[[caption-width-right:350: Not pictured: Horrifiying noise courtesy of The Great Old Ones' grandfathers.]]
->''"You're an evil man, Vaughn. You're sadistic. You're a megalomaniac. You're insane. I pity you. But if I get half a chance, I'll kill you."''
-->-- '''Professor Watkins''' tells it like it is.

The TARDIS arrives in UsefulNotes/{{London}} in the near future, the Doctor and companions are enlisted by Isobel Watkins to find her uncle, a computer scientist working for International Electromatics.

IE's director, Tobias Vaughn, arouses the Doctor's suspicions - confirmed by meeting Lethbridge-Stewart again (see "[[Recap/DoctorWhoS5E5TheWebOfFear The Web of Fear]]"), recently promoted to Brigadier, who says that numerous other IE staff have gone missing. Lethbridge-Stewart now heads the British arm of UNIT, the United Nations Intelligence Taskforce, charged with investigating alien sightings and other such things.

The Doctor discovers that Vaughn is acting on behalf of the Cybermen, who plan to invade Earth, but is also planning to double-cross them and seize power for himself. The Cybermen send a hypnotic signal through all IE-made equipment, immobilising most of Earth's population, and start emerging from London's sewers to take over. However, the Doctor has managed to protect himself and the others from the signal and succeeds in defeating the invasion, thanks in no small part to Zoe's phenomenal brain, plotting guidance trajectories for nuclear missiles to destroy the incoming fleet.

"The Invasion" presaged the shape of many ''Doctor Who'' stories to come, specifically the [[Creator/JonPertwee Third Doctor]]'s era, featuring an alien invasion of Earth aided and abetted by a corrupt industrialist, with UNIT providing cannon fodder and the military connections to fight back.


