Doctor Who/Recap/S16 E6 The Armageddon Factor

""Men out there - young men - are dying for it!""

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 is a growing movement wanting to arrange a peace, but the Marshal of Atrios is determined to keep fighting until the Zeons are wiped out.

It turns out that all is not as it seems on Zeos (which is in fact completely deserted), and that a third party has been keeping the war going for his own purposes. The Doctor manages to get hold of the final segment - disguised as the Atrian Princess Astra - and complete the Key, while ending the war along the way.

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 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.

Tropes

 * Ad Dissonance: the opening scene.
 * Anticlimax: The "Key to Time" story arc, which spans six stories and an entire season, ends without a proper resolution.
 * 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.
 * Barrier Maiden
 * Dangerously Genre Savvy: The Shadow knows the Doctor's impatience is his weak spot.
 * Electric Torture: Used by the Shadow on both the Doctor and Romana
 * 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.
 * Everyone Went to School Together: Drax was at school with the Doctor - class of 92, even.
 * Everything's Better with Princesses
 * 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
 * Incredible Shrinking Man
 * 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.
 * MacGuffin Delivery Service
 * MacGuffin Girl
 * Malaproper: The Marshal. "Taste the moment of victory. Any second now, beautiful mushrooms will blossom and burst!"
 * 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.
 * Space Cold War
 * Stellar Name: Princess Astra
 * Stern Chase: The beginning of one is the reason for the randomizer
 * Stylistic Suck: The recruitment video seen the first scene of the story is deliberately produced and acted poorly.
 * Title Drop
 * 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 emmenating 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.