The Zygons are properly unveiled and they’re very impressive: the decision to build out the heads but leave the actor’s eyes and mouth largely visible makes it easier to distinguish them as characters rather than a whole host of rubbery aliens like the Silurians or Sea Devils (whose MO, shapeshifting aside, the Zygons have appropriated). But beyond the costumes, there’s a concerted effort by all departments to make them a coherent design, from the sets, which reflect the suckers on the costumes, through the props, which look like the products of the same culture, and even the voices, which sound phlegmy – moist and sticky as the Zygons generally seem to be. It’s a shame the exterior of their ship looks too smooth and metal – otherwise this is as good as Axos.

Their background is interesting as well. Like the survivors on the Ark, they’re refugees, their ‘world destroyed in a stellar explosion’. There’s also a vague implication that they can time travel when Broton claims, ‘We shall change the destiny of Earth.’ It may just be a figure of speech, but it’s enough for Zygor to plausibly be a casualty of the Time War in The Day of the Doctor. To Harry’s surprise, they’re something like mammals, dependent on the milk of the Skarasen to survive. Interestingly,and often overlooked in spin-off sequels, it’s stated that this Skarasen has been altered: ‘We have converted the Skarasen into an armoured cyborg of devastating power.’ The inference is that the Skarasen aren’t normally huge armoured monsters at all – I’m imagining something more like a blobby cow with numerous suckered teats for the Zygons to feed at.

This episode is tremendous. There are hints of Robert Banks Stewart’s unfamiliarity with the series – the Doctor’s investigations at the hospital skirt with crime thriller, and Sarah Jane’s showdown with the fake Harry is pure Hickson Miss Marple horror. Douglas Camfield’s direction is brilliant: the scene of a soldier patrolling through the mist belongs in a movie with several times the budget. The sheer bravura of the show tackling the Loch Ness Monster head on, in broad daylight, and almost entirely pulling it off is a mark of supreme confidence. It’s such a shame this didn’t close out Season 12 as planned, given it gives Benton and the Brigadier some decent, dignified material, and thematically it ties up the ideas of desperate survivors battling for control of a planet by bringing the struggle right back to contemporary Earth.

Next episode: Terror of the Zygons – Part Three