Once upon a time…there were….three doctors…two Osgoods…and one peace treaty…(back in “Day of the Doctor”)
Two Osgoods describe Project Double, which is Unit’s attempt to resettle the Zygons masquerading as humans. There’s a flash of one Osgood mourning at the grave of “My Sister.” They say the Zygon’s shapeshifting is not a weapon, but a survival mechanism. If one Zygon or human goes rogue, the treaty breaks, so the Doctor left “the Osgood box” as a means of last resort.
Cut to Osgood running into a building, hiding under a desk, and puffing her inhaler. Chaos and gunfire can be heard outside. A Zygon walks in the room as Osgood calls the Doctor on her phone. The Zygon drags Osgood out from beneath the desk and sends some sort of power current into her brain.
The Doctor is playing amazing grace on his guitar in the TARDIS when he gets the call on the “Nightmare Scenario.”
The TARDIS arrives in Brockwell Park, London. The Doctor calls Clara but gets her voicemail. He says he’s staking out dangerous creatures, and goes to speak to two little girls.
Kate, Jac and Unit are mobilizing in search of Osgood. They spot her with a satellite, being dragged away in shackles by two Zygons. Jac says their encryption has been broken.
The Doctor talks to the two Zygons disguised as children. He says he knows there are rogue, extreme factions of Zygons, but the children say the extremists are their problem. The Doctor gets a call from Kate. While he’s distracted, a smoke bomb goes off and a Zygon kidnaps the girls, taking them away in a van. Symbol is left behind.
The same symbol is hanging over Osgood in a video message released by the extremist Zygons. The Doctor calls Clara and gets her voicemail again. He tells her to call him back now.
Clara is notified to 127 missed calls from the Doctor, but she runs into a lost child in the stairwell before she can listen. She goes looking for the boy’s parents. She finds them, but they’re acting odd and the boy screams when they take him. She calls back the Doctor.
Unit brings the Doctor and Clara to a preschool that was also the secret headquarters of the Zygon high command. Kate says the Zygons have been difficult to deal with since Osgood went missing, and that there’s been radicalization of the younger brood. They a Zygon communication command computer that’s been compromised somehow. The Doctor gets it working again. They explain to Clara that there were two Osgoods that were both human and Zygon. One died, and the other went mad with grief. They get a video message from the Zygon high command, the children Jemima and Claudette. The morph into Zygon form and are killed. Another Zygon claims to be the new Zygon high command and says “truth or consequences.”
Kate refuses to negotiate with the rebels. She says she’s going to order Colonel Walsh to bomb the high command. The Doctor warns that bombing the extremist minority will only radicalize the Zygons as a whole. Clara says “truth or consequences” is a town in New Mexico. The Doctor sends them there.
Kate tells Clara about a gas that kills Zygons that was developed by Unit, but was taken by the Doctor
Catelyn and Jac return to Clara’s apartment building and try to follow a mysterious figure, but he disappears in an elevator. Fluid begins to seep out from behind the control panel. Clara opens it up. Inside is a Zygon tech control. She touches it and the elevator shakes. The lights go out. The door opens and she and Jac find themselves in a secret underground area.
The Doctor arrives as Unit prepares a drone strike on the Zygon town. The drone operator hesitates and finally pulls away.
Kate arrives in Truth or Consequences and finds the Zygon extremist symbol. She pulls out her gun and begins to investigate. She finds the sheriff’s department abandoned. An officer pulls a gun on her. She asks if Kate came alone. Kate asks why she’d need it.
Colonel Walsh explains that they don’t know how to track the Zygons. She believes they should have the gas to kill the Zygons.
Jac tells Clara about strange activity in elevators across the countries, people being dragged in bundles and then disappearing.
The officer recounts to Kate about how the Zygons slowly came into the town. She tells Kate that Osgood was here, in a hotel, but they’re all gone now.
Unit moves in on the Zygon village. Walsh believes they’re holed up in the town’s church. They call for the Zygons to come out and surrender. A woman walks out alone. The woman looks like a soldier’s mother. She tells them that the commanders are the imposters. More people walk out of the church. The mother tries to get the soldiers to come inside. The soldiers follow them in. Walsh shoots her way into the headquarters. There’s a map of the Zygon invasion and electrified bodies.
Walsh leaves and the Doctor finds Osgood in a cave. He frees her, and a Zygon appears behind him. The bombs start coming down, and rubble falls on the Zygon.
The officer shows Kate a bin of electrified people.
Clara and Jac lead Unit underground. They find something glowing red.
The Doctor’s group whell out the captive Zygon from the cave onto the Doctor’s “President of the World” plane. The Doctor comments on Osgood’s question mark collar. He asks her which one she is, and she rejects the question. The Doctor believes she must be human because the other is dead, but Osgood explains that’s not the case anymore.
Clara and Jac walk into a cavern full of Zygon pods.
The Doctor interrogates the captive Zygon. The Zygon says they want the world.
Unit soldiers march into the cavern. Clara says they’re growing duplicates and finds one of herself. Jac realizes that these are the originals, which means Clara is a Zygon. She and Unit find themselves surrounded. “Clara” gives the order to kill the traitors. It is revealed that Clara was captured after finding the boy in the stairwell.
The Zygon prisoner tells the Doctor that they want a world where they can live in their true forms. He says they’ve already taken the UK>
The officer recounts how a single Zygon was seen by the humans and the humans saw them as monsters. She realizes Kate has no backup coming and transform. “Clara” calls the Zygon in New Mexico, now in the form of Kate. “Kate” tell her she’s been neutralized. Clara reports to another that Unit has been neutralized in the UK.
“Clara” calls The Doctor. She tells the Doctor that Clara and the Unit troops are dead. She then fires a rocket at the Doctor’s plane.