[[caption-width-right:350:Huh, [[CompletelyMissingThePoint Sarah did something with her hair...]]]]
->''"Is that finger loaded?"''
-->-- '''The Doctor''' sizes up the threat.
The TARDIS lands in a typical English village that Sarah recognises as Devesham, home of the British Space Defence Station. However, it soon becomes obvious that things are not all as they should be, and the Doctor and Sarah realise that the village is a replica populated with androids.
An alien race -- the Kraals -- are aided by a brainwashed human astronaut, Guy Crayford, and have built the replica to rehearse for an invasion of the Earth. Styggron, a Kraal scientist, has perfected a lethal virus and plans to use his androids to disseminate it around the world, paving the way for the Kraal fleet.
Crayford, whose rocket had been damaged and who was rescued by the Kraals, returns to a hero's welcome. But with the Brigadier off in Geneva, no-one at UNIT believes the Doctor when he tells tall stories of android replicas -- which now include himself, Harry and RSM Benton. The Doctor manages to use the radio equipment at the base to jam the android control signal and stop Styggron releasing the virus. He then uses his own android replica to fight Styggron, who infects himself with the virus in the process and is killed.
* AllThereInTheManual: The novelisation clears up the PlotHoles, such as what happened to Chedaki.
* AllThereInTheScript: According to the script, Corporal Adams' first name is Richard.
* ArmCannon: See quote at the top of the page.
* BackFromTheDead: One of the many clues that something is terribly, terribly wrong in town is the fact the Doctor and Sarah see Corporal Adams up and about... despite him having just run off of a cliff earlier and had no pulse when they checked him.
* BondVillainStupidity: Styggron on at least two occasions.
* TheBusCameBack: Harry Sullivan and Sergeant Benton make their final appearances in ''Doctor Who''.
** The Doctor gets off a good one when tied to a post and left next to a bomb.
-->'''Doctor''': "Don't go. Stay, then we can [[BlackComedy all go together!]]"
** Sarah herself gets a good one in the pub, when talking to the android Adams, telling him to make sure he doesn't fall.
* DifferentlyDressedDuplicates: Androids impersonate the Doctor and Sarah in each other's presence on two separate occasions. The Doctor realizes that the Sarah duplicate is not real because she was wearing a scarf that the real Sarah lost.
* DeathGlare: Android Doctor has a terrifying one when Android Bentor shoots him, thinking he was the Doctor.
* DistressedDude: Sarah busts the Doctor out of his cell.
-->"This time I'm saving you!"
* DoesNotLikeSpam: Sarah Jane apparently hates ginger pop. It's how the Doctor recognizes her ginger pop-drinking android impostor.
* EvilRedhead: The android Adams. The real one, though, being a dutiful UNIT soldier, is a good redhead.
* EyepatchOfPower: Crayford wears one. It later turns out he doesn't need it.
* TheFuture: After many stories in which the original idea of the UNIT era being in the near future had been almost forgotten, the last but one of the 1970s UNIT stories fully revives it, being set in a world where the UK has a major functioning space programme, "freighter" flights out as far as the asteroid belt are routine, and crewed exploration journeys have reached Saturn at least.
* GambitRoulette: Styggron's plot is almost as ludicrous as the way he goes about executing it. The DVD production note subtitles helpfully point out all of the holes in his plan and all the baffling and counter-intuitive moves he makes. Most fanon and ExpandedUniverse suggests Styggron didn't even understand his own plan.
* HighVoltageDeath: Sarah busts out of her cell by taking a cable, waiting for Android!Adams to come in, and then frying him with it. He explodes quite spectacularly.
* HoistByHisOwnPetard: Styggron ends up getting killed by the virus he intended to infect humanity with.
* ImpersonatingTheEvilTwin: The Doctor pretends to be his android double and successfully fools the other androids. He also succeeds in reprogramming his double to assist him, a fact which is only revealed after the double has done a Heroic Sacrifice in front of his horrified companion.
* LargeHam: Two Creator/TomBakers? It's amazing any scenery was ''left''.
** Styggron has his moments, too.
* LikeAnOldMarriedCouple: Styggron and Chedaki, full stop.
* LiteralCliffhanger: When Sarah Jane and the Doctor are fleeing from the androids, she ends up falling and dangling off of a cliff.
