The Three Doctors

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

A gel-like plasma creature arrives on Earth and hunts down the Doctor, who calls on the Time Lords for help. The Time Lords themselves are in crisis as their energy is being drawn off into a black hole. They send the Doctor's earlier selves to join him.

The first Doctor, caught in a time eddy and able only to advise, deduces that the creature is a time bridge. The third Doctor and Jo then give themselves up to it and are transported to a world of antimatter beyond the black hole.

On Earth, the second Doctor is forced to take refuge in the TARDIS along with the Brigadier and Sergeant Benton. On the advice of the first Doctor he switches off the ship's force field, and the whole UNIT building is transported through the black hole.

Behind these events is Omega, a figure from Time Lord history whose solar engineering provided the power for time travel. He has been trapped in the black hole ever since and now wants the Doctor to swap places with him, but it transpires that the corrosive properties of his domain have already destroyed his physical form, leaving only his will.

He threatens to destroy the universe but is tricked into touching the second Doctor's recorder - the only thing not converted to antimatter when the TARDIS passed through the black hole - and is consumed in the resulting supernova. Everyone else is returned home.

11 out of 42 (26.2%) raters say this story requires a previous story.