[[caption-width-right:350:An incredible badass. [[BaitAndSwitch And her boyfriend.]]]]
->'''The Destroyer:''' Pitiful. Can this world do no better than you as their champion?\\
'''Brigadier Lethbridge-Stewart:''' Probably. I just do the best I can.

Two great British institutions meet at last, as ''Series/DoctorWho'' delves into the KingArthur stories to begin the final season.

The Doctor and Ace follow a signal of some sort to arrive TwentyMinutesIntoTheFuture (a ShoutOut to the tradition of having UNIT stories set in a non-specific near future). They land near an archaeological dig somewhere in England and try to hitch a ride to the dig -- the source of the signal -- from a UNIT convoy passing through with a nuke ([[Recap/DoctorWhoS8E2TheMindOfEvil Not ones to learn from experience, are they?]]), but are left in the dust.

Fortunately, archaeologist Peter Warmsly, the guy in charge of the dig, is nicer and ''does'' give them a lift. Though he's just as baffled as they are about the whole UNIT nuke thing. UNIT, led by new brigadier Winifred Bambera, is equally weirded out by the people claiming to be "the Doctor" and [[ContinuityNod "Liz Shaw" (the only other UNIT pass the Doctor could find in his clothes)]], and decides to recall Brigadier Lethbridge-Stewart out of retirement and into active duty. The Brigadier's wife, Doris, is less than thrilled.

The Doctor, Ace and Warmsly repair to a [[MyLocal watering hole]] / hotel, where Warmsly bitches about the UNIT hardasses and Ace spends £10 on a lemonade. She also befriends a girl named Shou Yuing. Meanwhile, someone is performing a magic ritual of some sort that
# causes a sword scabbard to fly across the room and embed itself into the wall mere inches from Warmsly's alarmed face, and
# lets evil witch Morgaine enter our universe from a parallel dimension where the Arthurian legends are a bit real.

Then Pat the innkeeper hears his microbrewery go boom, and the crew rushes to investigate, where they find... a knight in armor. Who looks up at the Doctor and goes "Merlin!"

As it turns out, the Doctor will become known as Merlin at some point in a future regeneration, and he'll have to pretty much wing this adventure -- as he has no clue yet how the first half of it will go. He does find a lot of notes, hints, contraptions and archaeological evidence left for him by his future self. Also, the Brigadier brought Bessie!

The next day, the Doctor and Ace dragoon Warmsly into taking them down to the dig. There, the Doctor quickly translates a mysterious inscription as "Dig hole here"; which, he says, he knows, because it's in his own handwriting. Then he tells Warmsly to stand guard while he and Ace investigate the hole, which leads to a tunnel, which leads to a spaceship under the lake. It houses Excalibur, which Ace promptly decides to guard.

While Bambera and the knight (Ancelyn) get into a fist fight and subsequently get their flirt on, the Doctor sets about convincing Morgaine to stop trying to nuke the planet. Things get a little bit complicated when Morgaine decides to summon an EldritchAbomination to help her out -- but luckily, Brigadier Lethbridge-Stewart is willing to make a HeroicSacrifice and shoot it back to hell. He survives, too, because the writers love him (we do too, so it's okay).

