Doctor: Peter Capaldi (Twelfth Doctor)
Michelle Gomez – Missy
Anthony Calf – Godsacre
Ferdinand Kingsley – Catchlove
Richard Ashton – Friday
Adele Lynch – Iraxxa
Glenn Speers – Sergeant Major Peach
Ian Beattie – Jackdaw
Bayo Gbadamosi – Vincey
Ian Hughes – Knibbs
Lesley Ewen – Coolidge
Ysanne Churchman – Voice of Alpha Centauri
Directed by Wayne Yip
Written by Mark Gatiss
Produced by Nikki Wilson
Executive producer(s) Steven Moffat
Incidental music composer Murray Gold
Series Series 10
Length 44 minutes
Originally broadcast 10 June 2017
In the present, NASA finds the words "God save the Queen" made of rocks buried under the Mars ice cap. Witnessing this, the Doctor, Bill and Nardole venture to Mars in 1881 to investigate and discover that the planet is occupied by human soldiers from Victorian Britain. Nardole goes back into the TARDIS, but it departs for unexplained reasons, returning to the university in present day. Nardole asks Missy for help getting back.
The humans have befriended an Ice Warrior they have named Friday. Captain Catchlove says they rescued Friday from his crashed spaceship while on patrol in Africa. In exchange, Friday allowed the soldiers to use his species' technology to travel to Mars, establish a small breathable atmosphere within a cavern, and mine the planet. The Doctor surmises that Friday is using them to access his hive. They soon unearth the tomb of the Ice Empress. Despite Colonel Godsacre's order to not approach it, one guard inadvertently revives the Ice Empress, Iraxxa.
Iraxxa's awakening leads to a confrontation with the humans. Friday tells Iraxxa it has been 5,000 years, and Mars' surface is uninhabitable; the Doctor asks Iraxxa to show mercy and gain help from the humans. Dismissing the Doctor and the soldiers, she asks Bill for her opinion, as a fellow woman. She decides to relent based on Bill's word, but then a soldier suddenly fires his rifle, the shot harmlessly glancing off Iraxxa's helmet. Provoked, she returns fire. Catchlove takes command, traps Iraxxa and Friday within the tomb, and locks up the Doctor, Bill, and Godsacre. Inside the tomb, Iraxxa starts reviving more Ice Warriors.
As the Ice Warriors launch their attack, Friday allies himself with the Doctor and Bill and helps the Doctor gain control of the mining device. The Doctor threatens to use it to bury them all under the ice cap. Catchlove holds Iraxxa at knifepoint and attempts to force her to help him pilot a spaceship back to Earth. Godsacre manages to surprise Catchlove and kills him, then begs Iraxxa to kill him. Iraxxa calls off the attack in exchange for Godsacre pledging himself to the Ice Warriors. The Doctor covertly contacts Alpha Centauri to assist the Ice Warriors.
The Doctor and Bill help Godsacre leave the message that NASA will discover. Nardole then re-appears with the TARDIS, with Missy piloting the ship. She expresses concern about the Doctor's well-being.