That Peter Harness, eh? Given the opportunity to go back to the events of The Day of the Doctor, and he hasn’t just written a sequel to the Zygon solution in that story, but in doing so has written a sequel to the themes and resolutions of the whole thing – and has made The Day of the Doctor look even cleverer than it already is as a result.

And that Peter Capaldi, eh? Every week finding a new way to play the Doctor, that not only feels consistent with all the weeks that have gone before, but still manages to seem fresh and new every time he’s caught in the lens – and who this week has delivered a scene that not only finally confirms him as the Doctor, but perhaps even confirms him as the Doctor. The definite article, so to speak.

That’s the second story in a row that Peter Harness, without shying away from the grisly deaths that have often been the series’ stock-in-trade, has managed to defy the odds and find a peaceful solution – and in so doing has reaffirmed Doctor Who’s central ideology. His understanding of the series’ core conceits, unlike his understanding of real world science maybe, would appear to be second to none in the modern era, and his hat is surely now firmly in the ring as and when Steven Moffat ever steps aside. If there’s a quibble to be had with The Zygon Inversion (and there are two), it is perhaps that in order to bring the episode to that scene and give it enough room to work, the pacing was a little Russell T Davies in nature. But the way the episode suddenly stopped dead to allow Peter Capaldi his most Doctorish of Doctorish moments was a small sacrifice to make in order to get that moment. And it was exceptional, the author and by extension the actor reiterating what it is that makes the series so different, so enervating, so inspiring – and using the series as a platform to ask the same questions and put the same proposals to the real world too. Some of the politics in the first half of this story might have felt a little heavy-handed, and this was hardly subtle, but maybe sometimes it behoves the series to be that way to get its point across. It was a point well worth making anyway, and Peter Capaldi delivered hands down his best performance (as the Doctor, perhaps of his career) when making it.

Jenna Coleman was also exceptional, particularly in her two-hander scenes with herself – and this is where that second quibble comes in, because in the last ten minutes the real Clara felt rather sidelined in a way that was completely understandable, but that flagged up the manner in which the character has felt a little shoehorned into this series; odd, given how fundamental she was to the last. There’s a sense that having done something so out of the ordinary as Series Eight, the writers are relearning how to write a ‘regular’ companion again, and there have been a couple of occasions during this year’s episodes when Clara has been there in body but the spirit has felt forced. Having said that, Coleman’s performance as ‘Zygella’ was sublime, and the chemistry between the actress and Peter Capaldi will be sorely missed when she leaves (and was the reality-within-a-reality beginning to this episode another clue as to the manner of her departure…?).

Elsewhere, Harness’ second half was all about ambiguity and deception, and fulfilment. When the first missile misses the plane, you think the cliffhanger has been evaded, but then when the second scores a direct hit you realise what Harness is up to: leading you up the garden path in order to show you what a wonderful place the garden is.

The episode even employs the same trick that Forest of the Dead did seven years ago in order to fool the audience into thinking, however briefly, that Harness was ‘doing a Moffat’ and moving the second instalment on from the first; rather, Harness was using The Zygon Inversion to tie up those elements of the first episode that might have felt somewhat clunky. The symbiosis spoken about between the two Osgoods would have seemed unwarranted if we hadn’t seen the two Oswalds living a similar experience, and if it felt unworthy of Harry Sullivan to have created a genocidal gas, then the resolution – in which it is absolutely fundamental that such a gas doesn’t exist – resolved that; but the Doctor needed to give the notion of such a gas verisimilitude in order to create the belief among all involved parties that the Zygon Solution was a very final possibility. And if the global nature of The Zygon Invasion might have left the first episode lacking a little in clarity, then the way in which Harness focussed the second half – so much so that if she hadn’t appeared in the reprise at the beginning, we might even have forgotten that Jaye Griffiths’ eminently agreeable character had even been involved – gave the story as a whole a sense of homing in on its objective.

The Zygon Inversion resembled nothing so much as a modern duplicate of a classic era story; channelling not only Philip Kaufman’s 1978 Invasion of the Bodysnatchers (the scene in the city centre in which one of the contented Zygons is unwillingly revealed was very much out of the Donald Sutherland film’s repertoire), Harness’ story was very much of a piece with 1970s Doctor Who; the fake policemen were straight out of Terror of the Autons and the body horror was an updated Hinchcliffe and Holmes conceit, while the political backdrop was the kind of thing the Jon Pertwee Doctor was forever getting involved in. This was Invasion of the Dinosaurs mashed up with The Ark in Space and given the ending from The Day of the Doctor (or any other name-your-Moffat poison) for good measure. Harness also managed to use the way that story concluded as the inspiration for his own resolution in a way that paid homage to the Zygons’ original use as foreshadowing in the anniversary special. It’s almost as if Harness opened up a conversation between the two stories and was replying to Steven Moffat’s motifs. As if the Zygon portion of The Day of the Doctor was always meant to finish on such an ellipse.

But The Zygon Inversion is really all about Peter Capaldi, and that dialogue with Bonnie. It’s a scene that Capaldi was almost begging to have written for him after the ‘idiot’ realisation in Death in Heaven, and by providing him with such a beautifully expressed paean for peace, Harness has expanded upon the achievements of The Witch’s Familiar (in which Davros was the better served) and The Woman Who Lived (in which Lady Me got the meatier stuff) to give the Twelfth Doctor a coming of age moment of his own. The only possible niggles were the almost self-congratulatory comments from the Doctor regarding whether he was getting through to the Zygon, but these were offset by the naturalness of Bonnie’s responses and the handling of the way both Kate Stewart and her Zygon opposite stood down. It was a scene all about the importance of talking, a finding a solution that doesn’t involve mutual destruction – and if the first episode of this two-parter promised an explosive ending, then it was the acting and the ideas that provided the fireworks. And that’s the real Zygon inversion, taking an audience’s expectations and replacing them with a far superior reversal of those expectations instead.