The Doctor and Billy arrive in London, 1814, for the Great Frost Fair on the Thames. After looking around, the Doctor takes Billy back in and explains where they are, and dematerializes only for them to reappear again. He's ready to go exploring, and Billy points out that slavery is still a thing. The Doctor directs her to the wardrobe room and Billy goes back in. Meanwhile, a man drops a shoe into the Thames and a giant creature awakens.
Soon, the Doctor and Billy reemerge wearing period garb. They're unaware that the TARDIS is warning something gigantic in the nearby Thames. Meanwhile, Billy wonders what the rules are for time travel. The Doctor assures her that there's nothing to worry about. A girl, Dot, offers them a flier for the fair and the Doctor assures Billy that she won't alter history by taking it. They go to the fair on the Thames and Billy steps hesitantly onto the ice. They walk through the booths and Billy notices how many blacks there are.
As they go through one of the tents, Billy notices lights under the ice. They then go to a fish pie tent, and the Doctor notes that the pie-man congratulates him on being a trickster. Meanwhile, Billy notices the lights again and asks the Doctor if there are symptoms to time travel. He's already noticed the lights, and says that he figured they'd get to work eventually. They go further out on the ice and the lights follow them.
A drunk sees the lights and follows them out onto the thin ice. He disappears into the fog and a few seconds later there's a scream.
A girl, Kitty, comes up to the Doctor and Billy and asks if they've seen her dog. The Doctor realizes that she's conning them, just as Kitty's accomplice Spider takes the Doctor's sonic. When the Doctor grabs him, Kitty kicks his leg and the two children run off. The time travelers give chase and the children run out onto the ice. The lights circle around Spider and then gather at his feet, and he stops to look at them. Something pulls him beneath the ice and then the ice freezes, leaving only Spider's arm exposed. The Doctor grabs his sonic as Spider's arm is pulled down with the rest of him. He tells Billy that Spider is gone, but Billy asks him to save Spider.
The Doctor asks Kitty where they live and says that they can help her if she does. Kitty points out that Billy is gone and runs when the Doctor turns to look. He finds her sitting nearby and crying, and she says that she's never seen anyone die before. Billy asks him how many people he's seen die, and the Doctor admits that he doesn't know. He explains that he cares and then moves on, and Billy asks him if he's killed anyone. The Doctor tries to avoid the question, but Billy persists and he finally admits that he has... and he's moved on. He says that if he doesn't move on then more people die, and explains that he's two thousand years old and has never had the time for outrage. The Doctor warns that there are other children nearby and asks Billy if she's going to help him or let them be next on the menu. Kitty comes up and overhears him, and she takes them to where she and Spider live. Dot and the other children are there and Kitty assures the girl that the Doctor and Billy are all right. The Doctor wonders who pays them to get people onto the ice, and the Doctor hands out the fish pies he stole before Kitty has to tell Dot what happened to Spider.
Later, the Doctor reads stories to the children. Billy assures Kitty that the lights won't hurt anyone else, and Kitty wonders if Billy and the Doctor are still fighting. After a moment, Billy says that she moved on and the Doctor glances over at her as he hears. The Doctor then asks the children who paid them, and Kitty tells the children that can tell them. They explain that a man with a tattoo of a ship on his hand paid them. The Doctor says that he has a better way to find him and leaves with Billy. He says that his plan is to get eaten.
The Doctor and Billy get diving suits and go to the Thames, and admits that they're going to be defenseless but she doesn't need to worry about it. They put the suits out and go out on the ice to wait for the lights. The lights approach Billy and she calls to the Doctor, who doesn't hear her through the suits. She throws a lantern at him, just as she gets pulled through. The Doctor runs over and jumps into the hole just before it seals over, and sinks to the bottom. He scans around and sees Billy, who points out the fish circling above them. They see chains binding a gigantic creature to the bottom, and Billy finds Spider's hat floating in the water.
The creature opens one enormous eye. They then crawl out of the ice to the surface, where the startled pie-man is fishing for supplies. He runs off, and the Doctor explains that the creature is a prisoner. They notice that the pie-man was capturing the bioluminescent fish, and the Doctor figures that they're helping the creature and they evolve into it eventually. Billy figures that the creature is alien, but the Doctor is more interested in who is capturing the creature.
The pie-man is hiding nearby, and the Doctor finds him and asks about the man with the tattoo on his hand. The pie-man finally admits that he did, and Billy asks him if he's seen anyone acting suspicious. The man says that the tredgers, who gather people for the poor house, have been about.
