Before I say anything else, the post below contains spoilers (lots of them) for The Zygon Invasion. If you haven't seen it I recommend you stop reading now and go watch it.
You still here? Ok, I'll be honest and say I really wasn't expecting much from this first-parter. Actually, I really wasn't looking forward to it at all.
The Zygons have just never really had that kind of impact for me. Of course, 1975's Terror of the Zygons was a good story but in The Day of the Doctor I just felt they were... kind of average. The story involving the Zygons was kind of weak, in my opinion, and there were issues I had with their story and the resolution. The "decision moment" between the two Kate Stewarts towards the end was the highlight of the Zygon part of that episode for me. The rest was mediocre.
So when I heard that we were getting a two part story involving on the Zygons I felt a bit of a gnawing at my stomach. Season 9 has been blisteringly good to date, I think, and I really didn't want to see a story that would detract from that right in the middle of the season.
I'm very happy to say, I couldn't have been more wrong.
The Zygon Invasion was, on its own merits a masterpiece of tension, suspicion and intrigue. The Zygons, themselves, were menacing and actually scary. Part of that fear came from not knowing who you could actually trust. There were moments that had me scratching my head a little. These were the soldiers who just walked into the church after a little persuading from the (obviously)_ Zygon duplicates. The second was Kate Stewart, the head of UNIT, walking into Zygon occupied Truth & Consequence on her own. The first can be explained away satisfactorily but that second point... never, ever would the head of a para-military organisation like UNIT be allowed to march into a potentially hostile region with just a sidearm. No way.
That aside, and they are perhaps just nit-picky points, the rest of the episode was spectacular. The Sheriff in Truth & Consequences was kind of obvious as a Zygon plant but the obviousness of that didn't detract from the encounter. They played that out well enough to leave just the tiniest smidge of doubt, enough to make you go, "Yes! I picked it" when the truth was revealed.
The one that really got me was Clara.
Some of you will, no doubt, have picked up on her replacement but not me. I was totally shocked to see Clara in the Zygon pod. Looking back, there are signs, subtle ones perhaps, but they're there. But they're subtle enough to fool the likes of me. It was a surprise and it was brilliantly played out. Now, am I the only one who thinks Clara plays a magnificent "bad guy"? This is something that we've never seen of her character before and Jenna Coleman played it brilliantly. I'm really going to miss Clara.
It was good to see Osgood back too and they handled her return, post-death by Missy, just as I expected they would. Her being there felt seamless and natural. The Osgood Box has me a little perplexed by I have no doubt we'll see some resolution to that in the next episode.
Speaking of... I harbour a dreadful fear that the next episode will let down the first. This may just be a remnant of my initial misgivings but the name... "The Zygon Inversion"... conjures thoughts of a resolution to the whole story that involves a horrible deus-ex-machina "switching" of Zygon and Human counterparts. I sincerely hope I'm wrong. I want to see the second part be every bit as dark and tense as "Invasion" was.
Time will tell but, for now, I'm still basking in the awesomeness that was "The Zygon Invasion".