In the very final analysis, this is a bit of a cop-out ending. The Doctor’s spent the last week and a half coming up with a clever plan to foil the Intelligence, but ultimately he doesn’t get to carry it our because Jamie jumps the gun, uses the hacked Yeti to attack its peers, and wreck the Intelligence’s machine. There’s a lot of chaos, a big explosion, and then it all wraps up in about three minutes as the Doctor bemoans Jamie’s actions and warns that the Intelligence is still out there: ‘It might come back.’

This is all, obviously, set-up for a third encounter with the Yeti – which was planned but, for various reasons mainly due to a falling out with Haisman and Lincoln, never happened. Which is a real shame, because we’re denied a ‘final end’ moment – an omission not addressed until Steven Moffat wrote The Snowmen and The Name of the Doctor as part of the 50th anniversary series.

That’s not to say this is in any way bad – it’s well paced and Haisman and Lincoln keep us guessing about the identity of the Intelligence’s human pawn right until the final minutes, with Evans remaining shifty and untrustworthy, Lethbridge-Stewart again falling under suspicion (notably because of his attempt to guilt the Doctor into handing himself over to the Intelligence) and Chorley making a remarkable return (‘I’d forgotten about you,’ says Arnold, speaking for the audience). But the reveal that it’s the dependable Arnold himself is a good twist – the Hartnell sergeant major type figure turning out to be an animated cadaver could almost be an in joke, particularly given Jack Woolgar’s passing resemblance to Hartnell.

Overall, this is even better than The Abominable Snowmen, even though it has a slightly cynical whiff to it. The two stories share a common theme – the sin of intellectual hubris, of over-reaching beyond what man is supposed to know, punished by supernatural malevolence. It’s very M.R. James. In The Abominable Snowman it was Padmasambhava’s delving into the secrets of astral projection; here it’s Travers’ technological tinkering – and in both cases the men are horrified by the evil their curiosity has unleashed.

But between Camfield’s direction, the astonishing design, the well-structured script, and Troughton’s performance this is very strong. Perhaps its main weakness, beyond sacrificing a proper ending for sequel-hunting, is its failure, again, to give Jamie or Victoria much to do. Fair enough, Jamie gets to play the role more usually associated with Ace – only half briefed by the Doctor, doing his best and then chastised because he doesn’t know exactly what the Doctor has planned.

But Victoria gets practically nothing to do for the third serial in a row. Whereas Anne is clearly an intellectual foil for the Doctor, and Jamie the comedy partner, Victoria’s initial spark has faded and she spends all her time trailing round, wailing. It’s almost a surprise that she doesn’t stay behind with the Professor while Anne goes off with the Doctor. I’ve already commented on how much Anne anticipates Liz, but there’s a lot of Zoe in there as well.

As for the Intelligence: it’s such an iconic villain that it’s name is mentioned perhaps more than any other monster, including the Daleks. As we will soon learn, after this incursion the United Nations sets up a special paramilitary organisation in anticipation of its revenge. This Intelligence Taskforce, under the leadership of newly-promoted Brigadier Lethbridge-Stewart, ended up fighting more than the monster it was explicitly named after, but its primary function, now and always, remains to defend the Earth against the return of the Intelligence.

Next episode: Fury from the Deep