Exploring Mars remotely, NASA finds the words God Save the Queen made of rocks under the Martian ice-cap. Noticing this, the Doctor (Peter Capaldi), Bill (Pearl Mackie), and Nardole (Matt Lucas) travel back to 1881 and find British soldiers there. When Nardole goes back into the TARDIS for a moment, it suddenly returns to Earth in the present, and Nardole asks Missy Michelle Gomez) for help.
According to Captain Catchlove (Ferdinand Kinsley), the humans rescued an ice-warrior who crash-landed in his ship in Africa. They called him Friday (Richard Ashton), and he used his tech to transport the troop to Mars, create an atmosphere in a cavern, and mine the planet, which of course they claim in Victoria’s name. They unearth the tomb of the Ice Empress, named Iraxxa (Adele Lynch). Colonel Godsacre (Anthony Calf) orders his men not to approach the tomb, but one guard accidentally awakens her.
Friday tells Iraxxa it has been 5000 years and Mars is now uninhabitable. The Doctor asks Iraxxa to show mercy toward the humans and recruit their help. She dismisses the Doctor and the soldiers as noisy men and asks Bill for a woman’s opinion. She decides to relent, but of course an accidental shot strikes her helmet, and a gun-battle breaks out. Catchlove traps Iraxxa and Friday in the tomb, and locks up the Doctor, Bill, and Godsacre. Iraxxa begins reviving more ice warriors.
During their attack, Friday frees the Doctor and Bill and helps the Doctor take control of the mining machine. The Doctor threatens to bury everyone under the ice-cap. Catchlove attempts to force Iraxxa at knifepoint to help him pilot a ship back to Earth. Godsacre kills him, then pledges himself to the ice warriors. The Doctor secretly contacts his old friend Alpha Centauri to help the ice warriors. The Doctor and Bill help Godsacre to leave the message for NASA (and the Doctor) a hundred years later. Nardole and Missy turn up in the TARDIS, worried about the Doctor.
The portrait of Queen Victoria is actually that of Pauline Collins, who played her in the Tooth and Claw episode, in which Torchwood was founded. The voice of Alpha Centauri is that of Ysanne Churchman, who played the character in The Curse of Peladon and The Monster of Peladon, in which ice warriors appeared to the Third Doctor Jon Pertwee. Churchman came out of retirement to play the character again at the ago of 92. This was the first appearance of a female ice warrior. 5000 Martian years is about 9500 Earth years. The Victorian soldiers wear cool steampunk spacesuits. Lurking in the background, of course, is A Princess of Mars by Edgar Rice Burroughs.