Brigade Leader Lethbridge-Stewart: Your identity is being checked with Central Records. When we know who you are, the real interrogation will begin.Doctor Who's first dabbling in an Alternate Universe. Professor Eric Stahlman theorizes that a new energy source, "Stahlman's Gas", lies beneath the Earth's crust and UNIT are providing security for the drilling operation to exploit this useful resource. The drilling strikes a green goo which regresses anyone who touches it into a beastlike "Primord", which is drawn to heat. Unfortunately, Stahlman is driven to disregard safety precautions, and he ignores every attempt at slowing down and being more cautious.The Doctor's attachment to the project is two-fold. He's curious about the scientific effects of penetrating the Earth's crust, and he's using the power from the project's nuclear reactor to run experiments which aim to get the TARDIS operating again. While taking an accidentally overpowered test flight, he ends up discovering a barrier he can't get through, and his curiousity is piqued. However his altercations with Stahlman reach the breaking point, and Stahlman cuts off his nuclear power. The Doctor manages to restore it just long enough to take off again, but the power is again cut, trapping him in a parallel universe where Britain is a fascist dictatorship. The Brigadier is now Brigade Leader Lethbridge-Stewart (with an eyepatch and no moustache), while Liz Shaw struts around in kinky dominatrix boots. The Doctor does everything he can to stop the drilling, which is more advanced than it is in his universe, but he is ultimately unsuccessful and penetration zero is achieved with disastrous results.One by one, the alternate versions of the base's staff make a Heel–Face Turn and join the Doctor's cause, but honesty compels him to reveal that he can't possibly save them — they'll all have to make a Heroic Sacrifice to get him back to his own dimension, and the only thing they can do is maybe save their regular world counterparts. The Brigade-Leader is killed while trying to force the Doctor to save him, and the Doctor escapes as the parallel Earth is destroyed.Returning to the "real" world, the Doctor warns the others of the dangers, but is not believed. That is, until Stahlman himself is transformed into a Primord. The Doctor kills him with icy blasts from a fire-extinguisher and manages to stop the drilling just in time.Finally, the Doctor announces that he's finished repairing the TARDIS console, that the Brigadier is a "pompous, self-opinionated idiot" and that he's leaving. He flips a switch and vanishes, only to reappear moments later through the door, explaining sheepishly that he materialised on the local rubbish tip.
The Doctor: But I don't exist in your world!
Brigade Leader Lethbridge-Stewart: Then you won't feel the bullets when we shoot you.
The Doctor: But I don't exist in your world!
Brigade Leader Lethbridge-Stewart: Then you won't feel the bullets when we shoot you.
The production team had been looking for an excuse to get rid of the old console prop, and this seemed the perfect opportunity...They also disposed of Liz Shaw in the gap between series — Caroline John had become pregnant, though no-one knew that at the time and it was just felt that the character wasn't working out. However, this decision hadn't been made at the time of filming, so Liz never got a "goodbye scene". Expanded universe tries to deal with that issue, with the most dominant take at a departure story being the appropriately titled Missing Adventures series, in the novel The Scales of Injustice. However, a new generation of writers for the 2013 audio "Vengeance of the Stones" apparently didn't know about this novel's existence, and we now have two conflicting accounts of how Mike Yates started working with UNIT (wouldn't be the first time UNIT continuities got knotted in a twist), who debuts in the next episode.The Doctor Who Expanded Universe uses it quite a bit by having the tyrannical overlord of Earth be an alternate Doctor. (There is no indication of this in the actual story, which on the contrary heavily implies that the Doctor was only able to travel to that universe because he didn't already exist in it, but this is Hand Waved as it being a different incarnation, much like in the other multi-Doctor stories.) This theory comes from the fact that Jack Kine, whose face was used for posters of the dictator, was also one of the faces offered for the Second Doctor to regenerate into in The War Games.
- Alternate History: in which Britain has been a Republic since at least 1943. When the Doctor asks what happened to the Royal Family, the Brigade Leader says cheerfully that they were all executed.
