The third Doctor and Jo find themselves on an artificial world inside the black hole while the second Doctor tries to find a way to control the organism.
Quest roles:
William Hartnell(The Doctor), Patrick Troughton(The Doctor), Nicholas Courtney(Brig. Lethbridge-Stewart), John Levene(Sergeant Benton), Clyde Pollitt(Chancellor), Roy Purcell(President of the Council), Graham Leaman(Time Lord), Stephen Thorne(First Ogron), Rex Robinson(Dr. Carter), Denys Palmer(Corporal Palmer), Laurie Webb(Mr. Ollis)