Season 16, and the Key to Time arc concludes this week with this six-part story written by Bob Baker and Dave Martin that aired from 20 January to 24 February, 1979.
The Doctor (Baker), Romana (Mary Tamm) and K9 find themselves caught up in an intergalactic war, as they arrive on the planet Atrios, which has been at war with the planet Zeos for decades. Atrios is ruled by a young Princess Asrta (Lalla Ward – who appears next series as the regeneration of Romama), and is hoping to establish a peace finally.
But there is more going on here than anyone suspects, there is someone behind the scenes machining everything at the behest of the oft-mentioned Black Guardian. He is The Shadow (William Squire), and much like the Doctor has been gathering the pieces for the Key to use for good, the Shadow has been trying to do the same, but for evil. But there is only one piece left up for grabs.
The Shadow has been using a leader on Atrios, the Marshal (John Woodvine) to continue the war and take control of everything.
While the Doctor and Romana investigate they end up being captured (surprise) and labeled as spies, but the Doctor of course proves his wit, and out thinks everyone, with Romana’s help, and shows he is on the side of the right.
Unfortunately, Princess Astra ends up under the control of the Shadow, as does K9, but even at that point, the Doctor has things figured out, almost getting one step ahead of his nemesis, and turning things around.
To help save the planets, he jury rigs a time loop using the five pieces of the Key to Time that he and Romana do have, stopping an attack by the Marshal long enough for them to stop the Shadow.
But it’s not going to be easy, and there are some interesting revelations as the story rolls on, including the reveal that the Shadow has a fellow Gallifreyan in his prisons, Drax (Barry Jackson), who may or may not be still in service to the Shadow.
It’s fun and when the last part of the Key is revealed, the Doctor has a great idea, to keep the key safe he stows it in the TARDIS and activates the randomizer… Explaining why for the most part, the TARDIS goes where she wants to.
The season ends, with the Key to Time safe, probably hidden in one of the many levels and rooms of the TARDIS (I can’t remember if it gets mentioned again, I’ll have to wait and find out I guess).
Next week we start a new season, with a new version of Romana (though there is no regeneration sequence – something obviously happened to her between episodes) and an old enemy with Destiny of the Daleks.