The Doctor, Bill, and Nardole stop in at NASA and are present when a Mars probe discovers something unexpected in the polar ice: a message clearly reading "God Save the Queen."
They immediately head off to Mars to discover the source of the message. Unfortunately, Nardole accidentally gets swept away when the TARDIS acts up, sidelining him for most of the story until he asks Missy for help.
Back on Mars, the Doctor and Bill are very much surprised to discover Victorian soldiers, with an Ice Warrior dubbed Friday helping them. When the soldiers break into what looks like a tomb, the Doctor realizes that it's not a tomb but a sleeping chamber.
The queen of the Ice Warriors awakens, and despite the Doctor's best efforts, hostilities commence. The second-in-command of the British soldiers reveals that the colonel, Godsacre, is actually a deserter and effects a coup, locking up Bill, the Doctor, and Godsacre.
When the now-awake Ice Warrior hive attacks the soldiers, it quickly turns into a slaughter. Friday asks the Doctor to help stop the needless war. Even as they do their best, though, Catchlove takes the Ice Queen hostage.
Godsacre surprises Catchlove and kills him. After he offers up his own life for the sake of his remaining men, the queen spares them in exchange for Godsacre's fealty, which he offers.
The Doctor contacts and old acquaintance, Alpha Centauri, to help the Ice Warriors, since Mars is uninhabitable now. The Doctor, Bill, and Godsacre set out the "God Save the Queen" message as a signal to Alpha Centauri.
Nardole returns with the TARDIS and Missy, much to the Doctor's shock.