UFO: Enemy Unknown

The player manages X-COM throughout the game, battling aliens. As the game progresses, the player learns more and more about the enemy, their species, sub-species, mutated creations and technology. It is revealed that the aliens are led by creatures of immense mental power called Ethereals, and that their main base is located on Mars, in a region known as Cydonia. It is up to the player to fully prepare the final assault team, attack Cydonia and destroy the mastermind behind the alien invasion, the Alien Brain.