None.

One of the worst Dalek stories of the 70s. In fact one of the worst Dalek stories in general.

Episode 1 is nowhere near as tense as it wants or needs to be, and gives the impression that the reason everything is shrouded in fog is that they want to keep the dismal sets and costumes hidden for as long as possible. I don't blame them. Imagine using sets instead of shooting in a quarry—and your sets look like a quarry anyway!

None of the human crew members make an impression and I don't remember a single name despite having seen this more than any other Pertwee story. (They used to show it on TV a lot, for some reason.) Awful practical effects are everywhere to be seen, and Bellal—a truly dreadful character with saucer eyes and visible seams—inadvertently crystallises the whole adventure when he crosses a threshold and gasps "an empty room!".

Remarkably, it gets worse. There's something dire involving a Dalek but I can't currently remember what it is: partly because all I have in my head is Carey Blyton's monstrous, parping score. Inept cliffhangers only compound the misery. On first watch this was the only Pertwee story I thought worthy of only one star out of five. Since then it's been joined by The Time Monster (worse) and The Claws of Axos (maybe about the same).

This story I find dull on latter rewatches I prefer it but am of the belief that the Daleks should be removed from this story. And using the extra time with out the Daleks properly flesh out the cast of characters give us one of the Exillons who worship the City who has a personality. Focus on why they worship. The narrative without the Daleks could be an exploration of religion and of science.

What aren't my problems with this story would be a better question.

To begin with, the fantastic premise of how the Daleks would operate without their weapons is completely squandered by giving them their weapons back in the same episode where they lose them. Despite this, the Daleks are still made absolute jokes of throughout and are at no point even vaguely threatening (bursting into flames after being repeatedly hit with sticks and immediately committing suicide for no apparent reason after discovering that it's prisoners have escaped).

The production values are also laughably bad all around, with obvious stuntmen, plainly visible wires above the roots, and the completely hysterical costumes of the Exillons that don't even come close to hiding the appearance of the actors inside. It also has the worst incidental music of any Doctor Who story (who would've thought that the guy who would go on to write the Bananas in Pyjamas theme song would be a bad fit for Doctor Who?), and the worst cliffhanger of any Doctor Who story (the tile floor of terror).

It's at least mercifully brief and fascinatingly bad unlike The Mutants, so I can't quite call it the worst Pertwee story, but it is still what I imagine what all of classic Doctor Who looks like to people who don't like classic Doctor Who.