Zygons. Shapeshifting, perfect mimics. Now, granted, going into this episode, I didn't know anything about Zygons except what I saw in "The Day of the Doctor", but the concept that there's this race of aliens that can turn itself into such a perfect copy of someone that with memory suppression, the alien itself cannot tell if it's an alien or not is fascinating. Add to that an unknown population of them living among humans, and this episode should have been an epic tale of deception, subterfuge, and distrust.
But that's not what we got. It was a mess of stupid concepts and poor characterizations. The problems and inconsistencies have already been combed through on the internet, so I'll only mention the ones that really killed the story.
I think, in the end, the episode feels like the climactic scene, where the Doctor monologued to convince Kate and Bonnie to stand down, was written first, and then the writers said, "What can we set up to get the characters to this point?" Don't get me wrong, the Doctor's speech was fantastic and Capaldi was amazing during it, but it came down to a very simple point: it doesn't matter what you're fighting over, war is devastating and you should never resort to it, but instead sit down to talk over the differences. It was a bit heavy-handed, but we're used to that with Moffat, and the Doctor did come across as more holier-than-thou than usual, but Capaldi's performance was spot-on and heartfelt.
The problem is that for this final scene to work, for the Doctor to change Bonnie's mind with a morality speech, the Zygons' grievance had to be trivial (or at least trivial in the audience's eyes). If the Zygons were suffering a true, ongoing injustice - for example, the humans were enslaving the Zygons and so the Zygons were trying to start a war to end their slavery - having the Doctor come and tell them "You shouldn't go to war, because people will die and you'll feel bad about it later" would ring really hollow: How dare the Doctor act all high and mighty, telling them how superior his morality is, when Zygons are truly suffering? And it would make no sense for Bonnie to decide that he was right and that her life was just fine the way it was. So, the Zygons' grievance had to be something that the audience would perceive as something they could live with if they just put on an optimistic attitude.
This made it easy for the Doctor. Rather than having to actually solve, or even address, the root problem, he basically told Bonnie to forget the thing she and her people were upset about and don't push the button because you're going to be upset about it later. And forget she did: the Zygon who was ready to fight and kill to throw off her oppression turned completely around and embraced it. In the speech, the Doctor even said that the two sides should sit down and talk it out, but that never happened: one side's memory was wiped, and the other side simply capitulated. I know that the message of the Doctor's speech is supposed to be that war is terrible and people should find an alternative solution, but the story's message is that oppressed people should just suck it up and learn to enjoy it, because the morally superior alien said so.
To set up all of this, we have a cascade of stupid. This has been hashed on the internet in great detail: the absurd numbers of Zygons living in Britain, the addition of powers to the Zygons so that they can pull off spectacular stunts rather than the Zygons using their established powers to great effect, the stupidity of the UNIT troops and of Kate Lethbridge-Gullible (who had no suspicion that the only person left alive in the town might be a Zygon), Clara's inexplicable ability to control her copy, the lie-detector thing, Bonnie's failure to return to Clara to get her to tell her about the boxes, the need to wipe Kate's mind sixteen times to keep her from knowing about the boxes (has this rebellion really happened sixteen times in the last two years, or is Kate randomly punching buttons on doomsday devices?).
There's plenty more, but I'll stop here. Rather than getting what Zygons should do best, which is infiltration and intrigue, or a political commentary on oppressed populations (which is what the first episode looked like it was shaping up to be), we get a meandering, implausible story, a Doctor monologue, and the villain conveniently forgetting she was ever upset. Everything is tied up with a neat little bow.
Doctor Who episodes are not perfect. Name any episode in the modern show and I will point out tons of plot and characterization problems. The thing is, small inconsistencies, even if they're numerous, can be and usually are overlooked if the overall story is consistent. However, if the story is not believable, the inconsistencies start to add up. I know that right now, this episode looks a lot worse than it actually is because of that magnification, but when it comes down to it, it's still not good. And because I had such high hopes for a story of deception and intrigue, I'm doubly disapointed.