Warriors' Gate

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

The TARDIS is hijacked in the vortex by a time sensitive Tharil named Biroc, and brought to a strange white void. Biroc wants to free the others of his race who are being transported in a slave ship, captained by Rorvik, which is also trapped in the void. The only other thing present in the void is an ancient gateway leading to a decrepit banqueting hall.

This is the domain of the Tharils, who in a previous time were cruel masters to their human slaves but have now repented. The humans built the Gundan robots to kill the Tharils, and this led to the latter race's downfall.

Rorvik tries to break through the gateway, and thereby gain access to N-Space, by blasting it with his engines. The blast is simply reflected back, however, destroying the ship and freeing its prisoners.

Romana and K9 elect to remain in E-Space to help liberate the remainder of the Tharil race, while the TARDIS - now occupied only by the Doctor and Adric - is able to return to N-Space.

59 out of 88 (67.0%) raters say this story requires a previous story.