Colony In Space

From http://www.bbc.co.uk/doctorwho/classic/episodeguide

The Time Lords discover that the Master has stolen their secret file on the Doomsday Weapon and decide to send the Doctor to retrieve it for them.

The TARDIS takes the Doctor and Jo to the desolate planet Uxarieus in the year 2472. There they become involved in a dispute between some beleaguered colonists and the crew of an Interplanetary Mining Corporation (IMC) spaceship over the ownership rights to the planet. The Doctor learns that the indigenous Primitives and their High Priests worship a large machine tended by a creature called the Guardian.

The Master meanwhile arrives in the guise of an Adjudicator sent from Earth to decide the fate of the planet. He forces the Doctor to take him to the Primitives' underground city, where they learn that the machine is in fact the Doomsday Weapon, capable of destroying entire planets. Its radiation emissions have brought about the decline of the Guardian's race and are also responsible for the crop failures that the colonists have been experiencing.

The Doctor persuades the Guardian to destroy the Weapon rather than let it fall into the Master's hands. The two Time Lords get clear just in time as the machine explodes, and the Master then escapes in his TARDIS. The colonists, meanwhile, attack the IMC men and force them to surrender.

7 out of 83 (8.4%) raters say this story requires a previous story.