The one where JNT fanboyed so hard about the Queen of England that he completely knackered the timeline of U.N.I.T. in the process.
Brigadier Lethbridge-Stewart has retired to teach maths at a boarding school in the English countryside, but he has his hands full there, too. Two schoolboys steal his car for a joyride; when it crashes, one of them — Turlough — has a vision of a man dressed all in black, who offers Turlough his heart's desire — passage off Earth (and back to his home world) — and all he has to do is kill some guy named "the Doctor"!
Aboard the TARDIS, Tegan is still freaking out over her experience with the Mara. Nyssa and the Doctor try to reassure her, but they aren't at it for very long before they nearly collide with a ship that looks oddly like a pointy clamshell. The Doctor avoids a collision only by dematerializing the TARDIS and rematerializing it aboard the other ship.
The clamshell ship is gaudily decorated in a sort of Art-Deco-by-way-of-Vegas way, but there's no one aboard, and nothing happening except that a teleportation capsule is listed as "en route". It contains Turlough, who sneaks into the TARDIS just in time to encounter the Doctor returning, whereupon he tries to look innocent in an "Oh, was this your spaceship?" way. For some reason, this works, and the Doctor enlists him to help figure out what the deal is.
Unfortunately, after the Doctor and Turlough leave for 1983 Earth, the TARDIS randomly decides that 1977 would be a great time to visit. Nyssa and Tegan wander around 1977, wondering what's going on; meanwhile, in 1983, the Doctor and Turlough eventually run into the Brig, delighting the Doctor but freaking the hell out of the Brig, who says he's never met him and doesn't know nothing about no aliens, and would he please quit babbling about UNIT because it's a top-secret organization. Now we know where Torchwood learned how to fly below the radar. The Brig's memory is eventually jogged, and he confesses to having had a nervous breakdown six years ago. Shortly after meeting a young Australian woman named Tegan.
Tegan, in 1977, finds 1977-Brig and brings him back to the TARDIS, where there's a badly burned guy they think is the Doctor. (It isn't, as is shown when the action cuts back to the Doctor and 1983-Brig, but the girls don't know that yet.)
The burned guy says his name is Mawdryn; the Art Deco spaceship is his, the seven other guys in suspended animation are his buddies, and they'd really really like the Doctor's eight remaining regenerations so they can put an end to their tortured lives. The Doctor refuses; the undead guys guilt-trip him, then end up triggering some time fluctuation contamination on Tegan and Nyssa, which they swear they'll knock off as soon as the Doctor gives them his lives.
The Doctor struggles to control the time fluctuations, as it washes back and forth over his companions and regresses Nyssa and Tegan to child-Nyssa and child-Tegan. Even though they are, basically, the cutest things ever (certainly the most adorable companions until wee Amelia), the Doctor prepares to sacrifice himself to save them... but the colossal energy that was to come from the Doctor's eight deaths comes instead from 1977-Brig and 1983-Brig meeting up at last and shorting out the time differential. 1983-Brig remembers just what caused his nervous breakdown. They drop the Brigs off in their respective times, and the Doctor continues on — with Turlough added to the crew, over Nyssa and Tegan's weary looks.
This story threw a colossal spanner into the works of dating the UNIT stories, since during the 1970s it had been strongly implied that the stories from "The Invasion" to "The Web of Fear" had been set a few years into the future compared to their broadcast dates. (This would finally get cleared up in 2013, when it was revealed that UNIT has various dating protocols.) The sad thing is, this was totally unnecessary — the serial was originally to feature First Doctor companion Ian Chesterton as the returning companion, but when his actor was unavailable it was changed to the Brig. Though it was suggested that the script change its scripted dates from 1977 and 1983 to 1983 and 1989 to avoid continuity errors producer John Nathan-Turner refused simply because he thought it'd be cool to set the earlier part of the story in the Queen's silver jubilee year.
- All There in the Script: The Headmaster's name, according to the script, is Mr Sellick.
- The script suggests that the Doctor takes Turlough on because he misses Adric.
