Doctor Who: Lost in Time S01 E06 "The Celestial Toymaker" - episode 4 "The Final Test"

Trapped in the domain of the Celestial Toymaker, the Doctor is made invisible and forced to play a game of intellect. His companions too must play the Toymaker's games: if they win, they will regain the TARDIS and may go free; but if they loose, they will become his playthings forever ...