The Doctor (Peter Capaldi) and Bill Potts (Pearl Mackie) have landed in London in 1814, at a Frost Fair on the frozen Thames. They dress in period clothing and look around, not noticing that the TARDIS has spotted a kilometre-long lifeform under the ice. A child pickpocket named Spider (Austin Taylor), under the tutelage of Kitty (Asiatu Koroma) swipes the Doctor’s sonic screwdriver. The doctor and Bill chase down Spider and Kitty and confront them outside the fair. Glowing lights appear beneath Spider’s feet and, before the Doctor can act, Spider disappears through the ice. Bill is troubled by the Doctor’s lack of remorse. They track down Kitty and learn that a number of people have gone missing. The Doctor and Bill put on diving suits and let the lights take them. The lights are caused by strange fish working symbiotically with a giant sea-creature in chains which has devoured the victims. They learn that Kitty’s team is working for Lord Sutcliffe (Nicholas Burns). He is making a fortune by selling the creature’s waste, which is a fuel superior to coal. Sutcliffe is rigging a bomb to shatter the ice and create many more victims, and he sends the Doctor and Bill to be eaten.
They escape, and while Bill helps Kitty and the other orphans clear people off the ice, the Doctor steals the bomb and places it on the creature’s chains. When it explodes, the creature escapes to the sea and Sutcliffe drowns. The Doctor rewrites the deed to Sutcliffe Manor and leaves it to the orphans. The time-travellers return to the present and Nardole (Matt Lucas) scolds the Doctor for leaving Earth, which he has promised not to do. Bill finds old newspapers about the orphans living long and happy lives, but nothing about a sea-monster. Nardole hears a knocking in the vault beneath the University.
During what is called the mini-ice-age, which lasted 200 years in Northern Europe, winter fairs were held off and on in the 17th to the 19th Centuries. The 1814 Frost Fair was the last. It was held between Blackfriars Bridge and the Old London Bridge. In fact, work on the latter contributed to the freezing of the river by slowing the flow of water. An Elephant was walked across the river, and there were many other carnival-like acts, such as wrestling, sword-swallowing, skittles, magicians, and ring-toss. Snacks were served, such as Shish kebab, sheep hearts, fish and meat pies, gingerbread, and gin.
The First Doctor took companions Vicki and Steven to the 1814 Frost Fair, where they met Jane Austen. The 11th Doctor took River Song to the 1814 fair, accompanied by Stevie Wonder, who sang for them, at least according to River Song. The 12th Doctor took Clara to a Frost Fair in the 1890’s in the Doctor Who novel Silhouette. The writer of the episode, Sarah Dollard, was writing fan fiction about Hannibal Lecter at the same time, and unconsciously named Lord Sutcliffe after a character in that book. She decided one was ancestor to the other, both of them despicable.