Having set up exactly what is happening, how and why last time, this episode goes ahead and shows it happening, and at great length. The scenes of scientist Senta extracting life energy first from Nanina and later the Doctor take up about a quarter of the running time. The telesnaps make the laboratory set seem impressive – with bubbling vats, and a glass extraction chamber – and it’s likely Christopher Barry maximised the vampiric horror of these scenes.

But, at its heart, the episode boils down to a battle of ideas between Jano and the Doctor, centring around their different attitudes to the value of life, and the balance of material progress versus individual freedom:

JANO: Do you not realise that all progress is based on exploitation?

THE DOCTOR: How dare you call your treatment of these people progress.

Ian Stuart Black’s nakedly political script clearly comes down on the Doctor’s side of the argument, with various pointed lines of dialogue highlighting the rottenness at the heart of the City: ‘Your rewards are only for the people who agree with you’ sounds like the kind of thing an anti-Capitalist protestor might come up with now. In a particularly telling moment, the effete Avon plaintively declares, ‘ This is a free state, and we are all equal here’ before one of the Guards (who seem to be the ones who actually do all the work, and take pleasure in bossing the hippyish citizens around) shoots him with a light gun.

It’s amusing to hear Hartnell, who we now know to be a fairly reactionary sort, deliver lines like, ‘They’re men – human beings, like you and me.’ But the Doctor’s furious denunciation of the Elders’ so-called “wonderful civilisation” is stirring, and Hartnell rises to the occasion with a much more assured performance (with far meatier material) than in Episode 1. This may be deliberate – perhaps Innes Lloyd was trialling the idea of using Hartnell in alternate weeks, minimising his participation in Episode 1, and letting him loose here, as a way to ease the burden.

Again, Jackie Lane gets more to do than usual, threatening to smash up the laboratory when she’s accidentally caught up in Senta’s experiments, having wandered into a section of the City that’s off-limits. This highlights an apparent inconsistency in the script, though – Jano proudly told the Doctor about the life-force transfers last week, but here it’s treated as if it’s a dirty secret. Why that should be isn’t explained – if Avon and Flower are typical Citizens, then it’s common knowledge, and it’s hardly like the Savages don’t know what’s happening to them. So who are the Elders afraid of finding out?

With a robust ethical conflict at its heart, and with a horribly protracted cliffhanger of the Doctor being drained of his vitality, this achieves the rare distinction of being a second episode that’s a lot better than the first.

Next episode: The Savages – Episode 3