The one where the Doctor gets stuck with a coat rack.
Tegan wants to see something of humanity's future. So when the Doctor takes the TARDIS to Frontios, where sometime in the distant future the last remnants of humanity huddle together under semi-primitive conditions and occasionally get devoured by the ground or squashed by meteor showers, she insists on going out and checking up on them.
The Doctor, as usual easily talked out of disregarding Time Lord rules about not interfering, tries to help out. The locals, also in the usual way, treat the Doctor as a threat rather than a helper, but Turlough saves him by waving a hatstand in a threatening manner. Then it turns out the problem is giant insect aliens that used to attack Turlough's people too, and he has a prolonged fit of scenery chewing. Eventually our heroes, together with the more reasonable Frontios-humans, fight the Tractators by outwitting their only articulate, thinking member, the Gravis. So that's all right. And they'll all get a nice break next, promise.
- Apocalypse How: Apparently the Earth collided with the sun somehow.
- All There in the Script: Two of the colonists are called Kernighan and Ritchie (see Shout-Out below), but the scene containing the dialogue establishing this was cut.
- The Ark: Brought the humans to Frontios, then promptly crashed, leaving them without their advanced technology.
- Big Creepy-Crawlies: The Tractators.
- Bound and Gagged: Norna in Episode Four.
- Briar Patching: The resolution of the story - the Doctor persuades the Gravis to use his powers to reconstruct the fragmented TARDIS around himself... which cuts off his telepathic link with the other Tractators.
- Colony Drop: The meteor showers engineered by the Tractators.
- Crapsack World: Frontios.
- Deadpan Snarker: Five, by this point, has become almost as prone to this as his squabbling companions. E.g. "Oh, marvellous. You're going to kill me. What a finely tuned response to the situation."
- Death Glare: Tegan manages a serious one when the Doctor, having passed her off as an android to save her from the Tractators, throws in a gratuitous bit about having got her cheap because "the walk is wonky and the accent's wrong".
- Family-Unfriendly Violence: People are dragged screaming down into the ground, and we see a digging machine controlled by a severed head. (The latter scene was actually toned down compared to the script, which had the entire machine made from dismembered human body parts.)
- Genetic Memory: With a large side of Incoming Ham, punctuated by BSOD-ing.
- Good Samaritan: The Doctor is trying to obey the rules about non-intervention when he sees there are wounded.
- Gravity Master: The Tractators are powerful enough to displace the TARDIS' walls.
- Gravity Sucks: And the Tractators use it to swallow people through holes in the ground.
- Hive King: The Gravis. Without him the other Tractators become harmless.
- Hollywood Acid: Turlough spills some acid on the metal floor of the colony ship, which creates a hole almost immediately.
- Humans Through Alien Eyes: Apparently we're quite pathetic compared to Trions. Says Turlough. And Tractators. Says the Gravis.
- Improvised Weapon: A hatstand, which also, thanks to the Doctor's rambling "Not hat people, are you?" talk, counts as Chekhov's Gun. It works, too.
- I Was Never Here: And he means it literally.
- Large Ham: Turlough in breakdown mode.
- Made a Slave: The captured humans.
- Mistaken for Spies
- Obstructive Code of Conduct: Time Lord rules about non-interventions go out the window early, for a change.
- One-Word Title
- Patrick Stewart Speech: Plantagenet delivers one in Episode 1, about how the humans' indomitable spirit will overcome the endless meteor bombardment.
- Plucky Girl: Norna.
- Purely Aesthetic Glasses: This story being written by the guy who gave the Fifth Doctor his specs, out they come again.
- Refuge in Audacity: Attempting to protect the Doctor, Turlough takes up the TARDIS' hatstand. Due to a single, small discharge of residual energy from it, the paranoid leaders believe it to be a lethal weapon. He does nothing to dissuade them from the notion.
- Ripped from the Headlines: The desolate, Blitz-like setting was influenced by contemporary reports of fighting in Beirut during the 1982 Lebanon War.
- Schmuck Bait: How the Doctor dupes the Gravis.
- Shout-Out: One in a deleted scene: two of the guards are called Kernighan and Ritchie, after the authors of The C Programming Language.
- The So-Called Coward: "Of course not...I'm Turlough."
- Think Nothing of It: Subverted. At the end, the colony thanks the Doctor for saving them, and he tells them, "Don't mention it." His companion Turlough clarifies, "He means it. Literally." i.e. the Doctor doesn't want the colonists to let the Time Lords find out about it.
- What Measure Is a Non-Human?: "You can't possibly take their side!"
- You Can't Fight Fate: Turlough's professed motivation for going back.