The next morning, the Doctor and Billy go to the poorhouse. The Doctor uses his psychic paper to get past the tredgers, who mentioned Lord Sutcliffe. They play along and tell the tredger to show them around, and he does so. The tredger explains that the "bricks" the people are making go to a "steel mill", and the Doctor points out to Billy that they're at the other end of the creature. He confirms that doesn't know what's going on. He has concluded that the bricks are fuel for the furnaces, and that it burns a thousand times longer and hotter than coal, and burns underwater.
Next, the Doctor and Billy go to Sutcliffe's townhouse. As they wait for Sutcliffe, the Doctor tells Billy to leave the talking to him because she has a temper and the situation requires diplomacy and tact. Billy agrees, and when Sutcliffe comes in, he orders Billy to get to her feet in the presence of her betters. The Doctor punches him and confirms that he's human, and Sutcliffe's guards come in and tie them up.
Once Sutcliffe wakes up, the Doctor asks him where the creature came from. He says that it's always been there and the secret has been passed down through the family for generations. The Doctor figures that Sutcliffe is hosting the entire fair to get as many people out on the Thames as possible. Sutcliffe claims that he is saving lives because people die in coal mines, and he's moving the Empire forward. The Doctor says that an age and a species are defined by the value it places on "unimportant" life. Sutcliffe congratulates him on the speech but is unmoved. He tells his men to move the plan forward in daylight.
Sutcliffe's lieutenant, Dowell, takes the Doctor and Billy to a carriage. Billy asks the Doctor if he's two thousand years old, and admits that she wonders how long it takes to make a speech like he did to Sutcliffe. Meanwhile, the carriage arrives at the fair, and Dowell and his men. There are barrels of homemade rocket fuel inside, and Sutcliffe is waiting. The Doctor figures that Sutcliffe is going to blow up the fair and drop more people into the Thames, and Sutcliffe says that the city will mourn briefly and then move on. He tells Dowell to be off the ice by noon, and Billy yells for help but the noise of the crowd drowns her out.
Once Billy is done and Dowell leaves, the Doctor has Billy get his sonic from his pocket. It falls out on the floor and Billy kicks it over. As he tries to free them, a fish comes in under the ice and the Doctor realizes that the sonic's noise is attracting it. More fish arrive, and Dowell comes in and sees what the Doctor is doing. As he takes away the sonic, the fish circle him pull him under just as he tosses it to the Doctor to shut it down. Bill stares at the ice in horror, and the Doctor asks her what they're going to do about the creature, which he's nicknamed "Tiny." Billy figures that the creature might burst out of the ice and eat half of London, and the Doctor wonders what her future is worth if it's built on that kind of suffering. The Doctor says that he serves at the pleasure of the human race and tells Billy to give him an order. After a moment, she tells him to save Tiny.
The Doctor tells Billy to get everyone out of the ice. She asks Kitty to get the children off the ice. They claim that someone fell off the ice and everyone runs for the shore. Sutcliffe sees them leaving and yells that it's a mistake, but the fairgoers ignore him. He runs down to where is men are planting explosive charges, and says that they're blowing it early. Billy sees them and tells Kitty and the others to get off the ice.
Sutcliffe triggers the charge... but nothing happens above the surface. The Doctor has reouted the charges to Tiny's chains, and they're blasted free. Furious, Sutcliffe runs to the charges while Billy continues the evacuation and the Doctor climbs out of the water.
Sutcliffe runs to the tent with the barrels and sees the Doctor's sonic. Tiny breaks through the ice and Sutcliffe falls in. Billy runs to the Doctor and he pulls her up off the ice just as it cracks beneath her. The Doctor says that she'll go somewhere colder, and they watch as it swims off... splashing Billy in the process.
Later, Kitty takes the children to Sutcliffe's townhouse where Billy is waiting for them. She brings them in and says that they'll be there for a long time. The Doctor is busy forging new ownership papers, and Billy has the children sit down to eat and the Doctor signs the townhouse over to Sutcliffe's long lost heir... one of the boys.
The TARDIS returns to the present and the Doctor's office, and Nardole brings in tea. He realizes that the Doctor traveled through time, while Billy finds nothing on the Internet about Tiny. The Doctor insists that he returned to his office, and tells Billy that humans collectively overlooked the inexplicable. There is a news article about Sutcliffe drowning in a snap thaw. Nardole reminds the Doctor of his oath, while Billy discovers that the children inherited. The Doctor tells her that it's due to her because she gave the order. He then tosses a coin to determine if the TARDIS stays put or Billy leaves the Doctor alone.
Later, Nardole goes the vault, complaining. He hears something knocking three times on the vault door from inside, and says that they're not going to open it. Nardole insists that he's still there and the occupant is going nowhere. He walks away as the occupant keeps knocking.
Written by Gadfly on Apr 30, 2017