Doctor Who/Recap/S7 E4 Inferno

"Brigade Leader Lethbridge-Stewart: Your identity is being checked with Central Records. When we know who you are, the real interrogation will begin.

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."

Doctor Who's first dabbling in an Alternate Universe. Professor Stahlman has discovered "Stahlman's Gas" beneath the Earth's crust and UNIT are providing security for the drilling operation to exploit this useful resource. Unfortunately, Stahlman is driven to disregard safety precautions, and things start going wrong. The drilling strikes a green goo which regresses anyone who touches it into a beastlike "primord" which is drawn to heat.

Meanwhile, the Doctor has moved the TARDIS console out into the lab and is attempting to repair it with the help of a skeptical Liz. He manages to accidentally zap himself into a parallel universe where Britain is a fascist dictatorship and The Brigadier is now Brigade-Leader Lethbridge-Stewart (with an eyepatch and no moustache), while Liz Shaw struts around in kinky dominatrix boots. Hindered by his parallel "friends", the Doctor is unable to stop the drilling, and the parallel world is destroyed as the Doctor escapes.

Returning to the "real" world, the Doctor warns the others of the dangers, but is not believed - 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 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 - the actress 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 didn't get a "goodbye scene"...

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 Handwaved 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.

Watch it here.

Tropes

 * 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.
 * 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 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.
 * Big Electric Switch - Two of them, the old-fashioned kind, control the power to the TARDIS console during the Doctor's experiments.
 * Bilingual Bonus - The alternate universe ranks are based on English translations of Waffen-SS ranks.
 * Chair Reveal - of the Brigade Leader.
 * Chuck Cunningham Syndrome - Liz Shaw, who simply disappears after the end of this serial.
 * Years later, she does return, as the head of the Preternatural Research Bureau (PROBE) in a couple of fan-made spinoff videos, and is mentioned in an episode of The Sarah Jane Adventures.
 * 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.
 * Evil Is Sexy
 * Eyepatch of Power - The Brigade Leader has one.
 * Faux Action Girl - 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.
 * 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.
 * Heroic BSOD - 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.
 * 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.
 * 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.
 * 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.
 * 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.
 * Putting on the Reich
 * Reverse the Polarity - the same catastrophe is averted in both universes by "reversing all systems"
 * Sinister Shades - The mirror Stahlman wears them.
 * 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