The one where Nyssa takes her skirt off.
The Doctor, Tegan, Nyssa, and Turlough visit the exact center of the universe. (How they measured this is anyone's guess, as it's like trying to find the center of a sphere's surface area; presumably Time Lords can see into one or more dimensions that make it possible, or maybe they were just using Apple Maps.) They find Terminus, a derelict old spaceship that's being used as a colony for the victims of the fatal Lazar's Disease. Which Nyssa has just been infected with.
She takes off her skirt (the script half-heartedly justifies it as "Nyssa feeling feverish", but even actor Sarah Sutton had it pegged as Fanservice) and proceeds to be taken into the custody of Terminus' enslaved employees, the Vanir. They leave her with the other Lazars until it's her turn to be treated by the Garm, a monster that looks like some sort of bulky wolfman with shiny red eyes.
There's a couple of Space Pirates aboard, too, and one of them has gigantic '80s Hair that nearly completely fills her standard-issue bubble-shaped space helmet. They're just there to give the Doctor someone to talk to, because Tegan and Turlough spend the entire story being stuck crawling through ventilation shafts and avoiding the ship's sterilization procedures.
Oh, and the old space hulk was responsible for creating the universe (one of its fuel pods fell back in time and kicked off the Big Bang), and it's now in danger of destroying it. Only by pulling a really heavy switch can the process be stopped. Nobody tell the Master about this thing, OK? The Garm ends up saving the day, and is set free, along with the rest of the enslaved crew.
For those keeping track, this is where Nyssa leaves. The good news is she survives the Decontamination Chamber's radiation treatment and is cured of Lazar's disease, but since the treatment procedure is a hit-or-miss, she nobly elects to stay behind on Terminus so she might help it become a proper hospital. She tearfully bids farewell to her friends, with a kiss on the cheek for the Doctor and a hug for Tegan.
And Turlough is still suspicious.
- Air-Vent Passageway: Tegan and Turlough spend an inordinate amount of time in them.
- Are You Pondering What I'm Pondering?: She would prefer not to.Turlough: If ever you had to kill someone, could you do it?
Tegan: No. Don't know. If it was important. Save a friend, defend my life.
Turlough: But cold-bloodedly?
Tegan: You're weird, Turlough.
- Anyone Can Die: Strangely enough, averted. despite the grim tone of the story and its themes of disease and death, it is one of only two stories from season 20 in which no-one dies.
- Bland-Name Product: Unwittingly inverted; the announcer aboard the ship was credited as "Tannoy Voice," which unbeknownst to the production team is actually a trademark of the Tannoy company rather than a generic term for a PA system, resulting in the BBC being threatened with legal action.
- Caused the Big Bang: Always remember to secure the fuel pods on your spaceship, so they don't fall back in time.
- Chewing the Scenery: All of the secondary characters chewed the scenery a fair bit.Olvir: This is a lazar ship, where all the lepers come to die! We're all going to DIE!
- Cloudcuckoolander: Bor
- Compliment Backfire: No version of "You're so sweet when you get angry" is likely to go over well, schoolboy.
- Continuity Nod:
- Turlough inherits Adric's old room, last seen in Part One of "Earthshock", the story in which Adric was killed off. It's virtually untouched and still contains a caduceus necklace from the Kinda tribe and the Terileptil android's skull mask from its "Death" costume. In addition, the star chart from "Logopolis" is visible in the back of the room.
- The Doctor uses the term, "Event One", to describe the Big Bang.
- Darker and Edgier: A dark serial of the dreary and depressing kind.
- The '80s: Feast your eyes on those space pirates - the '80s Hair! The Shoulders of Doom!
- Everybody Lives: And that's probably the weirdest thing about this story.
- Fire-Forged Friends: Tegan and Turlough don't quite get that far, but they do make it to Fire-Forged Grudging Mutual Toleration.
- Gentle Giant: The Garm. He's a good dog.
- Getting Hot in Here: Supposedly the reason Nyssa takes her skirt off.
- Go Out with a Smile: Bor spends most of the serial dying, but seems cheerfully resigned to it.
- I Choose to Stay: Nyssa elects to stay behind to help others.
- Musical Nod: Nyssa's leitmotif from "The Keeper of Traken" is played over the scene in which she declares her intention to stay behind on Terminus.
- No Indoor Voice: The Black Guardian, who apparently doesn't think making your mole conspicuous might in any way get in the way of success.
- One-Word Title
- Panty Shot: Nyssa.
- Parent Service: Nyssa's skirt falls off for no explained reason. Though official sources gave various explanations for this — either the writer only had her removing her belt, which had to be changed due to Nyssa getting a new costume that didn't have pants earlier in the season, or the script editor deleted a line explaining she was dropping her skirt because of stomach pains — years later Sarah Sutton admitted it was purely an act of fanservice (and her way of saying farewell to her character's fans) in a number of interviews after the episode aired.
- Personal Effects Reveal: Apparently Adric was quite a magpie, as his old room is chock-full of Continuity Nods including his pirate outfit from "Black Orchid" and a double-helix necklace from "Kinda."
- Portal Door: This story reveals that in emergency, the TARDIS can materialise as a door in a two-dimensional wall instead of as a three-dimensional object.
- Punch-Clock Villain: the Vanir are all enslaved criminals who only are obstacles to the Doctor and Co. because they think they'll keep them from getting their supply of hydromel. When Nyssa says she can synthesize it, they gladly say they'll work for her.
- Recycled Soundtrack: A good chunk of the episode's incidental music is recycled from the season opener, "Arc of Infinity."
- Shoot Out the Lock: The Doctor uses a laser pistol to shoot the lock off the Hydromel case at the end of the story.
- Sickly Green Glow: Hydromel
- Skeletons in the Coat Closet: The Vanir wear suits of armor resembling skeletons.
- Space Pirate: Kari and Olvir, who board what is seriously the wrong vessel to attack.
- Theme Naming: Mostly from Norse mythology.
- Wardrobe Malfunction: According to lore, Tegan has a few problems like this. (She and Nyssa were both very obviously not wearing bras around this point.)
- Waif-Fu: Nyssa, who is one of the smallest companions, manages to overpower Valgard, who is wearing armor, by pushing his head, knocking him back onto some stairs.
- Withholding the Cure: Terminus Incorporated pays the Vanir in hydromel, which keeps them immune to Lazar's Disease and incidentally, bound to working on Terminus. After examining the stuff, Nyssa announces she can synthesize it, freeing the Vanir from the control of the company.
- Vapor Wear: Nyssa is very obviously not wearing any bra under her skimpy white top, another bit of Fanservice in addition to the infamous skirt removal.