The episode begins with Professor Travers being dragged away from the Goodge Street base by Yeti, and concludes with his return, escorted by them. In between are some of the most visceral action sequences and gruelling horror the show has attempted to date, including the massacre of nearly all the supporting cast in a Yeti attack that shows off Camfield’s dynamic film direction and ability to make about four Yeti costumes look like a horde. Shock moments include the sight of Lane’s corpse shrouded in cobwebs, and Captain Knight’s body left, eyes wide open, where a Yeti leaves it.

Fittingly for such a grim episode, Troughton gives his most subdued performance to date: his voice an ominous murmur as he contemplates the revenge of the Intelligence, a ‘formless, shapeless thing’ that has brought him to Earth. He’s unusually forthcoming about the TARDIS – as he was in The Abominable Snowmen – revealing the truth with a slightly amused smile, a hint of Hartnell’s smug superiority when he first told Ian and Barbara what they’d stumbled into. He also looks very comfortable working with Anne – ‘Well done, Travers,’ he says to her after she convinces Captain Knight to fall in with the Doctor’s plan for an expedition to the surface, which reminded me of Matt Smith’s habit of calling his female companions by their surnames.

We also see the first hint that Colonel Lethbridge-Stewart has the potential to be more than a stolid military man: his willingness to believe the Doctor’s story about the TARDIS, and to act on it, stands out against Knight’s scoffing at the very idea. Later, he’s brutally honest about the fact that the Yeti have defeated the army: ‘You were right, Doctor.’ Though he’s still potentially a double agent – his failure to recognise Driver Evans is a clear red herring – by the end of the episode it’s hard to believe he will turn out to be a baddie.

Next episode: The Web of Fear – Episode 5