It’s Troughton’s first week off in a while, and Haisman and Lincoln use his absence to their advantage – his mysterious disappearance makes him a suspicious person of interest to Captain Knight and his men, none of whom knows exactly who or what is behind the Yeti attacks. This is made clear in an effective scene between soldiers Blake and Weams, who dismiss the idea that they’re ‘abominable snowmen’ for the more prosaic explanations of a foreign power’s robot army, or creatures from outer space

Troughton’s absence also allows Frazer Hines and Deborah Watling to get some vaguely more rewarding material than they’ve had in the last two serials. Having learned that Yeti are on the loose in Tooting Bec, Jamie smartly surmises that the Intelligence must be behind both the invasion and their arrival. This is only reinforced by a delighted reunion with Professor Travers (‘Hasn’t he got old?’ Jamie remarks, tactlessly). Hines then gets to join the exploration of the tunnels, while Watling lurks about in the base, overhearing conversations and drawing her own conclusions.

The main Yeti sequence in the episode is notably well directed: looming out of the dark, close ups of their rending claws and handheld camerawork make them look significantly more dynamic and frightening than the Mark One versions. No wonder this story passed into legend as the epitome of behind-the-sofa Doctor Who. But the whole thing is exceptionally well done, with all the characters convincingly cast (particularly Jack Woolgar’s entirely believable staff sergeant) right through to the unfortunate comedy Welshman.

While Lethbridge-Stewart is obviously the story’s main contribution to the canon, it’s also fascinating to see how closely Liz Shaw hews to Anne Travers – they even sound the same as they patiently rebut patronising men. In this episode, Anne gets a particularly spicy exchange with the vile reporter Harold Chorley, tearing apart the gutter press and sensationalist journalism. The best Chorley can throw back is Anne is a redbrick university jibe that she bats away. In her own way, Anne is as much the template for 1970s companions as Vicki was for the 1960s: as well as Liz, I think there’s something of Anne’s temperament in Sarah Jane, Romana, Tegan and Mel.

Next episode: The Web of Fear – Episode 3