The first half of this episode resolves the Millenius plot, and is the weakest part of the whole adventure. After last week relied on Ayden blurting out the truth under pressure, this week his widow, Kala, does exactly the same thing, revealing in the most hackneyed way possible a piece of information she couldn’t possibly have known UNLESS SHE IS THE VILLAIN. If Marco Polo did one thing well it was making it difficult for Susan and Ping-Cho to convince Marco of Tegana’s treachery on the basis of this kind of flimsy evidence, so it’s a shame to see Nation resort to it here. And not once – but twice, because Yartek reveals himself to Ian with a similarly clumsy slip of the tongue, claiming Altos is a stranger when he was a friend of the real Arbitan.

Fortunately, there’s enough else going on that this is just irritating rather than disastrous. As Kala, Fiona Walker gives a hint of the splendid monstrousness she’ll bring to Lady Peinforte, switching from sobbing for her dead husband to cackling at her own wickedness as she menaces the captive Susan. Meanwhile, Hartnell gives another excellent performance. He goes from abject misery at his failure to save Ian, sitting, broken, outside the courtroom, desperately re-thinking events to grasp at a straw of new evidence, barely even acknowledging Donald Pickering’s Eyesen. Then, when he has a eureka moment, he’s all giggles and excitement at the thought of catching the real killer red handed. And then, when he travels back to Arbitan’s pyramid, he has a brilliant bit of physical comedy business as he and William Russell duck down and peer round a pillar, then bump into each other. ‘You’re all running around here like a lot of scared chickens,’ he says indignantly. ‘My dear Chesterton, sometimes you drive me round the bend.’ All the Doctors have to have this ability to turn on a penny, but when Hartnell is so often singled out as an exception to the later rule, it’s lovely to see in almost every respect he’d actually already got there first.

The second half of the episode ties up the story of the Keys of Marinus. Arbitan was already killed five weeks earlier, and Yartek, wearing his robes, is in control when Sabetha and Altos arrive at the pyramid. He doesn’t get enough screen time to qualify as a major villain, but he’s reasonably effective, even if his methods – crowing over his captives and playing the victim – almost exactly mirror Kala’s to the extent that it’s almost a shame Nation didn’t reveal that the two of them were in league to get the last key. His interrogation of Sabetha and Altos is useful because it forces the two of them to recognise their feelings for each other.

It also leads to one of the most charmingly bonkers moments for any Doctor Who baddie, when Yartek, pretending to be Arbitan, asks Ian whether Altos is worthy of Arbitan’s daughter’s love:

Yartek: The young man, who attached himself to her while she was away. Is he a good man? Is he sincere in his feelings for her?

Ian: What is your impression of him?

Yartek There are many races of men on Marinus. He looks and sounds well enough, but I don’t know.

Ian: Naturally, we like and admire him, but since you don’t know him you must make up your own mind.

Thematically, Nation has inserted the dialogue because he wants to reinforce the importance of ‘making up your own mind’ – being able to exercise your own judgment rather than relying on the direction of others. Ian’s point reinforces the Doctor’s later statement that ‘I don’t believe that man was made to be controlled by machines. Machines can make laws, but they cannot preserve justice. Only human beings can do that.’ Both are meant to show that the Conscience, whoever operates it, is a Bad Thing, because it means no-one can answer the question ‘Am I a good man?’, no one can know if their feelings are sincere, and no-one can make up their own mind – it will all be decided for them. However, putting this idea into a last-minute, strange conversation between Ian and Yartek suggests Nation had forgotten to do much with it in his Millenius plot (which would have seemed an ideal moment – justice by heartless machine) and so just dropped it in here.

Visually, The Keys of Marinus is as impressive as the last five episodes. Gorrie includes a couple of very neat touches: the shadow of a Voord appearing on the lattice behind the Doctor and friends as they flee the pyramid, and a slow, rather operatic zoom out as the robed Yartek activates the Conscience (which gets brighter and hums at a different pitch every time he inserts a key).

Nation ends the whole serial, as in The Daleks, with a little moral lecture by the Doctor. That’s characteristic of his later scripts, from The Dalek Invasion of Earth through to Genesis of the Daleks. Other characteristic Nation touches include the fake key (which crops up as the fake Taranium core – and, from other authors in the next Key Quest plot, the fake sixth segment), and the idea of machines replacing free will (Death to the Daleks, Destiny of the Daleks). The episodic format might have been a necessity, but it’s one Nation returns to for The Chase and The Daleks’ Master Plan. It’s very effective. The 1960s were a golden age for horror short story anthologies, and The Keys of Marinus, with its haunted pyramids, evil brains, killer plants, frozen zombies and murderous mermen, feels like the TV equivalent of a Pan Book of Horror. It’s such a shame that Amicus never exercised their option to make a Peter Cushing movie of this one as it would have fit perfectly with their style: The Keys of Amicus? Doctor Who’s House of Horrors?

Overall, The Keys of Marinus has to be classed as a major success, both for design and direction, but also scripting. For all the faults of the Millenius material, Nation’s scripts are pacier than Lucarotti’s, and have the same mix of effective comedy, family-friendly horror, broad sci-fi ideas and pulp execution that made The Daleks such a success. It’s a shame from hereon his contributions are, with one exception, confined to Dalek adventures. Between his last contribution and this, Nation pretty much lays the foundations for the future of Doctor Who, and makes the Doctor a much more central, heroic character than he has been up until now – he saves Ian’s skin rather than the other way round. One of my friends swears this is his favourite Doctor Who story. While I wouldn’t go that far, I’d admit that this is the first adventure that actually feels like I’m watching Doctor Who, and not some strange prototype. It’s obviously going to take some time for Nation’s influence to thoroughly permeate the series, but monsters, mad scientists and over-mighty computers are the way it’s going to go. Trivial Pursuit got it right: in a very real sense, Terry Nation invented Doctor Who.

Next episode: The Temple of Evil