Ever since the Earth/Zygon War, the Tenth and Eleventh Doctors have preserved the peace treaty. There were two versions of Osgood (Ingrid Oliver), a UNIT scientist—one of them human and one a Zygon duplicate, but they didn’t know which. In the Death in Heaven episode, Missy killed one of them, but still no-one knows which. There are 20 million Zygons on Earth, disguised as humans. Now, a radicalised splinter group of Zygons wants to come out of the closet. Let Zygons be Zygons, I say. They take over and kill Zygon high command and attack UNIT.

Osgood is captured in Truth or Consequences, New Mexico. In London, Clara is knocked unconscious and stored in a pod underground. A Zygon named Bonnie takes her place. Being President of Earth, the Doctor (Peter Capaldi) takes a plane to Turmezistan. He and UNIT converge on the church Osgood is hiding in. In New Mexico, UNIT leader Kate Stewart (Jemma Redgrave) is threatened by the Sheriff, who is a Zygon. In London, UNIT finds Clara in a pod, so they realise a Zygon outside is impersonating her. Bonnie tells the Doctor his friends are all dead and fires a missile at his plane.

The Osgoods both wear a long scarf or a bowtie, and both have question marks on their clothes. The Doctor says he still wears question marks—on his underpants. The Doctor once kissed a Zygon impersonating Queen Elizabeth I. The Doctor plays Amazing Grace on his guitar. There are references to Invasion of the Body Snatchers and the Midwich Cuckoos. Reviewers liked Osgoode and the scary Zygons and applauded the refugee subtext. There is a portrait of William Hartnell on the wall of the UNIT safe house. Truth or Consequences, New Mexico, was named for the show after host Ralph Edwards asked for a town to do so on the show’s tenth anniversary. It had been called Hot Springs, which is not very imaginative.