In the past, Kate Stewart makes a treaty with the Zygons who have invaded Earth and taken the former of multiple humans.
The two Osgoods, human and Zygon, make a video recording about Operation Double, outside of UNIT strictures. They will secretly resettle 20 million Zygons on Earth after they have assumed human form. The Osgoods are making the video in case something goes wrong and UNIT is infiltrated, or something unmasks the Zygons... or if one or both of them die.
Later, the Zygon Osgood attends the grave of her human counterpart.
On the video, the Osgoods note that any one Zygon or human could go rogue. If that happens, then the ceasefire would break. They indicate a large wooden box and explain that the Doctor left them the Osgood box. It's the final sanction if something goes wrong, and the ceasefire breaking is the nightmare scenario.
In New Mexico, people run through the streets in terror as explosions break out. Osgood hides in a police station and starts to make a phone call. A Zygon in its natural form comes in and Osgood calls the Doctor on her direct TARDIS link. The Zygon drags her out and stuns her with an electrical sting.
Aboard the TARDIS, the Doctor is playing guitar and notices Osgood's transmission that the Nightmare Scenario is occurring. He lands at Brockwell Park in London and tries to call Clara, but gets her voicemail. He says that he's staking out some of the most dangerous creatures imaginable and is under deep cover, as two schoolgirls, Claudette and Jemima, turn and look at him. The Doctor then goes over to the girls and says that they need to talk.
At a UNIT safe house in South London, Kate and Jac coordinate efforts to deal with the Zygon invasion. Colonel Walsh calls in and shows Kate surveillance video of the Zygons with the captured Osgood. Jac confirms that the encryption system has already been hacked, and brings up another video that the Zygons have sent through.
The Doctor tells the girls that he knows that they're Zygons so there's no point in them pretending. When he complains that they're not following the ceasefire, the girls ignore him. The Doctor warns that he knows there are other Zygon factions they don't control, and that those factions are planning something. He warns that if they don't oppose them together then it will mean the end of all Zygons. One of the "girls" says that it is their jurisdiction and they are close to finding the rogues. The Doctor doesn't believe they can and says that he's taking it out of their hands, just as his phone rings. It's Kate, who tells him that the Zygons kidnapped Osgood and know the location of every Zygon on Earth by using Osgood's security access. As she says that the ceasefire has broken down, a Zygon gas bomb goes off near the girls. The Doctor gets the other children away, as Zygons grab the two imposter girls and take them away in a parks maintenance van.
When the Doctor gets to the UNIT safehouse, he and Kate watch Osgood's forced transmission. She reads a statement that UNIT troops will be destroyed all over the planet, and there must be truth or there will be consequences. The Doctor is still unable to contact Clara.
Clara returns home and notices that the Doctor has tried to call her 127 times. As she goes up the stairs and checks her messages, she finds a young boy sitting in front of her. The little boy says that he can't find his parents, and Clara offers to find them. She goes into his flat and a man comes up behind her. When Clara explains what is going on, the father goes to get his son and she hears the boy screaming. The boy's mum confronts Clara and insists that everything is fine. Clara leaves and calls the Doctor.
Later, the Doctor and Clara go to Drakeman Junior High in Dunwich. Kate meets them and explains that the Zygon High Command had their secret base there. She explains that under Operation Double, 20 million Zygons were born on Earth and allowed to take the form of the nearest human being. Most of the humans were members of the UK. The two Osgood leaders, posing as the girls, were secret and uncommunicative since Osgood's duplicate left. They said that they had the revolution among the younger brood under control. Kate leads the Doctor to the base in the basement and he identifies the control polyp for all the Zygons on Earth. It's been compromised, and the Doctor warns that the Zygons are compromised and wide open to attack. As he interfaces with the polyp, the Doctor says that both Osgoods maintained a live link, making them both human and Zygon. The surviving Osgood went mad with grief when her counterpart died, and then disappeared.
The Doctor confirms that things have gone wrong with the Zygon centers throughout the world. UNIT receives a video from Jemima and Claudette, prisoners. The Zygon rebel leader announces they have been betrayed and their rights violated. He demands the right for Zygons to be themselves and normalize, and reverts the two girls to their Zygon forms. Other Zygons then kill them. The rebel leader says that they are now the Zygon High Command, and promise that all traitors will die. He says the rebel motto, "Truth or Consequences," and cuts the transmission.
