Doctor: Jodie Whittaker
Bradley Walsh (Graham O'Brien)
Tosin Cole (Ryan Sinclair)
Mandip Gill (Yasmin Khan)
Others Phyllis Logan – Andinio
Mark Addy – Paltraki
Percelle Ascott – Delph
Samuel Oatley – Tzim-Sha
Jan Le – Umsang
Directed by Jamie Childs
Written by Chris Chibnall
Script editor Fiona McAllister
Produced by Alex Mercer
Executive producer(s) Chris Chibnall Matt Strevens Sam Hoyle
Incidental music composer Segun Akinola
Length 50 minutes
First broadcast 9 December 2018
The Doctor directs the TARDIS to a planet called Ranskoor Av Kolos, where a large number of signals are originating. They find a large number of wrecked starcraft scattered on the planet's surface, as well as a psychic field that alters one's perception of reality; the Doctor provides Ryan, Yasmin, and Graham with neurobalancers to counteract the field. They soon come across a pilot who has amnesia, but the Doctor provides him with a neurobalancer as well, allowing him to recall his name as Paltraki. As they help Paltraki, he gets a video signal from a woman named Andinio and Tzim-Sha. Tzim-Sha warns Paltraki to bring him an item aboard his ship in exchange for his kidnapped crew. Graham tells the Doctor his intent to kill Tzim-Sha in revenge for Grace's death, but she warns him that if he does, she will not allow him to travel in the TARDIS again.
The group collect the item – seemingly a rock floating in a protective shell, and arm themselves and enter the citadel. Graham and Ryan go to look for the kidnapped crew, while the Doctor, Yasmin and Paltraki seek out Tzim-Sha. They instead encounter Andinio first who takes them under gunpoint to Tzim-Sha and Delph. The Doctor recognizes Andinio and Delph as members of the Ux, a race able to control the fabric of reality with their minds, but which only two exist any time. They learn from Tzim-Sha that after their previous encounter on Earth, he had been transported to Ranskoor and found by the Ux who consider him their "creator". Over the next 3,000 years, he has had the Ux work for his demands, effectively taking revenge on those races that humiliated him by shrinking their planets to tiny objects, like the one from Paltraki's ship. Yasmin and Paltraki discover four similar planets, while Tzim-Sha sets his sights to do the same to Earth. The ships that litter the surface are vessels that have attempted to fight Tzim-Sha but lost due to help from the Ux. The Doctor warns that even five such planets in close proximity can tear the fabric of reality and adding Earth would accelerate it.
Meanwhile, Graham and Ryan find the missing crew, along with other crew from other ships, in stasis chambers. An alarm alerts Tzim-Sha to their arrival, and he sends an army of robots to stop them before going after them himself; while gone, the Doctor convinces the Ux to stop the process with Earth and help her to return the planets. With Paltraki's help, Graham and Ryan defeat the robots, and Ryan proceeds to lead the crew to safety. Graham waits behind for Tzim-Sha, and threatens to shoot him, but cannot come to take the shot. Ryan returns, and Tzim-Sha attempts to fire on him, but instead, Graham shoots him in the foot, stunning him long enough for Graham and Ryan to stow him in a stasis chamber. Ryan is glad to see Graham was a better man.
The Doctor, the Ux, and Paltraki's crew work together to return the five planets back to their original positions in space. As the Doctor and her companions leave, Delph asks Paltraki if he can help return others to their proper worlds.