The one where Mars attacks!
Earth is in the grip of a new Ice Age, and the Doctor arrives at a station tasked with keeping the ice at bay using an ioniser. As the glacier moves forwards, a frozen creature is dug out of the ice - a reptilian-looking humanoid jokingly termed an "Ice Warrior" by the base personnel.
The Ice Warrior thaws and wakes up, stating his name is Varga and he was the commander of a ship from "the Red Planet" which crashed on Earth thousands of years ago. Now that his own planet is dead, Varga plans to conquer Earth with the help of his similarly-frozen comrades.
The base scientists meanwhile realize that if the ionizer beam hits the alien ship, the engines could explode and kill them all - but the Doctor persuades them it's worth the risk as not doing so could doom the entire Earth. Luckily for all concerned (except the Ice Warriors), there's only a relatively small explosion - killing all the Ice Warriors and destroying the ship and also conveniently stopping the flow of the glaciers for a while.
The Ice Warriors would recur several more times under the Second and Third Doctors before disappearing from the show. They had been planned for a story with the Sixth Doctor before cancellation put paid to that. And the Seventh, before it was cancelled again. They most recently reappeared in the Eleventh Doctor episode "Cold War" and later returned again in the Twelfth Doctor episode "The Empress of Mars".
- Achilles in His Tent: Penley, the former Chief Scientist of the base, eventuals walked out in disgust at Clent's blind trust in computers and is living as a scavenger.
- An Iced Reptile: The Ice Warriors.
- Arm Cannon: How the sonic blasters are carried.
- Artistic License Biology: The stated reason for the Earth's new Ice Age is the reduction of plants reduced the amount of Carbon Dioxide in the air, causing global cooling. Plants consume Carbon Dioxide and turn it into oxygen.
- Artistic License Physics: The whole discussion about the workings of the thawing device.
- Additionally, the part about voltage, current and resistance. While it is technically correct, you need a low resistance to create a high current from a low voltage. Instead, the Doctor implies that resistance can still be controlled when you have a fixed voltage and current.
- Author Filibuster: Several rants (from different characters) scattered throughout the serial on the evils of relying too much on computers to make our decisions.
- Change the Uncomfortable Subject:Jamie: You see how those lassies were dressed?Victoria: Yes, I did. And trust you to think of something like that.Jamie: What? Couldn't help thinking about it.Victoria: Well, I think it's disgusting, wearing that kind of thing.Jamie: [Mocking] Oh, aye, so it is, so it is. You er, you don't see yourself dressed like that then?Victoria: Jamie!Jamie: Oh, I'm sorry, it was just an idea.Victoria: We will now change the subject, please.
- Ironically Deborah Watling really liked those outfits and lamented that she didn't get to look as glamorous as Wendy Gifford in the role of Miss Garrett.
- Crocodile Tears: Victoria does this in over-the-top fashion to cover up her and The Doctor's scheming. (It's actually kind of impressive given all the legitimate weeping she'd done in the story up to then.)
- Disney Death: Victoria witnesses Jamie's apparent death at the hands of an Ice Warrior. However, while the man with Jamie is killed, Jamie himself survives.
- Evil Cripple: Though he's not evil when compared to the Ice Warriors, the obstructive boss of the base is crippled.
- Explosive Decompression: The cliffhanger to Episode Four, according to the Doctor. Tsk tsk.
- FaceHeel Turn: Penley's friend Storr tries this. The Ice Warriors aren't interested.
- Form-Fitting Wardrobe: All the human guest characters. Jamie is impressed with how it looks on the female staff.
- Hidden Depths: At the end of the episode, Clent surprises Penley by telling him that he never uses a computer to write his reports for him.
- I Have Just One Thing to Say: The reconciliation between Clent and Penley at the end:Clent: You are the most insufferably irritating and infuriating person I've ever been privileged to work with.
- I Will Punish Your Friend for Your Failure: Varga threatens to kill Victoria if the Doctor doesn't tell him what kind of power source the base uses. (Victoria courageously tells the Doctor not to tell him.)
- Idiot Ball: Most of the plot tension is generated by the fact that Leader Clent is an Obstructive Bureaucrat completely dependent on his computer to tell him what to do, and refusing to either take the initiative or allow anyone else to. Multiple times the other scientists get a plan in motion only to have it scuppered by Clent's insistence on going through the computer. Dealt with realistically in that everyone else on the base hates him for this, and one has even left civilisation to become a scavenger due to how unbearable he is to work with. Also, bosses like that are Truth in Television.
