This is an awfully clever episode. At first it looks as though it's a traditional, RTD-style, alien invasion two parter. But it's also a very timely bit of political commentary about immigration and xenophobia, albeit this doesn't quite work. And the twist at the end, of course, reveals that everything we thought was wrong: Clara has been a Zygon since early in the episode. And... did they actually just kill Clara? Like that? A really low key exit for a massively important companion? I really think they may have done, you know.
For something that looks, on the surface, like the same sort of thing as The Sontaran Strategem and similar stories, this is a text full of, ahem, semiotic thickness. References and clever tricks abound. Yes, the method of bringing Osgood back may be predicable but a) Osgood's back, so yay, and b) there's a line about her being a "hybrid", which reminds us of what Davros said in The Witch's Familiar. This is just a mischievous bit of misdirection, of course, but it's clever. We get a nice bit of metatextual playfulness as Kate states that Terror of the Zygons took place in the "'70s, '80s", and it also turns out that it was none other than Harry Sullivan who invented a kind of Warriors of the Deep style genocide gas for the Zygons. Which the Doctor promptly nicked, no doubt declaring that Harry Sullivan was an imbecile.
What else? The Doctor, as a tribute to the JN-T era, wears question marks on his underpants. Kate summoning the Doctor at the start because of Zygons echoes her father doing the same forty years earlier. And, although UNIT is a much more feminine place these days, it still has Berks like Colonel Walsh who just want to blow stuff up. Walsh is played here by Rebecca Front who, with this and Humans, seems to mainly play hard faced bitches these days.
The cliffhanger is superb: Clara and Kate are both dead and Zygon Clara seems to blow up the Doctor. Get out of that one.
I'm impressed so far but I'll reserve judgement until I've seen the second part. There's a slight worry I have in that, although the scriptures to satirise the hysterical tabloid attitude to immigrants, there's no denying that immigrants are literally shown as the baddies free, much as the script tries to emphasise that both humans and Zygons encompass goodies and baddies. It's all very Malcolm Hulke, very Doctor Who and the Silurians (it's even the young Zygons specifically who are portrayed as the bigots), but... well, let's see what happens.