Usually when reviewing a two-part episode of any TV show, one has to wait for the second part to really say what they thought of the story as a whole. Through the years, Doctor Who has had some pretty great parts ones and some pretty stupid part twos. Series 9 has, so far, been really great at matching the quality of the two parts, even if the storytelling methods do change. Last week’s “The Zygon Invasion” was the first true part-one because its conclusion, “The Zygon Inversion,” takes place immediately following and doesn’t change up the type of story too much. Now, having seen “The Zygon Inversion,” I can say that in a series full of episodes I’ve loved, Peter Harness and Steven Moffat’s story of alien/human cold warring might be the best yet, and one of the best in years.

On paper, this episode has all the “Yes, Kyle’s bound to like this” stuff, just like last week – UNIT, Zygons, ’70s-inspired sci-fi paranoia, evil doubles – but it also needed to have a good story to keep me invested beyond simply being reference heavy. And, for my money, it totally did. This episode features some of the best debate-based dialogue the show’s ever done and allowed the lead actors to get to act opposite each other. It now makes sense why Jenna Coleman wasn’t in “The Woman Who Lived” very much considering she had to have conversations with herself in this one. And boy, was she great.

Bonnie, the Zygon general who looks like Clara, proved to be a very complex villain. She wants her people not to be slaves and she’s prepared to have literally all 20 million of them die if it means not being under the thumb of the humans, even though most of her people are perfectly happy to live in secret without taking over the planet. But, like the Osgoods said at the beginning of the last episode, there are bad people in any group, and not all members of the group are bad. This goes back to Harness’ theme of thinly-veiled terrorism metaphors. Very thinly. Like, almost not there at all. But, like the best science fiction, an otherworldly element can change the way things are seen.

Jenna Coleman got to play double duty in some really interesting scenes opposite herself in another room. This isn’t the first time this has happened on the show by a longshot, but she was able to play both characters distinctly and believably. I believed Bonnie was a character unto herself. I really enjoyed the episode’s version of what Clara sees when she’s inside the Zygon pod, the living room and the words she can’t really read and things that are almost normal but are just slightly off. I also liked how quickly Clara figured out that she could manipulate Bonnie’s body without her knowing, and I loved the little glimpse of Clara in the mirror that Bonnie notices. Oh, also the First Doctor portrait hiding a safe. Super funny. The only downside of Bonnie being such a great character is Clara is sort of sidelines during the episode’s climax.

And with Clara talking to herself for most of the episode, the Doctor (a.k.a. Dr. John Disco, a.k.a. Basil) got to hang out with his new best friend Osgood. She’d be a great companion, but she’s got bigger fish to fry, overall. The constant asking whether or not she’s the human or Zygon Osgood got sort of annoying by the end, but I think this was to drive the point home as much as possible that it doesn’t matter, that it’s the not knowing which she is that makes the peace work. She had to represent both and neither, and the denouement and reinstating of the two Osgoods at the end is testament to that. Just like there’s always two people at the keys to a nuclear warhead, there have to be two Osgoods to keep the peace.

Which leads me to my very, very favorite thing about this episode. Well, besides everything else. The whole thing surrounding the Osgood Box is a million kinds of genius. This is what Steven Moffat’s tenure on the show has given us, plotlines like this and scenes like the Mexican standoff in the Black Archive. The Osgood Box can unmask all the Zygons, causing panic on Earth and igniting a war only a handful of Zygon militants want. Trouble is, there are two Osgood boxes. One will unmask the Zygons and the other will destroy them using Harry “The Imbecile” Sullivan’s gas. (The Doctor called his companion Dr. Harry Sullivan an imbecile in the 1975 story “Revenge of the Cybermen.” Harry’s final story as companion was “Terror of the Zygons.” There are references to this throughout the two episodes.)

But even that isn’t entirely true. Each box has two buttons on the inside, one marked “Truth,” the other marked “Consequences.” In the one box, the Zygons will either all be unmasked or all left in human form forever; in the other, the Zygons will either all be destroyed by the gas or the entirely of London will be by a bomb. Are Kate “Five Rounds Rapid” Stewart and Bonnie prepared to let their own people die just to win a pointless war? This was one of the tensest and best-acted scenes in Capaldi’s time on the show so far. This was like BAFTA, Golden Globe-nomination reel stuff. He’s amazing, a mixture of flippant silliness and deep pain. This is what a war looks like – no matter how righteous you feel when you fire the first shot, all you’re doing is getting people killed while wasting time until you just have to sit down and talk. Summed up beautifully, and it got Bonnie thinking differently.

There of course was nothing in either box, but that isn’t the point, and the fact that Kate has apparently done this 15 times proves it works. And, truthfully, who better to be the new Osgood than someone who no longer wants people to die for their own petty beliefs? Such a wonderful ending. I can’t say enough about it. Maybe I’ll go watch those 15 minutes again, and then every day forever.

Next week, Mark Gatiss gives us his episode, “Sleep No More,” which looks like it takes place on a scary deserted spaceship. The title is a reference to William Shakespeare’s Macbeth so I’m hoping there’s some literary nerdiness in there, especially given its follow-up is Sarah Dollard’s “Fear the Raven,” a Poe reference. At any rate, Reece Shearsmith and a sleep monster? I’m in.

Eight episodes, eight wins. Let’s see what the last four have in store! Let me know what you thought of “The Zygon Inversion” in the comments below, or hit me up on Twitter @FunctionalNerd.

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Images: BBC

Kyle Anderson is the Weekend Editor, a film and TV critic, and the resident Whovian for Nerdist.com. Follow him on Twitter!