- Anti-Villain: Mawdryn and his pals aren't evil by any means, they just want an end to their Fate Worse than Death. That being said, Mawdryn has no problem with the idea of the Doctor being forced to use up all his regenerations to help them die, and while he makes it clear that he didn't mean to infect Nyssa and Tegan, he certainly doesn't consider the result to be a bad thing.
- Astral Checkerboard Decor: Turlough has an out-of-body experience following a car crash, and finds himself in a mysterious void with a checkered background, being recruited as a pawn in a Cosmic Chess Game. The optional CGI effects on the DVD replace this background with a more atmospheric (and less garish) cloudy void.
- Bedsheet Ladder: Turlough uses one to escape from the school sickbay.
- Bigger on the Inside: The teleport capsule — a first clue that the people who built it have knowledge of Time Lord technology.
- Boarding School: Brendon College
- Body Horror: The top part of Mawdryn's skull is missing, exposing his brain.
- Nyssa and Tegan suddenly developing the hideous traits of Mawdryn's mutation is jarring, to say the least.
- Borrowed Catchphrase: The Doctor says, "If I reverse the polarity of the neutron flow...", as sometimes said by the Third Doctor.
- The Bus Came Back: The Brigadier returns to the series after an eight-year absence.
- Card-Carrying Villain: "In the name of all that is evil". Seriously, Black Guardian?
- "I am the Black Guardian. His good is my evil."
- Cast from Hit Points: The title character and his seven companions were caught by the Time Lords while attempting to discover the secrets to their regeneration ability, and are punished by being granted a never-ending cycle of imperfect regenerations. Mawdryn tells the Doctor that the only way for them to die and end their torment would be for him to give each of them a surge of temporal energy taken from his remaining regenerations this being his fifth incarnation, he'd have none left for himself. Fortunately, the temporal discharge resulting from the Brigadier coming in contact with himself from a different era is enough to avert this.
- Chekhov's Gun: What would happen if the Brigadiers met each other
- Clingy MacGuffin: When Turlough tries to throw the Black Guardian's crystal away, it sticks to his hand.
- Continuity Cavalcade: During the Brigadier's flashback he sees Yeti (The Web of Fear), Cybermen (The Invasion), the Second Doctor (The Three Doctors), the Axons (The Claws of Axos), Daleks (Day of the Daleks), the Third Doctor (Spearhead from Space), the First Doctor (The Three Doctors), the K1 robot (Robot), a Zygon (Terror of the Zygons), the Fourth Doctor, and finally himself from The Three Doctors. All of the clips were shown in sepia-tinted black & white.
- The Brig also reveals what happened to Benton and Harry. The former left the army and became a car salesman and Harry got transferred to NATO, "Doing something hush hush at Portland Down".
- Continuity Nod:
- Tegan is still unsure if she is finally free of the Mara.
- When Tegan expresses doubt that Turlough could've just walked right into the transmat pod, Nyssa points out that Tegan herself had done just that to the TARDIS when she joined up with them.
- Nyssa mentions that she wishes the Zero Room hadn't been destroyed so that Mawdryn, believed to be the Doctor, could properly regenerate.
- When Nyssa and Tegan are caring for a decrepit Mawdryn, he is given the red greatcoat that the Fourth Doctor wore in his last season. Mawdryn later proceeds to wear this coat for some time.
- Contrived Coincidence: The Doctor lampshades the amount of coincidences the plot hinges on, not least of which is the two Brigadiers running into each other at the precise millisecond such an event would be most helpful.
- Dark and Troubled Past: Turlough clearly has this from the outset, but it won't be more than hinted at until his departure more than a season hence.
- Deal with the Devil: The Black Guardian's offer to Turlough: Passage off Earth in exchange for killing the Doctor.
- Death Seeker: Mawdryn and his buddies all just want to die.
- Deceptive Disciple: Turlough, in just about all directions.
- Dull Surprise: The Doctor, in response to Turlough's "oh, is this your spaceship?" appearance.
- Establishing Character Moment: Turlough's first scene has he and a friend steal the Brigadier's car.
- A Fate Worse Than Death: Mawdryn and his crew got the ability to regenerate-but not how they wanted it...