* AffablyEvil: Tobias Vaughn. Very courteous, even to trespassers, as long as one is not hindering his plans. [[LargeHam But when he gets upset...]]
* AnimatedAdaptation: The two missing episodes were remade in an animated format for the DVD release.
* CallBack: There are numerous references back to the events of "The Web of Fear" and many mentions of Professor Travers and his daughter Ann. When Lethbridge-Stewart first appears, the Doctor refers to him as "Colonel" before being corrected.
* CorruptCorporateExecutive
* CutAndPasteEnvironments: The same set is used for Vaughn's office in two different buildings (with a different view out of the window). {{Lampshade|Hanging}}d.
* CyberneticsEatYourSoul: Averted. Unlike the usual plot scheme, Vaughn is positively thrilled to have been given a cyborg body, and doesn't lose any of his personality.
* EvilIsNotAToy
* {{Expy}}: Professor Watkins and his niece Isobel, the SuspiciouslySimilarSubstitutes written in when the actors of [[Recap/DoctorWhoS5E2TheAbominableSnowmen Professor Travers]] and his [[Recap/DoctorWhoS5E5TheWebOfFear daughter Anne]] were unavailable for filming. Professor Travers and Anne are [[PutOnABus said to have moved to America]], and Professor Watkins and Isobel fill their shoes ''extremely'' well. This is quite possibly {{lampshade|Hanging}}d by the fact that the Doctor and companions mention repeatedly that they were hoping to meet Travers and Anne before finding out that the pair are gone, even though they are otherwise unimportant to the story.
* ForInconveniencePressOne: Written in 1969, a rare example of ''Doctor Who'' predicting the future effectively.
* GettingCrapPastTheRadar: Vaughn isn't exactly clear on what Packer will do to Isobel or Zoe if they're turned over to him, but it's as strongly implied as it can be in ''Doctor Who'' that they'll be raped.
* IdiotBall: Zoe, mainly to annoy Jamie, agrees to Isobel's insanely dangerous plan of photographing the Cybermen. Unlike Isobel, she's encountered the Cybermen before and ought to know better.
* ImmuneToBullets: Vaughn, thanks to voluntary partial Cyber-conversion.
* KidnappedScientist: Professor Watkins.
* LargeHam:
** Viewers may remember Kevin Stoney from his former role as [[Recap/DoctorWhoS3E4TheDaleksMasterPlan Mavic Chen.]] His hamminess has seen no decrease whatsoever.
** Vaughn's assistant Packer also qualifies; he takes so much pleasure in his actions that he constantly looks like he's holding back a snicker.
* LogicBomb: Zoe uses ALGOL to blow up the computer receptionist.
* MadEye: Vaughn's right eye is half-closed through most of the serial ([[RealLifeWritesThePlot due to injuries Kevin Stoney had suffered in a car accident]]), then opens wide when he turns against the Cybermen.
* MagicSkirt: Well, kilt - the girls' skirts [[PantyShot definitely don't do this]], but Jamie's kilt stays in place. It's not magic, though - it's weighted.
* ModestyShorts: A photograph taken while filming shows Sally Faulkner (Isobel) wore shorts under her dress for the scene where she's rescued by the helicopter. The animators reconstructing that sequence [[PantyShot had other ideas]].
* MultinationalTeam: UNIT
* PantyShot: The two girls start in very short dresses (Isobel was shooting pictures using Zoe as a model) and stay that way for three-quarters of the episode - which does not make running from a private army or climbing up and down ladders easy to do modestly. They fail.
* PutOnABus: A BBC episode intro by the actor who plays Vaughn reveals that the roles of Watkins and his niece Isobel were originally written for [[Recap/DoctorWhoS5E2TheAbominableSnowmen Professor Travers]] and his [[Recap/DoctorWhoS5E5TheWebOfFear daughter, Anne]]--but the actors weren't available at the time of filming, so Professor Travers and Anne are said to have moved to America, and [[SuspiciouslySimilarSubstitute Professor Watkins and Isobel]] fill their shoes.
* RankUp: Colonel Lethbridge-Stewart is now TheBrigadier.
* RedemptionEqualsDeath - Vaughn. Well, sort of, since he claims he just wants revenge.
* SeriesContinuityError: The animated first episode depicts Zoe wearing the clothes that she later gets from Isobel, when she should be still wearing her glittery catsuit from "The Mind Robber". This may have been an error, or may have been due to a desire not to have to create a second character model.
* SpoilerTitle: Unusually for Classic ''Who'', averted. A BBC episode intro by the actor who played Vaughn reveals that the serial was originally to be named "Invasion of the Cybermen", but was shortened to avoid spoiling the Cybermen, who don't appear until halfway through the 8-episode serial.
* StockEpisodeTitles: 40 uses
* SurroundedByIdiots: Vaughn has his incompetent henchman Packer -- unusual given his stated desire for efficiency.
* SuspiciouslySimilarSubstitute: A BBC episode intro by the actor who plays Vaughn reveals that the roles of Watkins and his niece Isobel were originally written for [[Recap/DoctorWhoS5E2TheAbominableSnowmen Professor Travers]] and his [[Recap/DoctorWhoS5E5TheWebOfFear daughter, Anne]]--but the actors weren't available at the time of filming, so Professor Travers and Anne are [[PutOnABus said to have moved to America]], and Professor Watkins and Isobel fill their shoes.
* TalkingTheMonsterToDeath: Zoe talks IE's computer receptionist to death with a LogicBomb. Vaughn finds it ActuallyPrettyFunny.
* TakeOurWordForIt - Partway through the story, the Doctor says that it's time that they rescued Professor Watkins. The ''very next shot'' features one of Vaughn's men describing in detail the incredible, action-packed rescue operation that UNIT carried out, yet we never got the chance to see (it was scripted, but there wasn't enough time to film it).
** Ian Marter, who wrote the novelisation of the story, admitted that he was tempted to play the scene for laughs and reveal that all that had really happened was that the Doctor distracted Watkins' guard for a few moments while Benton rescued the professor, with the whole "rescue operation" being made up by the guard in the vain hope that Vaughn wouldn't execute him for his failure. In the end though, Marter kept the scene as it had originally been written.
* TwentyMinutesIntoTheFuture: Takes place 4 years after "[[Recap/DoctorWhoS5E5TheWebOfFear The Web of Fear]]" which would theoretically put it in the very early 70s. (See that article for more, and for the beginnings of the infamous "UNIT Dating Controversy".)
* UncannyValley (InUniverse): The Doctor notices that Vaughn doesn't blink as often as a human should, a sign that he has a cyborg body.
* UtopiaJustifiesTheMeans - Unlike most [[CorruptCorporateExecutive Corrupt Corporate Executives]], Vaughn believes in a greater good that justifies what he's doing.
* WhamLine: The Doctor to Tobias Vaughn at the end of Episode Seven (and, by extension, the beginning of Episode Eight):
-->'''The Doctor:''' Is this what you wanted, Vaughn?! To be the ruler of a ''dead'' world?!?!
* WhatHappenedToTheMouse: Whatever became of Rutledge? The novelisation has Vaughn ordering him to shoot himself. According to some sources, this was meant to happen in the TV version as well, but didn't.
* WrittenInAbsence: Jamie and Zoe each miss an episode, due to their actors being on holiday. The in-story reasons are that Jamie was in hospital after being injured, and Zoe was imprisoned by Vaughn after blowing up his computer receptionist.
* YoureInsane: Professor Watkins says this to Vaughn about his plans.