The impressive model shot that opens the episode immediately indicates that this is going to be another space adventure. Like Terry Nation’s last set of scripts, it starts out on an apparently dead planet – instead of a petrified jungle there’s a glass beach in an acid sea, and the regulars spend the first part of the episode exploring this alien environment and narrowly avoiding some of its dangers. But there’s a real confidence to this that wasn’t quite there in The Dead Planet: by this point the series was a success, and perhaps buoyed by this and the knowledge that the Daleks had been massively popular, Nation has written a very ambitious episode that’s very demanding of the production team, requiring a beach set, a giant pyramid, catacombs, loads of model work, and a new set of monsters. Luckily, they’re absolutely up to the job: this one looks as impressive as the telesnaps from Marco Polo, and the enormous Conscience of Marinus set is particularly good.

This absolutely feels like the first story to be written after The Daleks was transmitted. Nation plays up the monster angle, which was notoriously absent from Sydney Newman’s original conception: ‘However inviting that water looks, we don’t know what sort of creatures might be lurking beneath its surface,’ the Doctor says, ominously, before the camera cuts to a web-fingered thing slithering into shot.

Nation sets up the quest with admirable economy: establishing that the Voord are a gang of criminals set on corrupting the whole of Marinus, perverting a machine that was meant to eliminate evil from the minds of men. This harks back to a theme from The Daleks – the moral dimension of free will, and whether intelligence is used for good or for evil. If you abdicate the moral responsibility for your choices to a machine, what happens if that machine goes wrong? As with many 1960s adventures, this one doesn’t really answer the question, but leaves it hanging as something the audience might engage with, as though its meant to provoke some discussion round the family dinner table.

Nation’s script is also quite funny: cutting from Arbitan begging the TARDIS crew for help to them all heading back to the TARDIS is slightly surprising in retrospect, because nowadays the Doctor would probably jump in to help straightaway. But it’s also quite amusing – and it means Arbitan is a definite precursor of the White Guardian, who similarly threatened the Doctor that if he failed to find a key nothing would ever happen to him again. The most deliberately hilarious moment comes right at the end of the episode though, with this exchange between Arbitan and the Doctor, which pokes fun at the whole notion of teleportation:

The Doctor: If you think I’m going to travel across that acid sea in one of these primitive submersibles, you’re very much mistaken.

Arbitan: I wouldn’t think of asking you to travel in such an absurd way. No, I’m going to give you a device which will enable you to move from place to place… Place that around your wrist, please. The principle is much the same as that of your ship you told me about, except this will enable you to cross space, not time.

Ian: What, this little thing?

Doctor: Oh don’t be ridiculous, my boy. This is a perfectly acceptable method of travel.

Nation, and director John Gorrie, also play up the horror aspects of the show, particularly in the scenes where the Voords stalk Susan, and then when the cowled Arbitan appears like a vengeful ghost inside the pyramid to kill off the invaders. The threat of the acid sea is also horribly illustrated by the dissolved Voord – whose grisly fate we can well imagine having already witnessed Susan’s melting shoe.

Barring a couple of visible stagehands and a couple of Billyfluffs, there is nothing about this episode that doesn’t ooze self-confidence and a sense that the show can achieve practically anything. The regulars are all superb, George Coulouris sells Arbitan’s isolation and power with just the right level of hamminess, and the Voord, in their weird rubber suits, are memorably strange. This is excellent.

Next episode: The Velvet Web