When the Doctor says that they have to open negotiations, Kate refuses and plans to have Walsh bomb the rebel settlement in Turmezistan. The Doctor objects, insisting that the rebels are a splinter group, and the rest of the Zygons want to live in peace. He figures that the rebels want Kate to bomb them because it will radicalize the others of their kind. Clara points out that Truth or Consequences is a town in New Mexico, and Kate notes that the last signal from Osgood's phone came from there. The Doctor tells Kate to go to Truth or Consequences and see what she can find out, while he goes to Turmezistan to negotiate peace and rescue Osgood. Clara and Jac will stay in the UK to protect the country. They go to the airport and the Doctor boards the Presidential aircraft. Once he leaves, Kate warns Clara that they don't have many troops and can't recruit more without revealing the project. One of their Naval surgeons developed Z-67 during a previous Zygon invasion. Z-67 is a nerve gas that unravels Zygon DNA. However, the Doctor took all of it that they had.
After Kate leaves for New Mexico, Clara tells Jac that she needs to stop at home to get a few things. They go to her flat and as they leave, spot the little boy's parents dragging a bag into the elevator. The boy yells from inside the bag as the elevator descends, and Jenna insists on running down the stairs to catch them. However, when the elevator opens there's no one inside. Clara and Jac enter the elevator to examine it, and the doors close. Slime oozes from the button panel, and Clara pries it open to reveal a Zygon bioorganic mechanism inside. She activates it and the elevator shakes. The doors finally open and the women find themselves in a sub-level tunnel below the street. They see the parents dragging the boy away and follow them, and see more Zygons in the tunnel. Clara says that they need to get more reinforcements and the two women go back to the elevator.
At the UNIT command base in Turmezistan, the Doctor finds Walsh preparing to bomb the Zygon rebels. Walsh refuses to stop the bombing, pointing out that it's impossible to track shapeshifters and Osgood is almost certainly dead. The drone operator sees her husband and son at the site, waving to her. The Doctor sees photos of her family on the panel as the operator aborts the strike against Walsh's orders.
Kate arrives in Truth or Consequences and finds abandoned streets and rebel marks on the walls. There's also a sign saying "No British." When she sees the marks, Kate draws her gun and goes to the police station. She finds a photo of Osgood on the wall, and files on a series of murders of Englishmen. The sheriff, Norlander, comes in and draws her gun on Kate. She asks if Kate is one of "them." Kate says that she's there to help, and admits that she didn't bring backup. She asks Norlander why she needs backup.
Walsh tells the Doctor that the settlement is apparently a training camp, and they don't know how many there are or how they come and go. She plans on taking a squad in, and asks the Doctor for the Z-67 gas. He warns that the Zygons are trying to panic her, and Walsh points out that anyone she knows could turn into a Zygon.
At the UNIT safehouse in London, the soldiers prepare to invade the tunnels. Jac says that there have been reports of strange activity in elevators all over London. The CCTVs confirm that people are in the elevators and then disappear, and Jac warns that something is very wrong.
Norlander drinks some water and tells Kate that the Brits came two years ago. They had no jobs and no money, and started getting into fights and killings. As Kate goes through Norlander's files on the incidents, Norlander says that after the murders, the survivors banded together and one day one of them turned into a Zygon in plain sight. Then the others transformed and came for the townspeople. No one could fight them because the Zygons could be anyone. Norlander recognizes a photo of Osgood and tells Kate that she was asking questions before the invasion. The sheriff says that everyone is gone and Kate asks where.
Walsh takes her men in and warns them that they have a 13-minute window before the airstrike. The Doctor goes with them, and when they arrive Walsh warns her people that they have intel that the Zygons are holed up inside the Church. The Doctor shows them a photo of Osgood and says that their objective is to rescue her. He tells the soldiers to avoid harming the Zygons when possible, and Walsh reminds them not to fall victim to the Zygons' shapeshifting tricks.
