The Celestial Toymaker

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

The travellers arrive in a strange domain presided over by the Celestial Toymaker - an enigmatic, immortal entity who forces them to play a series of games, failure at which will render them his playthings. The Doctor has to solve the complex Trilogic game while Steven and Dodo are faced with defeating a succession of apparently child-like but potentially lethal animated toys in contests such as 'blind man's buff', musical chairs and 'hunt the key'.

The Doctor finally overcomes the Toymaker by imitating his voice in order to complete the Trilogic game from within the TARDIS, which then dematerialises as his foe's universe is destroyed.

8 out of 94 (8.5%) raters say this story requires a previous story.