We can never leave here. Never! Never!
Perhaps nothing sums up Terry Nation’s return to Doctor Who in “Planet of the Daleks” (Story Production Code SSS) better than the fact that said planet has a core of, um, molten ice, a geological anomaly that inevitably plays a prominent role in the story’s outcome. It’s a typically outré Nation conceit. Presumably this odd planetary structure would make hollowing the core out simpler than if it were molten lava, but unlike that plan of the Daleks, this one involves learning the secret of invisibility from the inhabitants of the planet, Spiridon, a jungle world whose savage lifeforms presumably drove the natives to evolve this ability as a protective measure. They also wear purple fur coats when it gets cold, slightly defeating the invisibility adaptation.
But rather than simply establishing a small research outpost to exploit the Spiridonian’s prestidigitous power, the Daleks also store tens of thousands of their brethren in suspended animation there—effectively, their entire military force—in order to retrofit them with the invisibility power for the forthcoming “invasion of all the solar planets” alluded to in the prior story, “Frontier in Space.” A fortuitous single point of potential failure, then, and one which a band of brave Thals (q.v. “The Daleks“) discover and trigger to thwart their eternal enemies.
Though the Dalek War took place generations earlier, the Thals sent a mission from Skaro to hunt down the Daleks. Their two primitive spacecraft crash-land on Spiridon, killing several Thals instantly, but after much derring-do and many scenes of self-sacrifice, the dwindling band of tow-headed non-mutants manage to crack open the walls of the vast Dalek hibernation chamber, letting in torrents of molten, er, ice, freezing the regiments of pepperpots in place for centuries.
Oh, right, and the Doctor and Jo show up, but probably only because Terry Nation’s contract required him to write the Time Lord into the story.