The inspiration has suddenly run out. Although the middle episodes of Marco Polo were repetitive, this feels like the first one that’s a collection of clichés plucked from previous episodes. Susan and Sabetha getting stuck in a cave desperate to make fire; Ian and Barbara having to navigate a subterranean ravine; a fur-clad figure looming out of the snow; statues that come to life – just like last week. The bulk of the episode is all pretty run of the mill. Even the Ice Soldiers are hardly the ‘stuff that makes legends’ – their surprised ‘milling about’ acting, not once, but twice is hilarious.

Where the episode does score is Francis de Wolff’s appearance as Vasor. He’s the series’ most memorable villain so far: initially quite friendly, almost avuncular, he gradually becomes increasingly sinister – rubbing Barbara’s hands, to her obvious discomfort, before sending Ian off into the wolf-infested wilderness. The subsequent scenes are deeply disturbing, like something from a particularly Grimm fairytale, as Barbara is trapped, alone, with the monster. ‘That door will keep anything out. Or in,’ Vasor promises her, before he starts stalking her around the filthy hovel. ‘Don’t you dare come near me,’ Barbara warns as she backs away. ‘I’ll wait no longer,’ says Vasor, as Barbara falls to her back with a blood-curdling scream. The implications are pretty obvious, and horrible – you definitely wouldn’t be able to show this in the modern series.

Almost as disturbing is Altos’s vanishingly short robe: it’s almost as if they’re doing a Troughton’s trousers and taking it in an inch every week. This leads to a very homoerotic moment where Ian rubs Altos’s exposed legs. Altos doesn’t seem to mind.

Once the episode moves into the ice caves, it’s typical Terry Nation stuff. The hiding place of the key is quite neat: frozen inside a block of ice that’s melted by a pipe from a volcanic spring. This bit reminded me of something from The Legend of Zelda, where you have to solve a puzzle, spring a trap, fight some baddies and get the latest piece of the Triforce. Inadvertently, Nation invented the role-play videogame 25 years early.

William Russell and Jacqueline Hill do a fantastic job carrying these Doctor-absent’ episodes for the second week in a row. Even so, you get the sense that the production team were conscious the audience might be wondering where Hartnell’s got to, and there’s the promise he’ll be back next time: ‘We’ll keep our date with the Doctor.’ Nowadays, the ‘Doctor lite’ episodes dwell on how bad life is if the Doctor is not around to save us, or how the companions barely manage to muddle through on their own. Even when Purves or Hines have to carry episodes alone the implication of adventures like The Massacre is that they’ll just get into bother without the Doctor. So it’s quite nice to see Ian and Barbara just getting on with it.

The episode ends with the best climax in ages: rather than just turning up at the next location, Ian’s left in the frame for burglary of the last key, and murder. I love that the cliffhanger isn’t just this, but that the next episode caption reads Sentence of Death – an awesome example of extradiegetic narrative telling the viewers what to expect next time, and adding another level of suspense.