After two episodes of rare video material, it’s tough to switch back to telesnaps and audio for this final instalment, particularly as there are large chunks that involve fiddling with equipment, or water rushing through the caves. From the sounds of it, this was a suitably apocalyptic climax to a story that threatened the end of the world, with photos showing the statue of Amdo toppled and hurled into the sea: another fallen civilisation joining Troy and the Elders’ in the Doctor’s 500 Year Diary.

It would be stretching it to claim there was an elegiac tone to this, but Orme and Davis give some sense of the scale of loss: ‘The everlasting nightmare is here at last,’ bemoans Thous as the ocean washes away temples and laboratories alike – religion and technology helpless in the face of nature. Damon, who’s not had any nuance to this point, has a sudden change of heart, revealing a backstory not obvious from the previous episodes, and consigning the (unseen, as far as I can tell) Fish People to the dustbin of history along with the rest of the Atlantean civilisation:

DAMON: No more temples. It was temples and priests and superstition that made us follow Zaroff in the first place. When the water’s found it’s own level, the temple will be buried forever. We shall never return to it. But we will have enough left to build a new Atlantis, without gods and without Fish People.

The Doctor has a plan (although, as in The Highlanders, he’s not very confident it will work): to save the planet he’ll sacrifice a society: probably a necessary trade off, but not one that seems to trouble him greatly. He’s more worried about abandoning the mad professor, still trying to reach the plunger that will blow up the Earth: ‘I can’t leave Zaroff to drown there.’ Troughton’s performance is probably as good as in the previous episodes, but without being able to see his business in the laboratory it’s hard to guess whether he looks as impishly delighted in besting Zaroff as the dialogue sounds.

The companions continue to get almost nothing interesting to do. In particular, Polly is very badly served by the story, especially after her strong role in The Highlanders. This time, she’s wetter than Kirsty was, pathetically bleating about how tiresome it is running for her life. Jamie’s been a third wheel, although there’s a hint of his usefulness as Polly talks about radiation, while he looks for something practical to do. Ben follows the Doctor round, being sarcastic.

On the other hand, the last scene, back in the TARDIS, is lovely: the whole team reunited, and looking forward to new adventures just like used to happen at the end of a Hartnell serial. And Polly’s look, in the last appearance of the Doctor’s giant hat, makes it look like Sam Briggs has turned up eight episodes too early.

Next episode: The Moonbase