The TARDIS materializes in a place governed by the Celestial Toymaker.  He forces the Doctor and his companions to play a games.  The Doctor must solve the Trilogic game.  Steven and Dodo must compete against animated toys in children's games that could turn deadly if lost.  The Doctor imitates the Toymaker's voice and finishes the Trilogic game inside the TARDIS.  They escape just as the Toymaker's domain is destroyed.