The one with killer soap. Alternately, the one where Wendy got paint on her new outfit.
The TARDIS lands on 21st century Earth, and discovers that the T-Mat matter transportation system upon which the entire planet's economy now relies is broken. The Doctor and companions agree to take an elderly rocket to the Moon (which houses the relay station) and investigate.
When they arrive, they discover that Ice Warriors have taken over the station and are preparing to invade Earth. They have seed pods which will be T-Matted to selected points on the Earth's surface before emitting fungus that will remove oxygen from the atmosphere, killing all humans and rendering the planet habitable for the Ice Warriors.
The Doctor manages to get the T-Mat working and transports back to Earth, where he discovers that the seed pods are destroyed by contact with water (quite a big hole in the Ice Warriors' plan there). He uses the weather station on Earth to trigger a downpour, destroying all the seeds, then T-Mats back up to the moon to misdirect the approaching Martian fleet into orbiting the Sun, where they will be destroyed.
Not to be confused with "The Seeds of Doom".
- Absentee Actor: The Doctor is unconscious for the duration of the fourth episode in order to give Patrick Troughton a holiday.
- Alien Kudzu: The Ice Warriors employ pods that release a rapidly-multiplying spore, which sucks the oxygen out of Earth's atmosphere. The Doctor finds out that water counteracts the pods.
- Artistic Licence Biology: The seed pods emit the fungus. Fungi grow from spores, not seeds. Though to be fair, these things are from another planet. They could be called fungi because they look like it, not because they act like it.
- Artistic Licence Chemistry: After trying various corrosive substances on a seed pod with no effect, the Doctor discovers that it's vulnerable to "ordinary water". But several of the things he tried before that were acids, which consist mostly of water.
- Batman Grabs a Gun: The Doctor hunts and kills Ice Warriors with a heat ray.
- Break Out the Museum Piece: The rocket. Later, a motor car, though we don't get to see that.
- Call-Back: When waking dazedly, the Doctor mentions Victoria Waterfield.
- Corpsing: The sight of Patrick Troughton being swept off his feet by a wave of foam is too much for Wendy Padbury. She apologises profusely for it in the DVD commentary.
- Cucoloris: The launch countdown is projected onto Gia's face.
- Custom Uniform of Sexy: All the male technicians wear hideous, baggy jumpsuits. Miss Kelly gets figure-hugging Future Spandex.
- Destruction Equals Off-Switch: Averted when an Ice Warrior pulls a bunch of levers on a (very simple) control panel and then shoots it, so it'd melt and nobody could change the setting. As the system is otherwise undamaged, the Doctor is able to rewire a new setting.
- Dirty Coward: Fewsham collaborates with the Ice Warriors, fearing for his life, until he sees he's helping wipe the human race out. Then he grows a pair and gives the secret to stopping the Ice Warriors at the cost of his life.
- The Dog Bites Back: Even a Dirty Coward can be pushed too far.
- Doomed New Clothes: A behind-the-scenes example. Wendy Padbury liked her second costume (the leather trouser suit) so much that she bought it and BBC bureaucracy required her to pay before the start of filming. Then, in the scene where Zoe has to crawl through a tunnel, the paint on the set turned out to be still wet.
- Dying Moment of Awesome: Fewsham, who is shot down after the Ice Warriors discover he is broadcasting their activities to an Earth station. Especially awesome because he'd been a Dirty Coward up until then.Fewsham: Every word has been heard on Earth.
- Emotionless Girl: Gia Kelly.
- Fake Shemp: Patrick Troughton does not appear in Episode Four as he was on holiday during the week it was recorded. The Doctor is seen only as an unconscious figure, with Tom Laird doubling for Troughton with his face out of vision.
- Failed a Spot Check: An Ice Warrior completely misses Phipps while actively searching for him. All it had to do was turn around and that would've been that.
- Greater-Scope Villain: Grand Marshall is Slaar's boss but Slaar has a much heavier hand in the Ice Warrior plot.
- Heroic Sacrifice: Early in the story, Osgood sabotages the moonbase's equipment while pretending to help the Ice Warriors. They kill him for it, but it serves to slow them down for a good episode or two.
- Hostile Terraforming: The Ice Warriors attempt to xenoform Earth using a fungus that will extract oxygen from the atmosphere.
- Hurl It into the Sun: The humans send a signal from a communications satellite to draw the invading Ice Warriors off course into the sun.
- Info Dump: Zoe pushes a random button in the "Space Museum" and, boom, we get a two-minute rundown of the major technology of the story.
- Kill 'Em All: None of the people originally stationed at the T-Mat moonbase survive this serial. Phipps and Fewsham survive longer than the rest, but Phipps dies at the end of Part 4, and Fewsham dies in the middle of Part 5.
- The Living Dead: The spores' first victim not only breathes visibly, but his fingers twitch too.
- Man-Eating Plant: The killer Martian fungus.
- Mishmash Museum: The TARDIS lands in a museum where displays about Yuri Gagarin, Leonardo da Vinci's flying machine, and a futuristic teleporter are all in the same room. On the other hand, this museum was pretty much the work of one person, who probably did have limited funding and space. Though it looks like a mishmash at first, it is explained that it is in fact a single exhibit about the history of transportation - which is what early flying machine designs, the first man in space, and a teleporter have in common that results in them being displayed together.
- Plant Aliens
- Portal Network
- Redemption Equals Death: Fewsham
- Right Under Their Noses: An Ice Warrior misses Jamie hiding in a corner, behind nothing at all.
- Sequel Episode: To "The Ice Warriors".
- Special Edition Title: The story title, episode number and writer are overlayed on a specially filmed model sequence, showing a scene of the Earth and Moon, with a music cue unique to this story.
- Stock Footage:
- The footage of the rocket in Episode Five is stock footage of Mariner 4, a mission launched in 1964 to map the surface of Mars. Notably enough, the images taken by it were sufficient to dispel belief in intelligent life on the planet.
- The footage of the rocket launch in Episode Six is also library footage, from a Soviet launch in 1948 of a R-1 missile, a short-range copy of the German V-2.
- Technical Pacifist: The Doctor racks up quite a bodycount for someone who Does Not Like Guns and hates killing. He's saving humanity from genocidal invaders, but this doesn't fit well with later stories where he claims to shun violence.
- Tranquil Fury: The Doctor, having spent most of the serial clowning and panicking, is frighteningly calm and restrained as he explains to the Big Bad what he has done, and why.The Doctor: We sent up a satellite. That signal has sent your fleet into a false orbit.
Slaar: The heat of the sun will kill them! You have destroyed our entire fleet!
The Doctor: You tried to destroy an entire world.
- Weaksauce Weakness: If your species can be wiped out or your plans crippled by something like water, setting up shop on a world with a surface that's ~70% water is ill-advised.
- Weaponized Teleportation: The Ice Warriors want to teleport the Doctor into outer space. Refusing do this is the beginning of Fewsham's HeelFace Turn.
- Wrench Wench: Gia Kelly is the electronics-expert variant; she's the world's greatest expert on the planet-wide teleportation system, and also makes field repairs to an improvised Death Ray.
- The X of Y