The soldiers surround the church while the Doctor and Walsh go in the back. One soldier, John Hitchley, fires into the air and orders the Zygons to surrender. After a minute, a Zygon shapeshifted into the form of Hitchley's mother comes out and tells her son that he doesn't understand. She claims that the Zygons brought them there and it's Hitchley's commanders who are the Zygons. Walsh contacts Hitchley over the radio and warns him not to fall for it, and tells him to ask for details that only his mother would know. His mother refuses to answer his questions, claiming that she can't remember, and more Zygons come out posing as the soldiers' family. The mother asks them to come inside so that they can prove they're hostages, and tells Hitchley that she forgives him for what he's going to do. Hitchley and the others can't shoot, and go inside despite Walsh's warnings.
Walsh shoots the lock off of the rear door and breaks in. There's a map of the world and the ashes of the dead soldiers. The Doctor refuses to let her bomb the village, and Walsh gives him ten minutes to find Osgood. Once she leaves, the Doctor hears Osgood calling for help from beneath the floor. He finds a trapdoor and goes down to where Osgood is chained to a chair. She says that the Zygons are training and practicing new skills, and says that they're escaping through the tunnels. They're heading back to the UK now that they have the Doctor out of the way. A Zygon comes in behind them and prepares to kill the Doctor, just as the bombing strike begins early. The ceiling collapses, knocking out the Zygon.
Norlander takes Kate to a dumpster and shows her the remains of the dead humans.
Clara and Jac accompany the soldiers down the elevator and Jac wonders if things are coming to an end. They find a new tunnel leading off.
The Doctor and Osgood return to the presidential plane, bringing the unconscious Zygon with them. The Doctor tries to call Clara and gets her voicemail, and then asks Osgood which one she is: human or Zygon. Osgood says that she doesn't answer the question because there isn't a question to answer: She says that she and her sister were the living embodiment of the peace they made, and she will give all of the lives she has to protect it. Osgood tells the Doctor that she is human and Zygon, and the Doctor realizes that she's a hybrid. He says that he's proud to know her, and won't tell anyone that he knows that she's human. Osgood says that now the Zygons can pluck the memories of loved ones from those that see them, and she wouldn't revert just because the human Zygon died. The Zygons only need the original if they need more information. Once they have it, the original can die.
Clara and Jac enter a huge chamber filled with Zygon pods. Clara tells Jac that she was right about the end of the world.
The Doctor questions the Zygon prisoner, and he realizes that the Zygon knows him and wants something. The Zygon laughs and says that they want the world.
UNIT soldiers march forward from the elevator and bust into the pod chamber. Clara tells them that they have to neutralize them before they hatch, but Jac says that there must be more to it. When Clara checks one of the pods, she finds a human inside: herself. Jac says that they should wait, and Clara insists that they have to destroy them. Despite that, Jac says that the Zygons duplicate humans, they don’t grow them. So the things in the pods must be humans. “Clara” smiles and Jac yells to the soldiers that it’s a trap. The Zygons move in and the Clara-Zygon tells them to kill the traitors.
Earlier in the apartment, a Zygon stings Clara unconscious and the Clara-Zygon steps forward to take her place.
They Zygons step over the human ashes, and Clara-Zygon looks at her human counterpart inside of the pod.
The Zygon tells the Doctor that they want the truth of who they are acknowledged, and to live as themselves at any cost. The Doctor says that they can’t live in the UK, and the Zygon tells him that they already have taken it over. He says that the invasion has taken place over the last year and they have won the battle... and now they’re going to win the war.
Back at the station, Kate goes through the files and tells Norlander that they have to find out what forms the Zygons took. The sheriff says that one townsperson saw a Zygon child before it could learn how to maintain its bodyprint... and then “they” were labeled monsters. “Norlander” says that she had to be sure there no backup and reverts to its Zygon form
Clara-Zygon calls Truth or Consequences and asks what the commander’s status is. Kate-Zygon responds and says that UNIT is neutralized in North America. Satisfied, Clara-Zygon goes to the armory and takes out a case. She reports to her superiors that UNIT is neutralized in the UK and brings up the Doctor’s flight plan. She goes out to the country and watches the Doctor’s plane, and then calls him.
Aboard the plane, the Zygon tells the Doctor that the plane will never land. Clara-Zygon confirms that the Doctor is aboard the plane, and then takes a missile launcher out of the case. As he tells her to get to the TARDIS, Clara-Zygon aims the launcher at the plane and tells the Doctor that Clara and UNIT is dead. She says “Truth or Consequences” and fires...
Written by Gadfly on Nov 1, 2015