the planet, there is a lab containing a vaccine sample from a futile attempt to stop the Thera virus. This vaccine sample is vital to the survival of humanity and the president of the Trans-Federation of Worlds gave an order to wipe out all organic matter on the planet with a doomsday device called a Jolt Ship.
In the mission Vaccine, the SAS have only fifteen minutes to retrieve the vaccine and leave the planet before it is destroyed. This is changed to ten minutes and later nine minutes as the player levels up, For any match on multiplayer mode, the time is usually twelve minutes at higher levels. Once the SAS retrived the vaccine, the planet was destroyed by the Jolt Ship, taking all of the zombies and anything else possibly living on the planet with it.