* AdaptationExpansion: The novelisation adds a flashback sequence includes an unknown future incarnation of the Doctor, who has red hair and wears a camel-hair coat.
** There is an extra opening sequence of Bambera being summoned to Geneva to be breached on the nuclear missile transport. Excalibur's signal causes a storm, which is what grounds the convoy.
** Material from earlier drafts that was never filmed is included, such as Rowlinson going to help Lavel after the helicopter crash, the Brigadier taking charge of Lavel's remains and Doris attempting to get information on casualties. It also uses the idea of the Destroyer initially appearing as an aristocratic man in a 20th century business suit, with his shadow being that of his demon self.
** The Destroyer kills a group of soldiers who try to help Ace and Shou Yuing at the hotel.
* AffirmativeActionLegacy: Brigadier Bambera.
* AllThereInTheManual: The novelisation includes dates not given on screen: Walmsley's tax disc reads 30.6.99 (suggesting a setting of late 1998 or early 1999), while Liz Shaw's old UNIT pass expired on 31.12.75.
** There is extra background material of Morgaine and Mordred receiving Excalibur's signal, including Mordred being summoned from a tavern with his drinking partner.
** Mordred's initial companions are named as Sir Comus and Sir Madlamor.
** Ancelyn's back story is given: He was a Knight General in Morgaine's army who deserted because of an ancient family pledge to answer Arthur's call. He refuses to give UNIT information on Morgaine's army because of his old oaths.
** Shou Yuing is given the family name Li.
** Ancelyn is specified as the one who killed the Knight Commander.
** Rowlinson reveals he is a retired police officer.
* AlternateUniverse
* AmazonChaser: Ancelyn.
-->'''Bambera:''' If you don't start running, I'll kill you myself. Now come on!
-->'''Ancelyn:''' Winifred?
-->'''Bambera:''' What?
-->'''Ancelyn:''' Art thou betrothed?
* BattleCouple: Ancelyn and Brigadier Bambera develop into this pretty quickly.
* BellumInterruptum: The Doctor halts a battle by simply shouting "STOP!".
* BlackBossLady: Bambera.
* BlueAndOrangeMorality: Morgaine. Her big PetTheDog and KickTheDog moments occur back-to-back without her giving either a second thought.
* CasualKink: Ancelyn asking Bambera if she's single the day after their first meeting involved her beating him up and manacling him.
* CallBack:
** The Brig mentions [[Recap/DoctorWhoS20E3MawdrynUndead his teaching career]] and Sergeant Benton in his first scene.
** As a matter of tradition, the Brig offers to [[FiveRoundsRapid call in an airstrike]] and wipe the Destroyer out. When informed that "conventional weapons" are useless, he sighs, "No, I didn't think so."
** Doris was first mentioned way back in "[[Recap/DoctorWhoS11E5PlanetOfTheSpiders Planet of the Spiders]]".
** Mordred uses the Doctor's own words from [[Recap/DoctorWhoS25E2TheHappinessPatrol The Happiness Patrol]] against him. ''"Look me in the eye, end my life."''
** When UNIT doesn't recognize the Doctor, he says "[[Recap/DoctorWhoS5E5TheWebOfFear Yeti]], [[Recap/DoctorWhoS7E1SpearheadFromSpace Autons]], [[Recap/DoctorWhoS9E1DayOfTheDaleks Daleks]]! [[Recap/DoctorWhoS6E3TheInvasion Cybermen]] and [[Recap/DoctorWhoS7E2DoctorWhoAndTheSilurians Silurians]]!"
* CharacterFilibuster: Averted. The Doctor's anti-nuke speech originally ran for ''much'' longer and was wisely cut down.
* TheChessmaster: Played with. When the Doctor confronts Morgaine, she remarks that she could always beat him at chess when he was Merlin. He replies that he's not playing chess, but poker, and plays his Ace.
* CoolCar: Bessie, as always.
* CrazyPrepared: UNIT packs all sorts of ammunition due to their encounters with strange things -- high explosives for Yetis, armor piercing for robots, even gold-tipped bullets for "you-know-what". When the Doctor asks if they have silver bullets "just in case" (possibly recalling Mags the werewolf), the Brigadier asks his personnel about that, too. They certainly do, and it ends up saving the day.
* DestroyerDeity: Morgaine summons a demon called the Destroyer, which seeks to devour the world.
* EvenEvilHasStandards: Morgaine is genuinely guilt-stricken when she discovers her knights have been fighting UNIT next to a war memorial, and calls a truce to pay respect to the dead.
* FlashedBadgeHijack: The Brig.
* {{Foreshadowing}}: ''"He was supposed to die in bed"''
* GettingCrapPastTheRadar / GoshDangItToHeck: Brigadier Bambera's habit of saying "Oh, sh... ame" could arguably count as this. See also the CasualKink example above.
* GirlOfTheWeek: Shou Yuing for Ace.
* GodSaveUsFromTheQueen: Queen Morgaine, naturally.
* HaveWeMetYet: Morgaine has met the Doctor before, but this is the first time the Doctor has met Morgaine. That goes double for Ancelyn.
* ImprobableWeaponUser: Twice, the Doctor manages to pin Ancelyn's arm with the crook of his umbrella. The first time, he even uses it to threaten Anecelyn with his own sword.
* JediMindTrick: Used by the Doctor to get civilians out of the danger area.
-->'''Doctor:''' You're very angry.
-->'''Pat:''' Of course we're angry.
-->'''Doctor:''' And you want to leave.
-->'''Warmsly:''' No, we do ''not'' want to leave!
-->'''Doctor:''' Of course you want to leave ''[Gives Pat a Look]''.
-->'''Pat:''' Of course we do.
-->'''Doctor:''' I wouldn't stand for any nonsense, if I were you.
-->'''Warmsly:''' Look, Doctor, the situation is perfectly simple. We are very angry and we... ''[The Doctor gives him a Look]'' ...want to leave.
* LamePunReaction: Ace is not impressed by the Doctor's joke about having "an Ace up my sleeve".
* LargeHam: Pretty much everybody from the Arthurian universe.
