Timelash

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

On the planet Karfel a high ranking official, Maylin Tekker, uses threats against Peri to force the Doctor to go to Earth and bring back a young woman called Vena who, while holding a precious amulet, has accidentally fallen into the Timelash - a time tunnel through which the planet's tyrannical ruler the Borad banishes all rebels. The Doctor also inadvertently brings back Herbert, a man from the 19th Century, who stows away aboard the TARDIS.

The Borad was once a Karfelon scientist but accidentally sprayed himself with an unstable compound called mustakozene-80 while experimenting on a Morlox - a savage underground reptilian creature - and consequently became half-Karfelon, half-Morlox.

The Borad plans to bring about the deaths of all the Karfelons by provoking a war with their neighbours, the Bandrils, and repopulate the planet with creatures such as himself, starting with Peri. The Doctor uses a kontron time crystal to defeat a clone of the Borad and makes peace with the Bandrils. He then defeats the real Borad by banishing him through the Timelash to 12th Century Scotland.

4 out of 68 (5.9%) raters say this story requires a previous story.