The Doctor takes Bill on her first future trip-what looks like a future Earth colony. Except there appear to be no humans around, and the colony is inhabited by small, fly-like maintenance robots called the “Vardy” (who also make up the Colony structure) and their interface “Emojibots”.
The Doctor soon realizes that the Vardy have killed the first group of colonists and are using their bones as fertilizer for the plants. Investigating further, he begins to discover that the colony ship isn’t coming, but has already landed-and that these robots-due to a flaw in their programming designed to keep the colonists happy-misinterpreted grief-in this case the death of the colony’s leader-as a plaque to be wiped out and sterilized.
When the second wave of colonists awaken, they begin to discover that the first wave are dead, and try to take up arms against the robots. However, the Doctor is able to re-set their programming, and urges the colonists and the Vardy to live in peace.
The Doctor tries to return to the vault, but instead winds up in the frost fair in 1814-on top of the Thames river in London. Despite not ending up where he’s supposed to be (again) he and Bill decide to enjoy the festivities. However, unbeknownst to them, there’s something lurking under the ice…
However, both Bill and the Doctor begin to notice strange green lights underneath the ice, and find out they’re deadly when a young street urchin is killed. After being disturbed by the Doctor’s moral relativism at the time-as this is the first death she’s witnessed-she then talks her into discovering what the creature is.
Turns out it’s a whale like creature-with a group of symbiotic fish around it creating the lights-that is eating people-and it’s waste material-which has extraordinary fuel properties-is being collected and exploited by the fair organizer, Lord Sutcliffe. Despite the Doctor’s attempts to persuade Sutcliffe to stop-first by punching him for making a rude remark to Bill, and then by an impassioned speech….
Sutcliffe doesn’t listen and attempts to kill the Doctor and Bill with an explosive device , but the Doctor rigs the device to instead free the creature, allowing it to leave the Thames and go out to sea. Sutcliffe falls into the cracking ice and deaths. He also-with some clever forgery-gives the street urchins Sutcliffe’s inheritance.
Returning to the present, the Doctor is reprimanded by Nardole, who said he should’ve kept his vow to keep any eye on the vault. Nardole is disturbed when he checks on the vault, and it knocks a few times….