* MeaningfulName: Winifred is germanified version of Guinevere. (Ancelyn is the original, Celtic version of Lancelot.)
* MistakenForQuake: Morgaine's arrival in our universe.
* MoreExpendableThanYou: TheBrigadier says this word for word when he takes the Doctor's weapon by force and goes to face the BigBad.
* {{Not H|imself}}erself: Under Morgaine's HatePlague spell, Ace actually spews a racial slur. That is how she realizes she and her friend are being played against each other.
* NoNameGiven: We never hear Shou Yuing's family name -- [[NamedByTheAdaptation the novelisation gives it as Li]].
* OneWordTitle
* OpenSesame: After encountering a locked door in a spaceship dating from the time of King Arthur, the Doctor tries ordering it to open up, on the off-chance that it will recognise him as Merlin too. It works.
* OutOfCharacterAlert: Behind the scenes-example: When noticing that the glass in the water-tank that held Creator/SophieAldred was about to burst and pour the water onto the studio floor, potentially electrocuting her when it touched the live electric cabling, Creator/SylvesterMcCoy had an split-second to warn everyone and he knew that he had to say something that everyone instantly knew was serious and not him as the Doctor Ad-libbing. What did he say?
--> ''[[PrecisionFStrike SHIT!!]]'' -- somebody GET HER OUT!
* PaperBagPopping: How the Doctor wakes Ancelyn and Bambera from their SleepCute.
* PetTheDog: Morgaine cures Elizabeth's blindness. Admittedly it was because Mordred had all but drunk the bar dry and she probably felt obligated to pay them ''somehow,'' but restoring someone's sight is a lot better than ''some'' of the ways you could repay them when you're an evil witch.
** On the other hand, it comes right after a KickTheDog moment where Morgaine kills and disintegrates Lt. Laval (who was already badly hurt in the helicopter crash), just so that she can absorb Laval's mind and find out more about the situation.
** Elizabeth is so confused by the MoodWhiplash she is left sobbing, "She killed that girl and cured my eyes," over and over trying to process it.
* PinPullingTeeth: One of Morgaine's knights.
* PunctuatedForEmphasis: [[LargeHam "THERE. WILL. BE. NO BATTLE HERE!!!"]]
* ReCut: The Special Edition adds a lot of unbroadcast scenes.
* RidiculousFutureInflation: Lemonade ''still'' does not cost £10, thankfully.
* RummageFail: The Doctor pulls all kinds of ridiculous things out of his pockets while looking for his old UNIT pass.
* SlapSlapKiss: Ancelyn and Brigadier Bambera's relationship.
* SleepCute: Ancelyn and Bambera.
* SpikesOfVillainy: The evil knights have spiky decorations on their armor; the heroic Ancelyn doesn't. (This does help in keeping track of who's who when they're fighting with their visors down.)
* SwordOverHead
* TakeTheWheel: With the added detail that the passenger is from an alternate universe where they don't have cars, and the driver takes the time to confirm that this is the case before telling him to take the wheel anyway.
* TemptingFate: In the first scene, the retired Brig assures his wife that he doesn't miss his UNIT days at all. "My blood-and-thunder days are long past." Cue plot.
* ThisIsNoTimeToPanic: The Doctor, at the end of Episode 2. A few seconds later, he adds: "''[[YouCanPanicNow Now]]'' [[YouCanPanicNow we panic]]!"
* ThrowingOffTheDisability: Elizabeth is blind until she meets Morgaine.
* TimeyWimeyBall: One of the few instances in the classic series where the plot reaches Creator/StevenMoffat levels of time travel confusion. A future regeneration of the Doctor has already done the first half of the plot, and Seven has to rely on clues from his future self to complete the second half. Every single character from the other dimension has already met the Doctor, but he's got no idea what he's done (er, going to do) to provoke them.
* ATruceWhileWeGawk: There's a moment in Mordred and Ancelyn's final duel where they break apart for a moment -- and the Doctor walks right between them, casually tipping his hat as he passes, on his way to his own final confrontation with Morgaine. Mordred and Ancelyn stare after him in utter bemusement for a moment, before remembering that they were in the middle of fighting to the death.
* TwentyMinutesIntoTheFuture: The [[Literature/DoctorWhoNovelisations novelisation]] established the date as mid-1997. However, in this story the UK has a King as monarch and the five pound note has been replaced by a coin, neither of which has actually happened yet. [[note]]While [[https://en.wikipedia.org/wiki/Five_pounds_%28British_coin%29 officially minted £5 coins]] do exist, they're collectors' pieces not intended for general circulation.[[/note]]
* UncoveringRelationshipStatus: Ancelyn blurting out "Art thou betrothed?" to Bambera.
* VolleyingInsults: Ancelyn and Mordred:
-->'''Ancelyn:''' Is your ''army'' not enough to give you ''courage''?
-->'''Mordred:''' Courage? To face you, Ancelyn, who fled the field at Camlann? Ancelyn the ''Craven'' I call you!
-->'''Ancelyn:''' What care I for the words of a ''half-man'', who ''cowers'' at a woman's wrath?
* VulnerableConvoy: [=UNIT=]'s nuke.
* WarMemorial: Carbury has a memorial to the townspeople killed in the World Wars. When Morgaine and her men encounter it, they stop to pay tribute to the warrior dead, and Morgaine gives Mordred a dressing-down for having assumed their opponents to be savages with no concept of honor.
* WeirdCurrency: When the Doctor tips out a handful of change, there's a Franchise/{{Zoid|s}} in among it, which he describes as "a very valuable piece of coinage".
* WhatAPieceOfJunk: Ace's and Shou Yuing's initial reaction to Bessie. As per the trope, they change their tune once they see what it can do.
* WriteBackToTheFuture: The Doctor's notes.
* WorldOfHam