In 1973, Doctor Who was in it's 10th year. And to celebrate, the producers decided to do something a little different. First, we officially get the name of the Doctor's planet, Gallifrey. A mysterious force is draining power from Gallifrey and the Timelords realize they will require the assistance of the Doctor. Every other Timelord seems to be needed to combat the power loss.
Unknown to anyone, the culprit is Omega. The original Timelord who effectively harnessed the power of a black-hole, which is what gives the Timelords the power to travel in time. He became trapped in the black-hole and was thought dead, but regarded as a hero. Trapped alone in a world of anti-matter, Omega became bitter and angry feeling abandoned.
On Earth, Omega has begun to reek his vengeance on the Timelords, starting with the Doctor. Sending anti-matter blobs & formless creatures in an attempt to capture and bring him to the anti-matter universe. While with Jo and Benton, the Doctor is forced to flee to the Tardis (This is Benton's first visit into the Tardis).
Still in critical condition, the Timelords break their own rule of time and send a previous incarnation of the Doctor to help himself. This catches Jo by surprise as she was unaware of the Doctors ability to regenerate. The Brigadier and Benton warmly welcome the 2nd Doctor, as this was how he looked when they first met him. Understandably, the Brigadier is still confused.
The power on Gallifrey is almost exhausted, but when the two Doctors continue to argue with each other instead of help each other, the Timelords are forced to send the 1st Doctor also. With limited power, they are only able to send him in an advisory role. (Actually, the actor William Hartnell was too ill to perform as the producers would have liked). The Doctors finally put their heads together and devise a way to "nobble" Omega.
The Doctors realize Omega has been completely consumed by the Anti-matter and exists only because his will commands it. Now knowing he can never return to the real world, Omega vows to destroy the universe but the Doctors give him the only freedom he can ever have - destruction. Doing so, they reverse the power drain on Gallifrey and supply it with a complete new energy source. In gratitude, the Timelords release the Doctor of his banishment and supply him with a new dematerialization circuit.
Next: Mastering the Frontier