- And Then Harry Was A Primord
- Apocalypse How: The alternate Earth apparently experiences a Class X, as the Doctor tells the alternate Inferno team that within the next few days the world will blow itself apart. We don't see the actual destruction of that Earth, but we see it very shortly before the implied end.
- The Doctor Who Expanded Universe books retcon this into a Class 4, in which the planet remains physically intact but is reduced to a burnt-out wasteland, with those (un)fortunate enough to have survived the catastrophe having all become Primords in the meantime.
- Beard of Evil: Inverted, as the Brigade Leader is clean-shaven, unlike his moustachioed counterpart. Likewise, the parallel universe Stahlman also lacks his prime universe's beard, but his personality is 99% the same. The 1% difference is the willingness to commit murder so his project remains on track.
- Big Electric Switch: Two of them, the old-fashioned kind, control the power to the TARDIS console during the Doctor's experiments.
- Black Dude Dies First: Okay, so he actually dies second onscreen (and third overall) but Harry the technician is the first of the Inferno personnel to become a Primord in both universes.
- Bilingual Bonus: The alternate universe ranks are based on English translations of Waffen-SS ranks.
- Chair Reveal: Of the Brigade Leader.
- Chekhov's Gunman: Bromley, the scientist who gets turned in to a Primord. In both universes he rather tenaciously eludes capture and keeps turning up. This is especially true in the original 'verse, where he simply disappears after the second episode only to make a rather abrupt reappearance in episode seven.
- Combat Pragmatist: The Third Doctor, as usual, gets creative with whatever he's got lying around. And his fists.
- Conspicuous Gloves: In both universes, Stahlman conceals his mutated hands with white gloves as he starts turning into a Primord.
- Crossing The Burned Bridge: After the Doctor reappears a mere few moments after declaring the Brigadier "a pompous, self-opinionated idiot", and now in need of said Brigadier's help with recovering his TARDIS console. The Brigadier just repeats the Doctor's previous insult in a completely deadpan manner, to which the Doctor, deflated and humbled, says he shouldn't bare a grudge over a few hastily chosen words.
- Day of the Jackboot: The UK has been a fascist state since at least 1943. The story does not explain whether this was due to conquest by Nazi Germany, or a home-grown fascist movement that was more successful than the real-world British Union of Fascists and others.
- Decoy Hiding Place: Used by the Doctor when he's being pursued by the soldiers in the alternate world.
- Democracy Is Bad: A possible interpretation of the fact that the fascistic alternate Britain is a republic, although a republic and a democracy are not necessarily the same thing. More "Non-Monarchy is bad"
- Downer Ending: Episode 6 has quite possibly the most depressing ending in the history of the series.
- Dug Too Deep
- The End of the World as We Know It
- Evil Counterpart: Subverted. The RSF versions of the Doctor's friends aren't exactly clean-cut good guys. Platoon Under Leader Benton is vicious and ruthless, Brigade Leader Lethbridge-Stewart is paranoid and cowardly, and Section Leader Elizabeth Shaw... isn't a scientist, but when they realize how screwed their world is, they help the Doctor get back to his TARDIS console, though the Brigade Leader does attempt to kill him before escaping.
- The Doctor Who Expanded Universe suggests the leader of the fascistic alternate Britain was actually one of the possible persons the Second Doctor could have regenerated into.
- Expendable Alternate Universe: Subverted, as the Doctor is traumatized by its destruction.
- Eyepatch of Power: The Brigade Leader has one.
- Faux Action Guy: Amusingly inverted (well it is an alternate universe). The Brigade Leader is a cowardly loudmouth while Section Leader Elizabeth Shaw is a tough leader in a crisis.
- Gone Horribly Right: Stahlmann is absolutely right about there being an awesome amount of untapped power beneath the Earth's crust. In the parallel universe, he succeeds in releasing it. Unfortunately, there is enough there to destroy a whole planet.
- Good Republic, Evil Empire: inverted, in the good universe Britain is a constitutional monarchy (which, presumably like the real world Britain was at this time shedding itself of its imperial legacy) and in the "evil" one it's a fascist "republic".
- Good Scars, Evil Scars: The Brigade Leader has a nasty scar to go with his nasty personality.
