Dennis Spooner makes a smart decision to begin this story about a month after the previous episode’s cliffhanger. It means when we re-join them, Vicki has had weeks to get to know the Doctor, Ian and Barbara.

This is the kind of middle episode that the show sometimes struggles with, particularly in the longer serials: not a huge amount happens, but it’s still too far from the last episode to start pushing things towards a climax. Instead, Nation focuses on more world building, expanding the story beyond central London to reveal the mines of Bedfordshire mentioned during the previous couple of weeks, and showing us the characters’ various journeys towards what’s clearly shaping up to be the epicentre of the Daleks’ invasion plans.

This is a thoroughly grim episode. The opening battle sequence goes on for ages, and oddly benefits from Richard Martin’s slapdash direction because it looks genuinely chaotic and shocking, with explosions going off, and people running in all directions. It works really well. So does the later (film) sequence of Barbara, jenny and Dortmun’s desperate race across London, with the classic images of Daleks outside the Palace of Westminster, in Trafalgar Square and outside the Albert Hall.

The purpose of this episode is to reformat the Daleks from the vulnerable, confined, static-dependent outer space robot people, as seen in their first outing, to world-conquering Space Nazis. It’s as big a shift as the Doctor’s development from shifty antagonist to series hero. It’s not done in a particularly sophisticated way, but there is some effort devoted to answering the questions kids might have about how the Daleks can move outside their city (Ian notices the discs on their backs), and how they can still be alive (Skaro was ‘a million years ahead of us in the future’). Later, the Doctor references the Daleks’ use of static electricity. A lot of it might be handwaving, but it does gloss over the inconsistencies. Plus, it’s not like these Daleks are a complete departure from the ones we saw on Skaro: they still subject their prisoners to weird experiments (last time it was radiation drugs, here it’s some convoluted intelligence test), and they’re still genocidal. Only their weedy-sounding voices are less impressive than in their first appearance.

It wasn’t badged as such, but watching this, it’s inescapably a first anniversary episode. Twelve months ago, a mild curiosity / creepy stalking led Ian and Barbara into a junkyard in London. World’s End returns them to a scrap heap in London, where the TARDIS is incongruously parked in a decaying building site, evoking our first sight of it among a dilapidated pile of junk. The Doctor even comments on how curious people would be to discover a Police Box in such an odd, out of the way place – perhaps a nod back to that very first shot in An Unearthly Child, of a policeman discovering the TARDIS.