Doctor: Jodie Whittaker
Charlotte Ritchie – Lin
Nikesh Patel – Mitch
Daniel Adegboyega – Aaron
Darryl Clark – Police Officer Will
Connor Calland – Security Guard Richard
James Lewis – Farmer Dinkle
Sophie Duval – Mum
Callum McDonald – Teen 1
Harry Vallance – Teen 2
Laura Evelyn – Call Centre Polly
Michael Ballard – Sergeant
Nick Briggs – Voice of the Dalek
Directed by Wayne Yip
Written by Chris Chibnall
Script editor Fiona McAllister
Produced by Nikki Wilson
Executive producer(s) Chris Chibnall, Matt Strevens, Sam Hoyle
Incidental music composer Segun Akinola
Length 60 minutes
First broadcast 1 January 2019
In the 9th century, three opposing factions narrowly succeeded in defeating a seemingly unbeatable enemy and hacked its body into three pieces to be hidden away in farthest corners of the Earth. The holder of the last piece is killed before he can bury the piece; which is subsequently discovered by Lin and Mitch, two archaeologists, in Sheffield on New Year's Day 2019. The last piece is unintentionally revived as it remotely summons the other two fragments to become whole.
While watching New Years' fireworks from different time periods, the Doctor and her team are alerted to this alien presence, arriving in the present day and meeting the two archaeologists. Lin reveals that she saw a large squid-like creature on a wall; the Doctor finds only a blob of slime. Unbeknown to everyone, the creature has attached itself to Lin's back and has taken control of her mind and body.
Returning to Graham and Ryan's home, the crew discover that Ryan's hitherto absent father, Aaron, has returned, and wishes to make amends with his son for not being there, especially since Grace’s death. Graham reluctantly allows Ryan to go with Aaron to a local cafe so the two can talk. Meanwhile, the Doctor and Yaz try to follow the creature, which has made Lin steal a police car and uniform. After examining the slime, the Doctor discovers that she is following a scout Dalek.
The Dalek, still controlling Lin, arrives at an archive base, killing its guard and using his fingerprints to access to the archives. Recovering a Dalek ray gun from the archive, she then travels to a rural warehouse, killing the owner and constructing a makeshift Dalek casing out of scrap metal. Eventually, the Doctor and company find Lin freed from the now-rebuilt Dalek's control. After killing a military patrol, the Dalek flies to Government Communications Headquarters, massacring the staff, and attempts to summon the Dalek fleet in order to conquer the world.
The Doctor, her TARDIS crew, Aaron and the two archaeologists immediately go to GCHQ and scupper the fleet-summoning signal, destroying the Dalek's casing in the process. However, the mutant survives, and it possesses Aaron, threatening to kill him unless the Doctor takes it to Skaro. The Doctor agrees, but instead takes the Dalek mutant to a supernova. Ryan saves Aaron at the last moment as the Dalek mutant is cast into the supernova.