Doctor: Peter Capaldi (Twelfth Doctor)
Companion: Jenna Coleman (Clara Oswald)
Ingrid Oliver – Osgood
Jemma Redgrave – Kate Stewart
Rebecca Front – Colonel Walsh
Jaye Griffiths – Jac
Cleopatra Dickens – Claudette
Sasha Dickens – Jemima
Abhishek Singh – Little Boy
Samila Kularatne – Little Boy's mum
Todd Kramer – Hitchley
Jill Winternitz – Lisa
Gretchen Egolf – Norlander
Karen Mann – Hitchley's Mum
Aidan Cook – Zygon
Tom Wilton – Zygon
Nicholas Briggs – Voice of the Zygons (uncredited)
Directed by Daniel Nettheim
Written by Peter Harness
Script editor David P Davis
Produced by Peter Bennett
Executive producer(s) Steven Moffat
Incidental music composer Murray Gold
Series Series 9
Length 1st of 2-part story, 45 minutes
Originally broadcast 31 October 2015
In the aftermath of the previous Zygon attack on Earth and the Doctor's actions to ensure the creation of a peace treaty ("The Day of the Doctor"), there existed two versions of UNIT scientist Osgood: one human and the other a shapeshifting Zygon duplicate who do not divulge which is which since they considered themselves as both hybridized sisters and the personification of the treaty. Though the treaty allowed 20 million Zygons to remain on Earth, peacefully living out as disguised humans, the Doctor warned the Osgoods of the Nightmare Scenario should the treaty break, and left the Osgoods with a mysterious container they call the Osgood Box. Since that time, one of the Osgoods was killed by Missy; following her death the other left UNIT and disappeared from the grid.
In the present, the remaining Osgood is captured by Zygons in the town of Truth or Consequences, New Mexico just after sending a warning to the Doctor that the Nightmare Scenario has occurred. The Doctor arrives on Earth and speaks with two known Zygon commanders, disguised as schoolgirls, but they are kidnapped by Zygons before he can learn more. He attempts to call Clara as he travels to UNIT headquarters at the Tower of London. Kate Stewart and her assistant Jac show him a video of Zygons killing the two commanders, followed by Osgood being forced to read a message from a small village in Turmezistan, declaring the Zygons' intent to go to war. The Doctor surmises this must be a splinter group as the bulk of the Zygons want to live in peace. Kate reveals that she knows of Z67, a nerve gas that UNIT had developed that would only affect Zygons but that the Doctor had taken away which could end this current situation, as he refuses to let them have it.
Clara is about to answer the Doctor's call when she sees Sandeep, a little boy from a nearby flat, worried about his parents. At his flat, his parents insist all is well and take the struggling boy back in, amid other noises in the flat. Clara leaves and meets the Doctor, as he sends Kate to Truth or Consequences to learn about that town, while he will travel to Turmezistan via the aeroplane afforded him by being President of the World to rescue Osgood. He instructs Clara and Jac to keep the homeland safe.
In Turmezistan, the Doctor speaks to UNIT Commander Walsh, who explains how the Zygons have been able to take on the appearance of her soldiers' loved ones to prevent any attack on the village. With a bombing run planned, the Doctor joins Walsh and other troops as they converge on the chapel in town. The Zygons, appearing as relatives and friends, lure all the other soldiers into the chapel and kill them, against Walsh's orders. The Doctor and Walsh enter through a back door, finding all the Zygons have escaped. Walsh gives the Doctor a few minutes before she will start the bombing run. The Doctor finds Osgood safe under the chapel, but they are cornered by a Zygon when they try to leave. However, Walsh's early bombing creates debris that knocks the Zygon out. The Doctor and Osgood return to the plane and fly back to England, during which the Doctor attempts to pry information from the Zygon. The Zygon tells him that their invasion has already taken place and his plane will never land.
In New Mexico, Kate finds Truth or Consequences seemingly empty. While at the sheriff's office, she meets Sheriff Norlander who claims to be the last human alive after Zygons that lived among them turned and attacked them when their secret was discovered. While Kate is looking for more clues, Norlander is shown to be a Zygon, and prepares to kill Kate.
At UNIT, Clara has Jac help to get some things from her flat. While there, they see Sandeep's parents dragging a large bag into a lift. When the lift returns, they discover Zygon control units within it, which take them to a series of tunnels under London which many other lifts across the city connect to. They return with a troop of UNIT soldiers to explore the tunnels, and find thousands of Zygon pods which Clara says must be where the Zygons are cloning themselves, and they must be destroyed. When one pod is revealed to contain Clara, Jac realises too late that Clara is a Zygon herself, having been switched when she first went to Sandeep's flat. The Zygon, calling herself Bonnie, states that the invasion has already occurred, with many of the humans in London already captured and replaced by Zygons. Jac and the troops are killed by Zygons when they attempt to escape.
Bonnie travels to UNIT to obtain a shoulder-fired missile, en route learning that the threat in New Mexico has been neutralized. She goes to the coast and contacts the Doctor, informing him of the deaths of his friends. She fires the missile at the plane, and smiles when an explosion is heard.