The one with SATAN HIMSELF.
Alternatively- the one where the Doctor swore the tin off K-9.
The search for the last segment of the Key to Time takes the Doctor and Romana to the planet Atrios, which is embroiled in a war with its sister planet Zeos. The war has been going for years and is taking a serious toll, and there's a growing movement wanting to arrange a peace, but the Marshal of Atrios is determined to keep fighting until the Zeons are wiped out. The Princess of Atrios, Astra, tries to stop the war together with her boyfriend.
It turns out that all is not as it seems on Zeos (which is in fact completely deserted and run by a mad computer), and that a third party has been keeping the war going for his own purposes. This bat-guy, an unbelievably Large Ham named "The Shadow", is controlling people from a distance and gets his hands on just about anyone who's important to the plot.
The Doctor and Romana do a bit of MacGyvering with the nearly-completely Key to Time, and manage to create a replica of what the final segment probably looks like. It works, a bit, and the Doctor goes into full-on A God Am I mode when he uses the thing to create a time loop around an impending nuclear strike. While the loop slowly starts to deteriorate, he also runs into Drax — an old academy buddy who ran away from Gallifrey, picked up a bit of Cockney (innit) and has spent a few years doing odd jobs in this war. He calls the Doctor "Theta Sigma", sending the entire fandom into a frenzy until the Seventh Doctor would finally confirm that it was just a college nickname.
Romana, Astra and K-9 all get themselves hypnotized by the Shadow, and Drax decides that the best course of action is to shrink himself and the Doctor down to mouse size. They re-wire K-9 and sneak into the villain's base inside K-9's body. The Doctor manages to get hold of the final segment — Princess Astra — and complete the Key, while ending the war along the way.
While the Doctor demonstrates to Romana how it would look if he were to go mad with the power, the White Guardian appears on his viewscreen, congratulates him and asks for the Key. The Doctor refuses, reasoning that its power is such that no-one should possess it, and that this guy isn't the White Guardian to begin with. After all, a God of order would care about the fact that a human being like Astra would end up turned into a piece of a cosmic toy. He scatters its pieces again. The White Guardian is angry, and reveals that he is in fact the Black Guardian, who's been waiting around near the final segment for the Doctor to show up with the others. The Black Guardian threatens to kill the Doctor, who sets a randomiser on the TARDIS so that his travels will be unpredictable. Well, more unpredictable.
- Ad Dissonance: The opening scene.
- Anti-Climax: The "Key to Time" story arc, which spans six stories and an entire season, ends without a proper resolution. Why? It was a trap! The Doctor figures out that putting the Key together was a mistake, and decides to gallivant away from the Black Guardian.
- Arc Number: Six. Astra is the sixth child of the sixth ruler of the sixth dynasty of Atrios. She's also the sixth key fragment.
- As You Know: A particularly bizarre example occurs in the final episode, where two incidental characters recap the Doctor's current predicament for the audience's benefit although the Doctor is across the star system and out of contact, and has been for some time, so there's no way they could know the events they relate.
- Authority in Name Only: Astra doesn't seem have any real control over anything in Atrios. The Marshall appears to be the one who's really in charge.
- Badass Boast: The Doctor gives one of these to the Shadow early in the fifth episode, revealing that he and Romana are Time Lords, in the service of the White Guardian himself. Unfortunately, this quickly turns into an Oh, Crap! moment when the Shadow appears and reveals that not only does he know perfectly well who the Doctor is, he himself is in the direct service of the Black Guardian.
- Beneath the Earth: The whole third planet / planet of evil has very much the feel of this, in spite of the fact that it's an artificially constructed structure that looks quite space agey from the outside.
- Card-Carrying Villain: The Shadow admits that both he and the Black Guardian have no desire for political power, they just love watching stuff getting blown up and people killed.
- Continuity Nod: The Doctor recalls Troy while miniaturised and hiding in K9, as they head to the hideout of the Shadow.
- Electric Torture: Used by the Shadow on both the Doctor and Romana.
- Everyone Went to School Together: Drax was at school with the Doctor — class of 92, even.
- Everything's Better with Princesses: Astra plays the trope completely straight.
- Exposition Intuition: At the beginning of part 6, Shap and Merac conveniently recap for us some recent events and the predicament the Doctor is in. Which is strange because these events happened on a different planet and no one present has been in communication with them.
- Fakin' MacGuffin: The Doctor creates a false Key Segment in his plan to trap the Shadow's forces in a time loop. He knew, however, that the segment (and the time loop) would disintegrate over time; the goal was to buy himself more time.
