The one with killer insulation foam.
The TARDIS lands in a forbidding jungle, but with the Doctor slipping into a coma, Jo sets out to see if she can find help. She comes across a party of Thals, and stays in hiding aboard their crashed spaceship while they go to help the Doctor. After they have treated him, they tell him that they have travelled to this planet, Spiridon, to destroy a group of Daleks who have come to try and uncover the secrets of invisibility discovered by the natives.
Another Thal ship arrives and crashes (the Thals are evidently not great pilots) and the survivors tell of a force of ten thousand Daleks somewhere on Spiridon. The Doctor and the Thals find the Daleks in suspended animation in a cavern below the planet's surface. Using explosives, they blow up a rock wall separating the cavern from one of the planet's ice volcanoes, and the Daleks are destroyed.
The Supreme Dalek and his guards arrive on the planet, only to be stranded as the Thals steal their spaceship and the Doctor and Jo leave in the TARDIS.
- Aluminum Christmas Trees: Ice volcanoes, which sound like an oxymoron, actually exist, and were first discovered on Neptune's moon Triton.
- Author Appeal: Plagues become a part of Terry Nation's Signature Style in this story. They would feature in each of his subsequent stories for Who: "Death to the Daleks", "Genesis of the Daleks", and "The Android Invasion", as well as Survivors.
- Balloonacy: The Doctor and a group of Thals escape from the lowest level of the Dalek city by making a hot air balloon from plastic sheeting and being carried up an air shaft.
- Bottomless Magazines: Strangely averted with the Thals' energy weapons. When defending themselves from the jungle animals, every single Thal runs out of shots.
- Broken Aesop: The Doctor delivers a heartfelt speech that the Thals must tell their people War Is Hell, and not to make it sound like their adventure was a 'fun game'. The story involves, amongst other things, them escaping fun, toyetic Always Chaotic Evil nasty pepperpot people by dressing up in purple fur coats and MacGyvering a hot air balloon. The reason for this discrepancy is because the scene was appended to the end by Terrance Dicks at the last minute because the script was underrunning.
- Jo mentions in the log that she had seen the Doctor recover once before after a sudden rise in temperature.
- The Doctor's legs are temporarily paralysed by a Dalek blast, just like Ian when he first encountered them. Also, people disguise themselves in Dalek casings like Ian did.
- Upon meeting the Thals, the Doctor mentions the events of The Daleks and The Evil of the Daleks. He also mentions Susan, Ian and Barbra.
- Continuity Nod: The episode starts with the Doctor being injured as per the ending of "Frontier in Space".
- Disney Death: The Doctor believes Jo has been killed when the Daleks blow up the Thal spaceship in which she was hiding. In fact, she had been rescued at the last minute by a Spiridon named Wester.
- The End... Or Is It?: The Dalek Supreme gives orders for his army to be freed from the ice. "WE HAVE BEEN DELAYED BUT NOT DEFEATED. THE DALEKS ARE NEVER DEFEATED!"
- Fearless Fool: The Doctor urges this on Codal while they are both imprisoned not to think himself such a coward. He even assures Codal that what he did was true bravery.The Doctor: Courage isn't just a matter of not being frightened, you know.Codal: What is it, then?The Doctor: It's being afraid and doing what you have to do anyway, just as you did.
- Glowing Eyes: The local beasts, about their campfire.
- Harmless Freezing: The Daleks can be problemlessly thawed.
- However, sudden freezing does kill the Dalek inside its casing.
- At the end of the episode the Supreme Dalek is giving orders for rescue ships to be sent to free his army from the ice.
- Invisible Monsters: The Spiridons. The Daleks are trying to imitate them.
- Kill It with Ice: Though the Daleks may not be dead.
- Made a Slave: The Spiridons.
- Off-the-Shelf FX: Commercially available Louis Marx Daleks were used to simulate the Dalek army, a technique previously used in "The Evil of the Daleks".
- People of Hair Color: The Thals are all blond.
- Pragmatic Villainy: When the Daleks are preparing to release their bacterial bomb, they administer vaccine to all their forces, including their Spiridon slave workers. This is not so much out of compassion but rather that they still need a cheap labour force to do the heavy work for them.
- Ripped from the Headlines: It's been suggested that this story was inspired by The Vietnam War.
- Rock Beats Laser: It helps to have gravity on its side.
- Same Plot Sequel: The story is notoriously a near-remake of the first Dalek story: Thals versus Daleks on a planet full of random monsters, with the Daleks planning to do something that will make it inhospitable to everyone but them.
- See the Invisible: The Thals have a spray that can reveal invisible Daleks.
- Sequel Episode:
- Science-Fiction Writers Have No Sense of Scale: The Dalek's plan to conquer the galaxy with ten thousand Daleks.
- Shout-Out: Rebec was named after Terry Nation's daughter Rebecca.
- Slave Race: The Spiridons.
- Stupid Sacrifice: Not only does Marat's attempted Heroic Sacrifice do absolutely nothing to delay the Daleks, it very nearly screws over the Thals completely, since the Daleks find the location of the Thals' explosive charges on his body. Only Jo's quick thinking prevents disaster for the Thal expedition.
- Suicide Mission: What the Thals think they face. For most of them it's right.
- Synthetic Plague: The Daleks are planning to use one to wipe out the Thal intruders on the planet.
- Technical Pacifist: see page quote.
- There's No Place Like Home: At the end of the episode Jo just wants to return to Earth, rejecting romance with Latep or more adventures with the Doctor.
- The X of Y: Humorously, "Planet" and "Daleks" are the most common X and Y. Really, this is the archetypical Doctor Who title!
- Trapped in Containment: The Daleks in the bacterial lab, again, since they are immune to the virus. Worse, they can never leave their confinement.
- Plot Hole: Note that the project leader was trapped in the lab, but an identical project leader is later in another room being exterminated by the Dalek Supreme. Either he suddenly gained magical escaping powers, or this second Dalek was the victim of a poorly-timed promotion.
- Trojan Prisoner: Used by the Doctor, Jo, the Thals and some Spidron rebels to infiltrate the caves.
- Visible Invisibility: Spiridons move bushes, wear coats or wave objects around, so we know they're there. If they perish, however, they turn completely visible. Their intangibility is biologically generated- those biological functions stop, so does it.
- Wham Line: "Somewhere on this planet are ten thousand Daleks!"
- Worth Living For: Latep is afraid after he finds this in Jo.