Doctor: Peter Capaldi (Twelfth Doctor)
Pearl Mackie (Bill Potts)
Matt Lucas (Nardole)
Nicholas Burns – Sutcliffe
Asiatu Koroma – Kitty
Peter Singh – Pie-Man
Simon Ludders – Overseer
Tomi May – Dowell
Austin Taylor – Spider
Ellie Shenker – Dot
Kishaina Thiruselvan – Harriet
Badger Skelton – Perry
Directed by Bill Anderson
Written by Sarah Dollard
Produced by Nikki Wilson
Executive producer(s) Steven Moffat
Incidental music composer Murray Gold
Series Series 10
Length 44 minutes
Originally broadcast 29 April 2017
The Doctor and Bill find they have arrived in London in 1814, in the midst of a frost fair on the frozen Thames. After dressing in period garments, they take to explore it, unaware the TARDIS sensors have identified a large life form, a kilometre long, under the ice.
The Doctor's sonic screwdriver is stolen by Spider, one of several orphan pickpockets led by Kitty. The Doctor and Bill chase down Spider and Kitty away from the fair. There, glowing lights under the ice encircle Spider's feet and before the Doctor can save him, Spider is pulled through the ice. Bill is troubled by the Doctor's lack of remorse for Spider's death. They track down Kitty and learn they have been paid to bring more people to the fair, with some having gone missing.
The Doctor and Bill get into diving suits and purposely let the lights take them; the lights belong to strange fish, and they find a giant sea creature ensnared by chains, which has eaten Spider and others. Returning to land, they learn from Kitty that their benefactor is the wealthy Lord Sutcliffe. Sutcliffe affirms his family has used the creature to amass a fortune by feeding it victims then collecting and selling its waste as a super powerful replacement for coal. Fearing the Doctor knows too much, Sutcliffe sends the Doctor and Bill to be eaten, while he rigs a bomb to cause the ice to shatter with maximum possible casualties for fish food.
The Doctor and Bill escape, and while Bill works with Kitty and the other orphans to clear people off the ice, the Doctor takes the bomb and places it on the creature's chains. When Sutcliffe sets off the bomb, the creature is able to swim free and Sutcliffe is lost in the freezing water. The Doctor rewrites the deed to the Sutcliffe mansion and its staff to the orphans, allowing them to live there as long as they want.
They return to the present, where Nardole scolds the Doctor for breaking his oath to stay on Earth. Bill finds from old newspapers that the orphans lived a fulfilling life, though surprised there was no mention of the sea creature. Nardole, while checking the Vault beneath the University, hears something behind it knock repeatedly and says he will only let it out with a proper request from the Doctor.