Castrovalva

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

The newly regenerated Doctor escapes with his companions back to the TARDIS. Suffering from post-regeneration trauma, he only narrowly manages to save the ship from destruction as it plunges back to Event One, the hydrogen in-rush that preceded the creation of the universe.

He then seeks sanctuary in the peaceful domain of Castrovalva, only to discover that it is an illusory, dimensionally paradoxical trap set for him by the Master with the unwilling aid of a kidnapped Adric. The Doctor eventually wins the day by enlisting the help of the Castrovalvan people who, although also part of the Master's creation, are nevertheless able to exercise free will.

60 out of 96 (62.5%) raters say this story requires a previous story.