Written by Sarah Dollard, directed by Bill Anderson
“So the Tardis has dresses and likes a bit of trouble? I think I’m low-key in love with her.”
In Regency England, beneath the frozen Thames, something is stirring. The Doctor and Bill arrive at the last of the great frost fairs and find themselves investigating a string of impossible disappearances – people have been vanishing on the ice! Bill is about to discover that the past is more like her world than she expected, and that not all monsters come from outer space.
The TARDIS has materialized in the middle of the last great frost faire on the frozen River Thames. The Doctor reminds Bill that you don’t steer the TARDIS, but you negotiate with her, and this is where she has brought them. Bill seems to be worried about the period and the fact that slavery still existed at the time. The Doctor helps her to not worry so much and they dress for the occasion. As a street urchin invites them to the faire, they gleefully attend.
Bill notes that Regency London is a lot more racially diverse than she remembers from the movies. The Doctor remarks that “history is a whitewash”. Whilst enjoying the festivities, Bill notices strange green lights under the ice and follows them; the Doctor reveals he had noticed them too, but let Bill have fun while she could before they get to work.
Elsewhere, a drunken man follows the lights across the ice to an area of thin ice before being sucked under leaving only his drink bottle behind.
The Doctor and Bill are approached by a girl looking for her dog that has gone missing, the girl wanting their help. However, the Doctor deduces that she is trying to con them, catching a boy’s arm as he attempts to pickpocket his sonic screwdriver from behind, only for the girl to kick his leg, enabling the pair to flee. The Doctor and Bill give chase across the ice but the boy, Spider, becomes distracted by the green lights and is sucked under leaving only his outstretched arm, with the sonic screwdriver, protruding through the ice.
Though the Doctor tries, he cannot save him and only manages to retrieve his screw driver. His relief at having done so and his seeming lack of care towards the boy’s death causes Bill to run off and then confront him. She demands to know how many people he’s seen die- he doesn’t know- and how many people he’s killed- he doesn’t say. The Doctor tells her that he is 2000 years old and doesn’t have the time for the luxury of outrage otherwise more people would die.
The girl, Kitty, takes the Doctor and Bill to meet the rest of the urchin gang- including the girl who handed Bill the flyer. After gaining their trust by giving the children the food the Doctor stole and reading to them, the urchins explain that a man with a tattoo of a ship on his hand pays them to lure people onto the ice by handing out flyers.
Bill and the Doctor put on dive suits and walk out onto the ice. They cannot hear each other with the helmets on and the lights begin to surround Bill. She throws her lantern at the Doctor to gain his attention before she is sucked under the ice. The Doctor jumps through before it reseals and underwater discover a giant sea creature chained at the bottom of the river.
Bill sees the hat belonging to the boy who was sucked under in its mouth before they come back to the surface. The Doctor discovers some of the fish causing the lights and determines that they may be terrestrial. Bill notes that the creature sounded miserable from its imprisonment.
The Doctor gets information from a local fisherman and the pair discover a group of dredgers harvesting the waste from the creature from the Thames. Using the psychic paper under the pretense of conducting an inspection, the Doctor and Bill learn that the waste of the creature is more efficient than coal and can even burn underwater making it a highly valuable resource.
They visit the manor home of the employer Lord Sutcliffe to determine whether he is an alien. The Doctor insists he do the talking as they must charm Sutcliffe and Bill is likely to become angered due to the creature eating the children. Under the guise of “Doctor Disco of the Fairbrook Society”, the Doctor meets Sutcliffe who instantly makes sexist, classist but mainly racist, remarks towards Bill even calling her a “creature” and demanding she show “respect in the presence of [her] betters”. In response, the Doctor punches him in the face, knocking the man down, determining that Sutcliffe is very clearly human.
Bill and the Doctor are quickly captured by Sutcliffe’s co-conspirators with a bruised Sutcliffe revealing the creature has been a family secret for years. He attracts people onto the ice by helping to fund the Frost Fair to be sucked under and provide fuel. He plans to blow up the ice to cause widespread panic and deaths before the ice thaws and keep the creature a secret. Leaving Bill and the Doctor tied up with the explosives, Sutcliffe prepares to blow up the ice.
Working together, Bill manages to hand the Doctor his sonic screwdriver who uses it to attract the glowing fish to them whilst also cutting through the ropes they’re in. Their guard rushes in and the Doctor hands him the sonic causing the fish to become attracted to him and suck him through the ice. The Doctor gives Bill the choice saying he only serves the human race; save the creature but risk it killing the people they’d worked to save or kill it even though it was innocent. Bill, after an emotional consideration, chooses to save it.
Knowing people will be harmed, the Doctor tells Bill to save the people on the ice which she does with the help of the urchins, managing to convince everyone the ice is melting to get them off the ice causing Sutcliffe to rush his plans and detonate immediately. However, the Doctor had used his dive suit to plant the explosives around the chains of the sea creature instead, using his sonic to reloop the wires, releasing it. The creature begins to swim away, cracking the ice above it and causing Sutcliffe to fall into the water to his death. The Doctor pulls Bill off of the ice before she falls through though they are drenched by the creature as it swims to freedom without harming anyone, pleasing Bill as it is no longer in despair.
Later, Bill and the Doctor invite the urchins to Sutcliffe’s home to be fed and cared for with the Doctor ammending Sutcliffe’s Last Will and Testament so that everything passes to Perry, one of the urchins.
They soon depart back to St Luke’s University, where Nardole, who had just finished making tea, walks into the Doctor’s office and is shocked when he notices their period clothing, giving away that they had been time travelling. He reminds the Doctor of his vow whilst Bill is surprised that googling the sea creature from 1814 turned up no results. The Doctor explains that humans tend to overlook the inexplicable and that the Frost Fair involved a lot of day drinking. He shows her the headlines which detail Lord Sutcliffe’s death and, despite the contestion of inheritance, Perry was eventually deemed the legitimate heir.
The Doctor and Nardole soon toss a coin to decide whether the Doctor keeps his vow and remains on Earth, or Nardole leaves him alone. Using a trick he learned from the con artist, the Doctor wins and presumably goes off in the TARDIS with Bill whilst Nardole checks the vault. As he goes to leave, knocking comes from inside, Nardole refuses to let the entity inside out before hurrying away looking very worried.