Uxarieus

Uxarieus was a mostly barren, rocky planet that was earmarked for colonisation by the Earth government in the 25th century.

Native life
Called "primitives" by humans, at least three different types of "Uxarieans" were seen on the planet. The Third Doctor hypothesised that perhaps they were all mutations of the same root species. Before he could conduct any tests to determine this, Jo prodded him to "stop philosophising" and return his attention to the more pressing matters at hand. There were no animals on the planet, just some bird and insect lifeforms.

Nevertheless, it is certain that the creation of the Doomsday Weapon plunged the native culture into disarray. Though never used, the potential of using it haunted the dominant culture and ultimately forced it to turn its back on scientific achievement; simultaneously, leaking radiation from the weapon left the soil of Uxarieus unable to produce crops. A pictographic history of the planet in the "primitive city" seemed to indicate that the creation of the Doomsday Weapon had plunged the culture into a kind of Dark Age.

History
Taking advantage of the earmark for colonisation, Robert Ashe led colonists to the planet in 2471. They had great difficulty in reclaiming the planet's barren topsoil for cultivation, which threatened the colony's survival. When the Third Doctor and Jo Grant were sent to the planet by the Time Lords, he was willing to offer his substantial knowledge of horticulture to address the colony's woes. Before he could do so, the colony's legal right to be on the planet was challenged by Captain Dent of the Interplanetary Mining Corporation. Citing IMC's authorisation to mine the planet's rich duralinium deposits, the two sides came to blows over who had the greater right to be there. They called in an Adjudicator to settle the matter, who initially ruled in IMC's favour. However, when the Adjudicator turned out to be an impostor — and when it became clear that the colonists' ship would likely not survive any trip away from the planet — many of the colonists stayed behind to fight IMC. This plan was ultimately successful.

Meanwhile, the Master, who was masquerading as the Adjudicator, revealed his interest in the planet as the one the Time Lords were worried about. The planet's indigenous culture had long ago created a Doomsday Weapon, which the Master longed to possess. The Doctor and the planet's native leader denied him this pleasure. The Doomsday Weapon was destroyed and revealed to be the thing that was inhibiting the colony's agricultural efforts.

Free of both the weapon and the IMC, the colony, under the leadership of David Winton, continued for an unknown period of time after the Doctor and Jo left. (DW: Colony in Space)

Behind the scenes
This is one of the few planets for which Doctor Who's notorious use of location filming in quarries actually made explicit narrative sense.