* MeaningfulEcho: "Don't be a fool, Benton, I'm one of you." First said by the real Doctor when he bluffs the androids, then by the Android Doctor himself.
* NoisyRobots: Generally averted, since the androids are intended to be perfect replacements. The only one who does make any of the typical mechanical whirring and clanking noises is the malfunctioning Adams (the UNIT soldier who does the really spectacular cliff dive), and that's only because there's something wrong with him.
* NotHerself: Sarah Jane apparently hates ginger pop. It's how the Doctor recognizes her ginger pop-drinking android impostor.
* OnlyAFleshWound: The Android!Doctor shoots Grierson, but it only seems to hurt him in his arm/shoulder area.
* OutOfCharacterMoment: The Doctor realizes Sarah Jane has been replaced when the android accepts the ginger pop he offers her, because he knows she hates ginger pop. In fact he figures it out the moment he sees her, because, as he put it, "The real Sarah wasn't wearing a scarf." Him giving her ginger pop is just his way of checking to make sure he's right.
* PlatonicCave: The fake Earth.
* PseudoCrisis: Episode Three has a cliffhanger where Sarah and the Doctor are getting blasted off into space and are overcome by the G-force... and then they escape the planet's gravity and everything's fine. Particularly sloppy since the scene immediately after this shows a pod opening to reveal an evil robot Doctor.
* RealLifeWritesThePlot: When a pack of dogs have his scent, the Doctor has to jump into a small pond to evade them. Creator/TomBaker performed the stunt for real while on location, and he got a minor throat infection from accidentally taking in some of the water. It wasn't too serious, but his voice sounds a little bit off thereafter.
* RedemptionEqualsDeath: Crayford.
* RobotMe: The android replicas include the main characters.
* RoboticReveal: When the Doctor works out android "Sarah" isn't the real one, she pulls a gun on him. He tackles her to the ground, and her face comes off, revealing the inner workings of the android. (See image).
* ServileSnarker: Chedaki is an evil version.
* ShoutOut: The Doctor says to Sarah, "Listen, once upon a time, there were three sisters, and they lived in the bottom of a treacle well. Their names are Olga, Masha, and Irina." This is a mash-up of lines from ''Literature''AliceInWonderland'' and Creator/AntonChekhov's ''Tre sestry''.
* SpoilerTitle: One of the worst of the many examples in ''Doctor Who'', which gives away something that is meant to be a mystery for two full episodes.
* SuspiciouslySimilarSubstitute: Colonel Faraday for TheBrigadier.
* SyntheticPlague: The Kraals plan to wipe the human race out with one.
* TableFlip: Done by the Doctor when escaping from Crayford's office.
* TwistedAnkle: Pleasingly subverted when Sarah goes down on her ankle while running away, the Doctor asks if she's alright, and she immediately is.
* UncannyValley: Almost certainly invoked with the robot duplicates, but especially the android Doctor, who acts almost exactly like the Doctor, but with ''just'' enough subtle malice that makes him profoundly creepy.
** Not to mention the pub scene, wherein the whole pub is filled with androids, just standing there, not moving. Then the clock goes off and they start acting like normal people.
* WakingNonSequitur: After being interrogated, the Doctor isn't exactly lucid.
-->'''Doctor:''' [[Literature/AlicesAdventuresInWonderland Once upon a time, there were three sisters, and they lived at the bottom of a treacle well. Their names were]] [[Theatre/ThreeSisters Olga, Masha and Iraina]]. Are you listening, Tilly?\\
'''Sarah-Jane:''' I'm ''Sarah''.
* WhatHappenedToTheMouse: We never see Benton again after he's knocked out by one of the duplicates. In fact, for many years some fans thought Benton had actually been killed in this story, until ''Mawdryn Undead'' revealed that he had retired from the army and become a used car salesman.
** Or Chedaki, for that matter. He doesn't go with Styggron to Earth. What happened to him?
** Creator/TerranceDicks noticed and cleared all this up when he wrote the novelisation.
* WholePlotReference: The story was inspired by ''Film/InvasionOfTheBodySnatchers''.
* YouHaveOutlivedYourUsefulness: Pretty much said verbatim by Styggron when he kills Crayford.
-->'''Stygron:''' I used you, Crayford, as I used the androids. But you are no longer of any value to me!
* YouSaidYouWouldLetThemGo / ILied