In the history of the planet Uxarieus, a highly-advanced society once existed. Through genetic engineering, they were able to develop a super-race; and this super-race created the most powerful weapon ever created, a doomsday weapon, which could target any object in the entire universe and annihilate it. It was never used, only tested on a star, which, as a result, became the Crab Nebula. But the radiation from the weapon's power source poisoned the soil of Uxarieus and their race began to decay.
By the year 2472, Uxarieus has changed greatly - the barren planet has no surface-dwelling animal life other than birds and insects, and the once-fine cities have turned to ruins. The natives of Uxarieus have evolved into a society containing three separate races. The Primitives have striped green skin and wear necklaces with gold pendants, and they always carry long spears and knives with them, although they may sometimes use bolos to lasso their prey. They have rudimentary telepathic ability, being able to understand what aliens are saying even if they do not understand the language. When the colonists from Earth first landed, the Primitives killed two of them; but then a truce is reached, and the Primitives work as assistants for the humans in exchange for food. The relationship is not always a smooth one, however; sometimes the Primitives capture stray humans and exchange them for more food. And when the colony is being attacked by what appear to be giant lizards, a man claiming to be the lone survivor from another colony says that the Primitives working for them were friendly until the humans were defenceless, and killed all the others who survived the lizard attacks.
The Primitives have a large underground city concealed within the rockface, in which the doomsday weapon is kept. All intruders to the city are sentenced to death, and put into a large reactor as a sacrifice. Inside the city dwell the second of the races on Uxarieus, the High Priests. These white-skinned creatures with large craniums wear hooded robes, and are practically blind. Once the super-race of Uxarieus, now they are merely the priests of a lunatic religion which worships machines instead of gods.
Finally, the doomsday weapon is watched over by a Guardian, who has advanced powers including the ability to teleport objects (or simply make them disappear?) and some kind of clairvoyancy - it senses that the Master will only bring unhappiness and destruction to the universe if he gains control of the doomsday weapon. The Guardian asks that the device be set to self-destruct, so the Doctor complies. When it explodes, it destroys the Primitive city with it.
