Doctor: Jodie Whittaker
Craige Els – Karvanista
Steve Oram – Joseph Williamson
Nadia Albina – Diane
Sam Spruell – Swarm
Rochenda Sandall – Azure/Anna
Jacob Anderson – Vinder
Annabel Scholey – Claire
Jonathan Watson – Ritskaw
Dan Starkey – Kragar
Matthew Needham – Old Swarm
Sarah Amankwah – En Sentac
Charlie Oscar – K–Toscs
Richard Tate – Wilder
Paul Leonard – James Stonehouse
Heather Bleasdale – Wilma
John May – Kev
Gunnar Cauthery – Jón
Barbara Fadden – Weeping Angel
Directed by Jamie Magnus Stone
Written by Chris Chibnall
Script editors: Caroline Buckley, Rebecca Roughan
Produced by Nikki Wilson
Executive producers: Chris Chibnall, Matt Strevens
Incidental music composer: Segun Akinola
Running time: 1st of 6-part story, 50 minutes
First broadcast 31 October 2021
The Doctor and Yaz are in pursuit of Karvanista, one of the dog-like Lupari species, for reasons unclear to Yaz. They escape from his capture and follow him to Earth in the TARDIS. En route, the Doctor experiences a psychic vision of Swarm, a mysterious being, escaping from its millennia-long imprisonment by the Time Lord Division.
In 1820, philanthropist Joseph Williamson constructs tunnels under Liverpool for unclear purposes. In present day Liverpool, it is Halloween and Dan Lewis arranges a date with museum worker Diane before attending a shift at the soup kitchen and heading home. Karvanista breaks into his house and transports him onto his Lupari ship. The Doctor and Yaz arrive after Dan's kidnap, investigating his house and discovering what appears to be a Lupari invasion fleet approaching Earth. They escape just before Dan's house is collapsed by a trap set by Karvanista. They then briefly encounter Claire, a woman who claims to have met the Doctor in the latter's future. Departing, the Doctor and Yaz are confused by the appearance of a second doorway in the TARDIS' console room. Claire is subsequently captured by a Weeping Angel.
On Karvanista’s ship, Yaz rescues Dan while the Doctor confronts Karvanista over his connection to the Division, before becoming distracted by the Lupari’s apparent invasion plan. Karvanista tells them that the Lupari are in fact saving humanity from Earth’s imminent destruction by the Flux, an unknown entity that defies all laws of space and time and consumes everything in its path. Dan was captured first because Karvanista hit a time disturbance and arrived on Earth ahead of the Lupari fleet. The Doctor, Yaz and Dan escape the ship and return to the TARDIS, where a third doorway has appeared, to investigate the Flux.
Meanwhile, the Flux causes Vinder, the sole crew member of a remote outpost in deep space, to evacuate his position. It also catches the attention of the Sontarans, who relish the prospect of destruction it poses. Swarm attacks a base in the Arctic Circle, killing one of its two crew members and reviving his "sister" Azure from the other. Azure later lures Diane into a derelict house.
The TARDIS takes the group to a seemingly empty area of space in pursuit of the Flux; as they watch, it appears in the distance. The Doctor experiences another vision of Swarm, who claims an old association with her that she does not recall. The Flux then accelerates its attack on Earth, before the Lupari fleet can rescue its population. The Doctor has Karvanista form a defensive shield with the other Lupari ships, protecting Earth from the Flux. However, the TARDIS is unable to transport behind the shield; the Doctor releases time vortex energy into the Flux, but this does not have any discernible effect. The TARDIS' three doorways fly open as the Flux approaches.