- Forever War: What the Black Guardian and the Shadow want to set up between the two halves of the universe. A lesser example is with the Atrians and Zeons, who have been fighting for decades.
- General Ripper: The Marshal.
- A God Am I: Played with. "I'm all right, but suppose I wasn't all right!"
- Hypocritical Humour: The Doctor lectures Romana on the importance of optimism and that she shouldn't always just assume the worst, she should wait until she has all the facts and then believe the worst. When Romana asks about the situation where you get the facts and it turns out not to be the worst, the Doctor insists that never happens.
- Imperfect Ritual: The Doctor and Romana have five of the six pieces of the eponymous key and make a makeshift sixth piece since they know what its shape is. It works, but not well.
- Incredible Shrinking Man: The Doctor and Drax.
- Inferred Holocaust: It's implied that the Atrians unknowingly managed to wipe out the Zeons very early on in the war, and that the subsequent conflict was engineered by the Shadow simply so that he wouldn't get bored waiting for the Doctor to arrive.
- Insane Admiral: The Marshal of Atrios is obsessed with achieving victory in an interplanetary war with Zeos, which has so far devastated Atrios to the point that its surface is virtually uninhabitable due to the radiation fallout.
- MacGuffin Delivery Service: Team TARDIS.
- MacGuffin Turned Human: Astra.
- Malaproper: The Marshal. "Taste the moment of victory. Any second now, beautiful mushrooms will blossom and burst!"
- Mind-Control Device: That little electronic chip on the Marshal's neck. It's even able to control robots.
- Mood Whiplash: The Doctor's A God Am I moment freaks Romana out, at which point he snaps right out of it and agrees that they need to get the Key back to the White Guardian.
- No MacGuffin, No Winner: The Doctor's decision to scatter the segments through time and space once more upon realizing that the Black Guardian was disguising himself as the White Guardian.
- No-Sell: The Shadow places a Restraining Bolt on the Doctor's neck. Unlike the Marshall, however, the Doctor is able to simply pluck his off, much to the Shadow's irritation.
- Omnicidal Maniac: The Shadow (and the Black Guardian).
- One True Sequence: Taken advantage of by the Shadow, who decided to just sit on the sixth segment while the Doctor ran around after the previous five.
- Out-of-Character Alert: The Doctor knows something is wrong with K-9 when the latter starts calling him 'Doctor' instead of 'Master'. He also identifies the Black Guardian by his disregard of human life.
- Ripped from the Headlines: The story was inspired by the Cold War.
- Robotic Reveal: K-9 reveals the fact that Zeos is governed by Mentalis, a computer.
- "Shaggy Dog" Story: One that lasts for an entire season. The Key to Time storyline ends with the Key only being fully assembled for the last five minutes of the serial's final episode, with only a Take Our Word for It that the White Guardian was able to use the Key for its intended purpose while the Doctor was talking with the Black Guardian. In fact, the only thing we really see the Key used for is putting the Marshall in a time-loop.
- Shout-Out to Shakespeare:Marshal: To halt the hated Zeons in their tracks, wipe their presence from our skies, and free this land, this world, this Atrios...The Doctor: (Interrupts) This Blessed Plot!Marshal: Good! Good! ...this blessed plot from the terrors of war and the evils of pestilence!The Doctor: Yes! ... No, I prefer the original.
- Shrink Ray: Drax uses his TARDIS's dimensional stabiliser to shrink the Doctor and himself.
- Space Cold War: With a computer involved.
- Stellar Name: Princess Astra.
- Stern Chase: The beginning of one is the reason for the randomizer.
- Stylistic Suck: The recruitment video seen in the first scene of the story is deliberately produced and acted poorly.
- Title Drop: The Doctor makes one after realizing that whoever is behind the war between Atrios and Zeos intends to allow both to be destroyed.The Doctor: There'll be a rather large bang, big enough to blow up Zeos, take Atrios with it, and make sure the whole thing ends in a sort of draw. That's the way these military minds work. The armageddon factor.
- Trojan Horse: With K-9 playing the role of the horse.
- Unwitting Pawn: The Marshal.
- Weakened by the Light: The Doctor holds off the Shadow with a beam of light emanating from the completed Key To Time. The TARDIS also produces a blindingly bright light as a form of defence against the Shadow, though his mooks are able to withstand it much more easily.
- Wire Dilemma: Disarming Mentalis before the time loop decays. Turns out it's the green wire.