- Flying Dutchman: Cursed to sail the skies for eternity, though once every seventy years one of them can go to the nearest planet to find help.
- Friendship Moment: After Mawdryn dies and the day is saved, Tegan makes a point of sincerely thanking The Doctor for being prepared to sacrifice himself for her and Nyssa. She doesn't snark once.
- Future Me Scares Me: Invoked and averted, insofar as the Brig seems pretty self-compatible, if somewhat freaked out to be doubled up.
- Ghostly Glide: Mawdryn's seven immortal companions move like this.
- Gone Horribly Wrong: Mawdryn's eternal cycle of mutative regeneration, until the Doctor ended it. Though he deserved it.
- Heroic Sacrifice: Narrowly avoided, but The Doctor is more than willing to give up his remaining regenerations to save Nyssa and Tegan. Tegan especially is very touched by this.
- Human Aliens: Mawdryn and company are essentially humanoid, only with exposed brains and eyebrows that can be swept back into their hair. They are still alien enough to give Tegan pause.
- Ironic Hell: Mawdryn and his friends sought the secrets of Time Lord regeneration and became doomed to immortality through eternal regeneration.
- Large Ham: The Black Guardian is an entire barnyard full of ham.
- Logic Bomb: "If I had amnesia, I would be the first to know."
- Meaningful Name (and/or Bilingual Bonus, plus possibly Department of Redundancy Department and This Is My Name on Foreign): "Mawdryn" is Welsh for "undead".
- Monkeys on a Typewriter: Discussed by Tegan and the Doctor.
- My Brain Is Big: Mawdryn and the other brainiacs.
- Never the Selves Shall Meet: Subverted. ZAP! It turns out to be just what was needed.
- No Indoor Voice: The Black Guardian. See the page quote, among others.
- Obvious Second Choice: The Brigadier's role in the story was originally meant for Ian Chesterton. This would have made more sense given the school setting and could have avoided that whole continuity issue.
- Only the Knowledgable May Pass: When the Doctor meets up with the amnesiac Brigadier, teaching at a boys' school, the Brigadier's attitude undergoes an abrupt change when the Doctor mentions UNIT he still doesn't know him, but anyone with sufficient secret clearance to mention it ought to know better than to talk like that.
- The Precious, Precious Car: Turlough convinces his classmate Ibbotson to join him in a joyride in the Brigadier's priceless antique car (a blue 1929 Humber 16/50 open tourer, Imperial model), which they accidentally veer off the road and crash. Being the decent chap that he is, the Brigadier is persuaded not to press charges.
- Properly Paranoid: Tegan is far from convinced by Mawdryn's claims of being a recently-regenerated Doctor, even when Nyssa and the Brigadier insist otherwise. Naturally, she is proven correct. She is also highly suspicious of Turlough, and she'll eventually be proven right on that front, too.
- The Punishment Is the Crime: The punishment for seeking immortality is immortality itself.
- Retired Badass: The Brigadier
- Same Character, but Different: The Brigadier appears in a story that was originally intended for science teacher Ian. The solution? Remove all references to Ian, and make the Brigadier a maths teacher.
- Stable Time Loop
- Stay in the Kitchen: The Brigadier very firmly insists that Tegan and Nyssa stay in the TARDIS while he searches Mawdryn's ship for The Doctor. Naturally, Tegan doesn't take well to this, and eventually follows after him.
- Timeshifted Actor: The girls playing age-regressed Nyssa and Tegan. Child-Nyssa — Lucy Benjamin — grew up to play Lisa in EastEnders.
- Unexplained Accent: Child-Tegan has British accent, not an Australian one.
- Who Wants to Live Forever?: Mawdryn and his comrades certainly don't. At least, not anymore.
- Wolf Whistle: A bunch of rowdy schoolboys whistle appreciatively at Tegan when she runs up to them. Luckily for them, she is too distraught over having lost The Doctor to chew them out for it.
- Virtual Reality Interrogation: Combined with Dream Weaver. In episode 2, Turlough thinks he's asking his headmaster for moral guidance. When he resolves not to kill the Doctor, the headmaster turns into the Black Guardian — it's a dream sent by the Guardian to test his loyalty.