- Lawful Stupid: Leader Clent as detailed on Idiot Ball above.
- Logic Bomb: A minor one happens when the science team asks the computer a question where any answer carries the risk of the computer being destroyed. Being programmed for self-preservation, the computer more or less crashes, but there's no actual physical damage to it.
- Ludd Was Right: On computers. Ironically the most outspoken critic of computers here is a scientist.
- Make Me Wanna Shout: The Ice Warriors specialize in sonic weaponry.
- Master Computer: A milder version than most. The computer doesn't run anything itself, but everyone relies on it to give instructions based on the data they input.
- Namedar: In subsequent stories the Martians are actually called the Ice Warriors.
- Obstructive Bureaucrat: Leader Clent insists on doing everything based on what the computer tells him to do, even though huge scaly defrosted green men from Mars have bunkered down in a dead spaceship and are trying to kill the scientists out of paranoia and there is no way the computer could possibly have a plan to deal with them. Several times in the story, characters get a plan in motion only to have Clent scupper it due to lack of input from the computer. Everybody else in the base is frustrated with his behaviour but powerless to do anything about it, and one has even dropped out of civilisation entirely and gone to live in an icy wilderness due to being sick of Clent.
- One-Woman Wail: Used a lot in the soundtrack for the scenes in the snow.
- Outside-Context Problem: Climate change scientists are trying to hold back a glacier, in a way that is clearly business as usual in the setting. Then one of the members finds a mummified body frozen in an ice floe and brings it back for research purposes. It turns out to be a Martian warrior downed in an ancient plane crash and trying to find its allies again. They are dependent on their computer to calculate probabilities and obsessively do what it says naturally, it has no programming to deal with alien invasions, leaving them high and dry and forcing them to rely on the Doctor, for whom these things are more normal.
- Panty Shot: A member of the female base crew's short uniform (Jamie wants to see Victoria in one, which more or less happens with Zoe [with a ladder-climbing scene] in The Invasion) doesn't quite cover for having to get up after a sonic blast that hits the base.
- Pet the Dog: Clent's reassuring Arden that the deaths were not his fault (due to him wanting to bring the Ice Warrior back).
- The Quisling: Storr tries to help the Ice Warriors destroy the scientists' base. It only gets him killed.
- Real Life Writes the Plot:
- Deborah Watling was unable to attend the complete recording of the final episode. Consequently, Victoria is asked (off-screen) to return to the TARDIS halfway through the episode. She was originally supposed to return to Brittanicus Base with him and appear in the final control room sequence. Jamie took on one of Victoria's lines as a result.
- Bernard Bresslaw used a hushed, hissing voice for Varga, as his normal voice was too recognisable from his recurring roles in the Carry On films and would have killed any chance of the audience taking the character seriously.
- Romanticism Versus Enlightenment: Penley, the former Chief Scientist, versus Clent, the boss of the base. A rare example where the "Romanticism" figure is a scientist.
- Science Is Bad: Storr is convinced of this. The serial as a whole downplays this a bit: science isn't bad, but relying solely on computers instead of common sense is.
- Scooby Stack: Done when the time travelers enter the base for the first time.
- Shaky P.O.V. Cam: There's a quick hand-held camera shot from the bear's point of view as it attacks Penley and Jamie.
- Similar Squad: The freedom-loving renegade scientist Penley and his younger, blunter Scottish sidekick Storr click instantly with the Doctor and Jamie.
- Snake Talk: The Martians speak in whispered, sibilant tones.
- Special Edition Title: The serial's title, writer and episode number are presented in an unusual manner where they are faded in over shots of the frozen wastelands. The episode numbers are also given as "ONE", "TWO", etc, rather than the usual "Episode 1".
- Stage Whisper: In the Martian spaceship, the Doctor makes Victoria fake crying when he needs to tell her something. Then he whispers louder than her sobs. Then Victoria stops crying and loudly whispers back. Of course, the Ice Warrior guarding them doesn't hear.
- Standard Female Grab Area: Done to Victoria, of course, when she's in the Ice Warriors' clutches. (Amusingly, it was actually Deborah Watling who was often leading the Ice Warriors around when they grabbed her because the actors had such limited vision in their helmets.)
- Terrifying Pet Store Rat: The fearsome bear that menaces Penley and Jamie is incredibly cute.
- Verbal Tic: Sorta. The hissing.