Enlightenment

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

The White Guardian warns of impending danger and directs the TARDIS to what appears to be an Edwardian sailing yacht, the SS Shadow, but is actually one of a number of spaceships taking part in a race through the solar system, the prize being Enlightenment. The yacht's Captain Striker and his fellow officers are Eternals who feed off the thoughts and emotions of their kidnapped human crew - Ephemerals - in order to fill their own empty existences.

Turlough attempts to escape the Black Guardian's influence by jumping into space but is rescued and taken on board the ship of Captain Wrack - another of the Eternals. Two of the other ships in the race are destroyed and Turlough discovers that this has been brought about by Wrack, using a concentrated beam of mental energy with the aid of the Black Guardian.

The Doctor boards Wrack's ship, finds her source of power and, with Turlough's help, ejects her and her number two, Mansell, into space. The Doctor and Turlough then pilot the ship into port - a glowing crystalline structure hanging in space - and, in doing so, win the race. The White Guardian offers a portion of Enlightenment to Turlough, while the Black Guardian demands that the boy give the Doctor over to him in exchange for a huge diamond within a glowing artifact - apparently the prize.

Turlough makes his choice: he sweeps the crystal from the table straight toward the Black Guardian, who vanishes in flames. The boy is now free, as Enlightenment was not in fact the crystal but the choice.

18 out of 22 (81.8%) raters say this story requires a previous story.