B is for... Black Guardian

Some time after the departure of Leela and K9 Mark I, the Doctor and K9 Mark II found themselves drawn off course by an incredibly powerful force. This was the work of the White Guardian, who sent them, with new companion Romana, on a mission to collect the six segments of the Key to Time. This had to be reassembled in order to restore balance in the universe between the forces of light and darkness. The White Guardian warned that he had an equal but opposite peer who represented the forces of chaos - the Black Guardian. Should the Key fall into his hands, the cosmos would be plunged into eternal strife.
Of the various foes encountered by the Doctor and Romana on their quest, it is not known how many were knowing agents of the Black Guardian. It seems that he simply waited until it was time to collect the final segment and complete the Key. The sixth segment was a living being - the Princess Astra of the planet Atrios. The Black Guardian had an agent here - the Shadow - working to foment and prolong a war between Atrios and its twin world of Zeos. This would lead to the destruction of a computer which the Shadow had commissioned from the captured Time Lord Drax - Mentalis. The machine's destruction would trigger a cataclysmic explosion that would spread out far from the warring worlds.

The Doctor defeated the Shadow and completed the Key, and the White Guardian appeared on the TARDIS scanner to request that it be handed over to him. His apparent disregard for the loss of Astra prompted the Doctor to smell a rat, and he quickly activated all of the ship's defences. The Guardian on the screen was really the Black Guardian, hoping to trick him. The Doctor scattered the Key back through Time and Space, reconstituting Astra. The Black Guardian vowed to have his revenge on the Doctor, so he fitted the TARDIS with a Randomiser on its navigational circuits to make it harder for him to be tracked.

In his next incarnation, the Doctor met a schoolboy named Turlough, at a school where the Brigadier now worked following his retirement from UNIT. Turlough was really an alien, who had been exiled to Earth. Following a car crash, the Black Guardian contacted the boy and offered him his freedom if he killed the Doctor. He explained that he could not directly involve himself - as Guardians can only act through agents. Turlough agreed, albeit reluctantly, but his efforts to kill the Doctor failed. Turlough had been given a crystalline device through which the Guardian could contact him, but it could also be made to inflict punishment. At one point the Guardian took on the appearance of the school headmaster to learn Turlough's true thoughts and feelings.
The Guardian's first plan was for the Doctor to be forced to relinquish all of his remaining regenerations. When this did not work, thanks to the timely intervention of two Brigadiers from different time zones, the Guardian then set Turlough to work sabotaging the TARDIS. The defences would break down, but the ship locked onto another vessel which was bound for the Terminus space station, which lay at the very centre of the universe. Turlough's further sabotage should have triggered a second Big Bang that would have destroyed everything.

The Guardian's final gambit with Turlough was to have the Doctor encounter the Eternals, who were about to embark on a race to gain Enlightenment - the knowledge to do anything they desired. Turlough finally rejected the Guardian when he threatened to leave him stranded on the Eternals' sailing ship for ever. He threw himself overboard, only to be picked up by pirate Captain Wrack, who proved to be another agent. The Guardian had given her the power to destroy her competitors, knowing that were she to win she would relish seeing the cosmos in chaos. The Doctor and Turlough were able to win the race, and they found themselves facing both the Black Guardian and the White. Turlough was offered a share of Enlightenment, if he handed over the Doctor to his foe. He rejected it, as he had come to admire and respect the Doctor. The Black Guardian disappeared, consumed in flames.
The White Guardian warned the Doctor that his opposite number would try to destroy him again, and that he should always remain vigilant. Whilst he existed, so too did the Black Guardian.

Played by: Valentine Dyall. Appearances: The Armageddon Factor (1978), Mawdryn Undead, Terminus, Enlightenment (1983).

• The idea of the Key to Time / Guardians was one of the first which new producer Graham Williams had when he took over the series from Phillip Hinchcliffe. Story development on the next season was too far advanced, so he had to wait until the following year to realise it. In hindsight, he thought it a mistake as it did not allow for any flexibility in moving around the story order. His successor, who was working on this season, took note - and so only ever allowed limited story arcs - such as the three part Black Guardian arc for Season 20 outlined above.
• The exact nature of the Guardians has never been explained. Williams envisaged a pyramidal structure to the Universe, with the Time Lords high up, but the Guardians above them. 
• It is never specified, but the implication is that the Black and White Guardians are actually different aspects to the same being.
• Fan fiction has come up with a whole rainbow of other Guardians, however, and predictably Big Finish have had a third encounter for the Doctor with the Black Guardian.
• Turlough the killer? A bit rubbish. You would have thought that the Black Guardian could have picked a more reliable assassin from all of Time and Space.
• Dyall - best known for villainous roles thanks to that voice - had one more connection with Doctor Who, in the radio adventure Slipback. He died shortly after it was recorded. His appearance in the horror movie City of the Dead (US title Horror Hotel), alongside Christopher Lee, is about to get a Blu-Ray release. He was Dr Noah in the 1967 Bond spoof Casino Royale. He didn't just play evil characters, though. In 1960, he co-hosted a musical variety show on the BBC with Dusty Springfield, now sadly wiped from the archives.