Intriguingly, this episode explicitly takes place in real time: near the start the possessed Travers, moving awkwardly as a puppet controlled by the Intelligence, warns the Doctor that he has 20 minutes to make up his mind whether he will give his mind to the Intelligence for the sake of all the human lives, and be forced to live as a child with Jamie and Victoria as “parents”. And then, at the halfway point Anne remarks that they have 12 minutes remaining to come up with a way to re-wire a Yeti control sphere to regain the initiative.

Meanwhile, feeling useless while the Doctor and Anne tinker, Jamie and Lethbridge-Stewart venture out into the Underground network to see whether they can rescue the captive Travers and Victoria. At this point, simply wandering round the tunnels in the dark is starting to wear a bit thin, so Haisman and Lincoln throw in a scene of the Colonel opening the door on the horrid, pulsing mass of the alien fungus – and later, another scene of it smashing through the walls of Goodge Street to flood the base. These moments are reminiscent of the oozing horrors from Hammer’s Quatermass spin-off X-The Unknown, adding to the sense that this story fits into a tradition of John Wyndham-ish British B-movies.

Again, Haisman and Lincoln have nailed the six-part structure: the middle episodes have included a fairly transactional recap, an all-action diversion to the surface that kills off most of the extraneous characters and slims down the cast ahead of the climax, and this tense countdown to the finale. They’ve also thrown in enough nice twists – the reappearance of a character we thought dead, and the Doctor gaining control of a Yeti to even the odds a bit – to keep this interesting and ensure that we’re not just waiting for things to play out. And equally there are some great directorial touches (including a great jump scare of a Yeti looming out a tunnel mouth at Victoria and Travers), and a lovely turn from Troughton – who’s switched from last week’s ominousness to this week’s delighted tinkering with the Sphere. This is great.

Next episode: The Web of Fear – Episode 6