The Keeper of Traken

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

The Doctor and Adric learn from the wizened Keeper of Traken that a great evil has come to his planet in the form of a Melkur - a calcified statue. The Keeper is nearing the end of his reign and seeks the Doctor's help in preventing the evil from taking control of the bioelectronic Source that is the keystone of the Traken Union's civilisation.

The Melkur, via various deceptions, becomes the next Keeper. It is then, however, revealed to be the Master's TARDIS. Its owner, still blackened and emaciated, hopes to use the Source's power to regenerate himself. The Doctor manages to expel him and install a new Keeper in his place, but in a last minute ploy the Master traps one of the Traken Consuls, Tremas, and merges with his body before fleeing the planet.

9 out of 93 (9.7%) raters say this story requires a previous story.