He's got a printed circuit where his heart should be.
— Penley doesn't think much of his colleague Clent.
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 it 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 and 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"
- Achilles in His Tent: Penley, the former Chief Scientist of the base, has 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: 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?
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.)
- 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.
- Face-Heel 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.
- 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.
- One-Woman Wail: Used a lot in the soundtrack for the scenes in the snow.
- 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.
- The Quisling: Storr tries to help the Ice Warriors destroy the scientists' base. It only gets him killed.
- 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.
- Snake Talk: The Martians speak in whispered, sibilant tones.
- 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.
- Verbal Tic: Sorta. The hissing.