Aired Dec. 30, 1972 - Jan. 20, 1973
Written by: Bob Baker & Dave Martin
Directed by: Lennie Mayne
While investigating a strange transmission that appears to be traveling faster than light, the Doctor discovers he is being hunted by a strange plasma creature. Trapped in the TARDIS, the Doctor calls on the Time Lords for assistance, but they too are under assault, their energy supplies being drained into a black hole. Unable to directly help, their only option is to muster enough energy to send the Doctor's past incarnations to help him, and hopefully, save the Time Lords as well.
While the Third Doctor is joined by the Second Doctor, the First Doctor is caught in a time eddy and only able to advise his successors over the TARDIS scanner screen. Together they are able to deduce that the transmission, and the creature, is a time bridge. The Third Doctor and Jo are caught by the creature and transported beyond the black hole into an antimatter universe.
Back on Earth, the Second Doctor, Benton and the Brigadier are still trapped inside the TARDIS. The First Doctor advises turning off the force field keeping the creature at bay, and at once the TARDIS, along with the whole UNIT HQ, is transported through the black hole.
On the antimatter world, the Third Doctor has discovered the planet is ruled by Omega, a legendary figure from Time Lord history whose stellar engineering experiment made time travel possible for their society. Thought lost since that experimental mission, Omega has in fact been trapped in the antimatter universe, surviving and creating everything around him through his will alone. He wants to be free of his prison and return to his original universe, but as his will maintains his existence, he cannot leave unless someone replaces him. He hopes that the Doctor will do so, and brought him here to that end.
Now joined by the Second Doctor, the Third Doctor tires to convince Omega to change his mind or accept some other form of help. Things fall apart once Omega discovers that his entire physical form has been consumed by the effects of such a long period in the antimatter universe. There is literally nothing left but his will, which maintains his existence, and therefore no escape for him.
Driven mad by this realization, Omega plans to destroy the positive universe. However, the Second Doctor discovers that his recorder had fallen into the TARDIS force field generator and therefore was not converted to antimatter upon passing through the Black Hole. He and the Third Doctor conspire to have Omega touch the recorder, and the collision of matter and antimatter results in a supernova, which the Doctors and their friends escape in the nick of time.
Once home, the First and Second Doctors are returned to their points in time, and in gratitude for saving the Time Lords, they grant the Third Doctor his freedom, lifting his exile and repairing his TARDIS. The Doctor is free to roam time and space once more…