This episode notably introduces that old Terry Nation standby: aggressive vegetation, as seen in practically every one of his subsequent scripts. But whereas in The Chase or Planet of the Daleks it’s just alien flora, here there’s a conscious attempt to explain it in scientific terms. The scientist Darrius (an old bearded man in a robe, like something out of a Christopher H. Bidmead story) has been experimenting with accelerating entropy, nature’s ‘fixed tempo of destruction’, which is causing the jungle to encroach on the temple ruins increasingly quickly and aggressively.

I wonder how much of this might also have been inspired by the 1962 movie of The Day of the Triffids (starring Carole Ann Ford). John Wyndham was the pre-eminent British sci-fi author of the 1950s and I think there’s already a hint of his influence in some of the post-apocalyptic elements of The Daleks. Even this episode’s dialogue, ‘the tempo of destruction’, sounds like the title of something Wyndham would have written. Nation’s script is really atmospheric this week: ‘The jungle is coming. When the whispering starts, it’s death.’

The other obvious influence on this is The Sea of Death: both episodes see the travellers arrive in a dangerous environment surrounding a building full of secret doorways and booby traps, and occupied by one old man guarding a key. This could either be a neat echo of the first part of the adventure, or Nation’s infamous plot recycling. Either way, because of the intervening Morphoton episode, he just about gets away with it. Oddly, there are also echoes of Marco Polo – not only in the climactic arrival in a freezing environment, and the living statue which recalls the wall in the Cave of 500 Eyes, and also Darrius’s dragon-carved chest which looks like it’s been borrowed from Kublai Khan’s boudoir.

That got me thinking about Marco Polo, and how it acts as a kind of buffer between the first, very intense 13 episodes where Ian and Barbara were desperate abductees, keen to return home, and the rest of the series, where they seem like seasoned travellers. Given they carry most of the episode (Hartnell’s on a week off, and Ford only appears during the first 10 minutes), Russell and Hill can’t exactly play this like two frightened schoolteachers who’ve wandered into an Indiana Jones movie, and so they become Ian and Barbara, space adventurers. Having the long passage of time depicted in the Cathay episodes at least means this makes some sort of sense – presumably so many months have passed since they left 1963 that Ian and Barbara aren’t rookies any more, and have accepted this is business as usual for the foreseeable future.

Other things I noticed: Altos’s robe is obscenely short, and the final scenes of Ian and Barbara scrabbling around in Darrius’s laboratory searching for the key are exactly like one of the challenges in The Crystal Maze. The Amicus portmanteau horror movie Dr Terror’s House of Horrors began production exactly a month after this episode was broadcast, and features the very familiar-looking story of a sentient vine menacing Bond’s Bernard Lee and DJ Alan Freeman. Coincidence? We know writer/producer Milton Subotsky was aware of Doctor Who, and went on to make the two Peter Cushing Dr Who movies. This is a pretty daft episode, but with good design and plenty of memorable incident – the moving statues, the creeping vines, the descending knives – it’s also very entertaining.

Next episode: The Snows of Terror