Still there? You were warned.
Part one starts off with fighting happening in some unknown place. Fleeing from the fighting is a familiar face: Osgood from U.N.I.T., the Doctor's number one fan-girl. This is, of course, a bit confusing as we saw her get vaporized by "Missy" in last season, but it turns out that there's been two Osgoods since the events of "The Day of the Doctor". One Zygon and one Human. Nobody knows which is which and the Osgoods have been in charge of keeping the peace in the truce between Humans and the Zygons who live in-secret amongst us.
It turns out that a splinter group of Zygons has broken the peace, wanting to live as Zygons and not shapeshifted into humans. So they want "the Osgood Box", some MacGuffin that can be used to end the cease-fire somehow. So they captured Osgood and interrogated her, but not before she was able to send off an SOS to the Doctor.
Meanwhile, Clara finds something amiss in her apartment building and gets snatched by Zygons and replaced.
The Doctor comes and meets with U.N.I.T. and Clara. He wants to talk to the splinter group and negotiate. Kate wants to use some nerve gas Harry Sullivan made to kill Zygons but the Doctor stole. There's a lot of tension.
Clara (secretly a Zygon) and a U.N.I.T. flunky do investigation in London on weird disappearances of people while Kate goes to New Mexico to investigate where Osgood disappeared. The Doctor goes to Turkmanistan to where the renegade Zygons had a training camp.
There is a lot of paranoia resulting in a lot of dead U.N.I.T. people. Clara is revealed as a Zygon before she kills all the U.N.I.T. redshirts with her. Kate is cornered by a Zygon and the Doctor is on a plane returning to the UK when Zygon-Clara calls him to say Clara is dead just before she fires a missile at his plane.
Part two starts off where the cliffhanger ended. Turns out Clara is alive and in a Zygon pod. Being a Mary Sue, Clara is able to use her special Clara mind-powers to influence the Zygon holding her shape and interfere with Zygon-Clara's assassination of the Doctor... mostly. The Zygon-Clara still blows up the plane, but the Doctor and a rescued Osgood (retrieved in the prior episode) escape to safety. Presumably the pilot is killed, but no mention is made either way. Kind of cold, really.
So Zygon-Clara finds out that her interrogation of Osgood does not get her the "Osgood Box" that she believes to be a device to force all Zygons to assume their true form, forcing war between Zygons and Humans.
Frustrated, she goes to Clara's pod and mind-melds with Clara to interrogate her. Turns out Clara knows where the "Osgood Box" is. It's in the Black Archive (from "The Day of the Doctor"). Zygon-Clara meets with Kate Lethbridge-Stewart who returns from New Mexico, apparently replaced by a Zygon.
Zygon-Clara takes Clara's pod with her and they jaunt off to the Black Archive. Meanwhile, the Doctor and Osgood have a bit of banter. The usual thing with the Doctor rambling on (he claims his first name is "Basil", for what it's worth). He wants to know if Osgood is Human or Zygon and she's not telling.
Everything comes to a head in the Black Archive where Zygon-Clara finds out there are two Osgood Boxes, each with two buttons. The Doctor comes on-scene, as does Kate. We get a reenactment of the show-down from "Day of the Doctor" only with these boxes. The Doctor claims one will either release the Zygon-killing nerve gas or detonate the nuke under the Black Archive, depending on which button is pressed. The other one will either force the Zygons into their true form or disable their shapeshifting, turning them pure human forever.
There's a lengthy bit of speeches in which the Doctor tries to convince Kate and Zygon-Clara not to press the buttons. He talks about his role in the Time War and the terrible consequences. It's pretty much the same fare that's been rehashed since 2005, so nothing new there. In an interesting turn of events, his speech works. Kate closes the box, buttons un-pressed. Zygon-Clara is reluctant, but finally agrees to not press the buttons.
Turns out the boxes are fakes. They don't do anything.
The Doctor wipes everyone's memory save for Osgood, Clara, and Zygon-Clara. Turns out this is the fifteenth time this showdown has happened.
Zygon-Clara takes the place of the missing Osgood. The Doctor and Clara jaunt off in the TARDIS. The Osgoods go off for ice cream.
The summary makes this story sound ridiculous and stupid, but I have to say I enjoyed it. The paranoia of the episodes was well-done and the threat was kind of cool. The villain being a splinter group of idiot children was a nice twist. And it was especially-nice that the Doctor's preaching found an audience for a change. Overall, it was pretty enjoyable.
EDIT: so there are a few points that bugged me in this episode:
- Why did the Doctor keep asking Osgood if she was human or Zygon? What did it matter?
- Mary-Sue-ness aside, how is Clara able to perform her bit of psychic mojo against the Zygons?
- Why do they keep portraying Kate Lethbridge-Stewart as borderline bloodthirsty? I thought she was supposed to be more "science" and less "guns".