- Here We Go Again: Briefly employed when the Doctor ends up in the alternate universe - he finds himself encountering the same two Primords as before, in the same place (atop a huge gasometer). Wyatt, the soldier, even dies the same way he did originally. Only after Wyatt's death do things begin occurring differently again.
- Heroic B.S.O.D.: The Doctor sees the alternate world burn as he leaves it and goes into a coma for a good chunk of the following episode.
- We find out later that this becomes his greatest fear.
- Hooked Up Afterwards: After spending the entire serial flirting back and forth, Greg Sutton and Petra Williams are seen hugging at the end of the emergency and later drive back to London together. One of the Expanded Universe novels says they become lovers.
- In Spite of a Nail: In an alternate universe with thirty years or more of historical and political divergence, the same drilling operation is still being run by the same people, although some of them are in slightly different positions or ended up there by different routes. Even more this trope, minor details are the same: notably, the same people have been infected by the green stuff on the same day, even though it was a highly contingent accident.
- Insufferable Genius: Good lord, Professor Stahlman. Throughout the entire serial he acts like a complete Jerk Ass, blatantly disregarding valid and sensible precautions, insulting military and scientific personnel frequently in their faces and sabotaging equipment that might as well be saying "STOP DRILLING INTO THE EARTH, YOU MORON".
- He's so hostile towards everyone and everything with half a brain telling him to stop the project, he can come across as a Know-Nothing Know-It-All.
- Interdimensional Travel Device: The TARDIS console transports the Doctor to a parallel universe where Britain is governed by Fascists.
- Large Ham: The normally calm and subtle Brig becomes the wonderfully hammy Brigade Leader.
- Malevolent Mugshot: Of Jack Kine.
- Mirror Universe: Complete with mirror counterparts with sinister shades, eyepatches, and/or scars. Although, demonstrating that Star Trek's version was not yet ubiquitous, the mirror universe actually has less facial hair than the usual one.
- Mutagenic Goo
- No Indoor Voice: Greg Sutton is this. Even when you know he is talking normally, he speaks louder than is necessary.
- One-Word Title: The first one for the series.
- In serial titles, at any rate; the series has previously used one-word titles for some of the individual episodes during the William Hartnell era.
- People's Republic of Tyranny
- Perp Sweating: Used by the Brigade Leader on the Doctor to find out who he is and where he came from, complete with the traditional desk-lamp-in-the-face.
- Pride Before a Fall: Stahlman is too proud to admit his theories could be wrong. He ends up a witless beast in both universes.
- Putting on the Reich: If the uniforms didn't sell it, the goose-stepping and vaguely Nazi like salutes should.
- Railing Kill: Done to Wyatt by himself when he overbalances while attempting to hit the Doctor with his rifle.
- Red Right Hand: Stahlman's hand turns green in both universes, slowly spreading the goo through his body. He hides it with gloves.
- Reverse the Polarity: The same catastrophe is averted in both universes by "reversing all systems"
- Schmuck Bait: The Doctor to his lone guard: "Have you ever seen anything like this?" Cue Venusian Karate when the dope walks over to see what the Doctor means.
- Sealed Evil in a Can: The mutagen under the Earth crust.
- Sinister Shades: The mirror Stahlman wears them.
- Technically Living Zombie: The Primoids
- Truth in Television: In the 1930s, Britain really did have a burgeoning fascist movement, the British Union of Fascists, led by Oswald Mosley. Fortunately, they were a bit too right-wing for the times and never got elected. But if they'd been better at playing on the public's fear of a Communist revolution like Hitler did, who knows how history might have turned out....
- Villainous Breakdown: The Brigade Leader.
- The Virus: The Primords.
- Stahlman's Ooze is how it is conveyed; if you get some on your hand, you'd better cut your arm off in the next few seconds, before The Virus has time to spread though your bloodstream.
- The Wall Around the World: The Doctor pushes through a barrier in time and ends up in a Mirror Universe.
- Was Once a Man: Again, the Primords
- Zee Rust: The Doctor has invented an automatic door for Bessie's garage, which everyone marvels over.
- Towards the beginning of the serial the Doctor is assured by one of the scientists that everything is safe... because everything is coated in layers of asbestos!