Thin Ice (TV story)

 was the third episode of series 10 of Doctor Who.

The events of this story follow on directly from Smile, with the TARDIS arriving in London just moments after the Doctor and Bill left the human colony planet.

As was the case in Cold Blood and Kill the Moon, the Doctor shows a refusal to do anything that could drastically alter the history of Earth on his own. Instead, he makes his companion make the decision. However, this time the Doctor takes a different approach; instead of leaving his companion to solely decide on what to do, this time the Doctor offers advice and encouragement to help his companion decide.

Synopsis
Trying to get back to the university before Nardole notices he broke his oath, the Doctor accidentally lands himself and Bill in the past, at the final frost fair in 1814. He decides to enjoy the unexpected trip.

However, something sinister is lurking below the frozen Thames.

Plot
The Twelfth Doctor and Bill are shocked to find themselves on the ice of one of the frost fairs, seeing an elephant. Before Bill has more time to come to terms with what she's seeing, the Doctor rushes her back into the TARDIS. As the Doctor works at the controls, Bill inquires if they've gone to a parallel world. The Doctor tells her that it's NOT; its Febuary 4th, 1814, the last frost fair. Bill questions why they aren't back, asking the Doctor why he can't steer the TARDIS. The Doctor states you don't steer the TARDIS, but negotiate with it rather unsucessfully. "She's a bad, bad girl this one. Always looking for trouble."

The TARDIS moves to a bridge above the River Thames. They exit back out, with the Doctor explaining that the thaw comes tomorrow, so he needed a more reliable parking spot. The Doctor is up to going to the fair, but Bill points out that they're in an era where her race was enslaved. He dismisses her worries, giving a series of complicated directions of the TARDIS interior. When Bill asks him what he's talking about, the Doctor tells her that's where the wardrobe is located and to go pick out a dress. Bill then laughs that the TARDIS has dresses and heads back in to find the wardrobe.

The Doctor bumps into a man carrying a basket of fruit as he heads back into the TARDIS. The man loses a fruit over the bridge. The screen follows the fruit to the ice, where it is passed by a drunken man, who stumbles onto an area of thin ice. He notices lights under the ice, which spin rapidly around him; in the following moment, he falls through the ice, which seals back up. The only thing left of the drunkard is his bottle.

Bill steps outside the TARDIS later, dressed in fancy period clothing. The Doctor exits next, wearing a suit and top hat. Bill wonders if there is anything they need to be on the lookout for, as the Doctor shuts the door. After they leave the TARDIS, the monitor gives off an alarm; a map of the River Thames appears on the screen with a large portion of it pulsing red. It warns that a large creature is living in the river.

As they walk, Bill asks about being in the past; she wonders if the future could be changed by the slightest things she does. The Doctor jokes their friend stepped on a butterfly and ceased to exist; now Bill can't even remember him. Bill notes the Doctor's joking; they were the only passengers in the TARDIS. As they walk, a street urchin offers them a flyer for the frost fair on the Thames; Bill is hesitant, prompting the Doctor to tell her that she's just taking a flyer, not altering the history of her planet. Reaching the Thames, Bill hesitantly walks onto the ice, becoming ecstatic at the prospect of being on the frozen river. The Doctor and Bill head to enjoy the fair.

Watching a wrestling match, Bill cheers enthusiastically; the Doctor, however, is not that impressed. He tells her that 'real" wrestling involves zero gravity, tentacles and magic spells. Amused by the idea, Bill notes that Regency London is a lot more racially diverse than she remembers from the movies. The Doctor remarks that "So was Jesus. History's a whitewash". Bill plays a bowling game and wins, telling the Doctor that she's on a team back home. They visit a pie shop, where the Doctor has a bet with the proprietor over a coin toss; Bill enjoys some pie, noticing green lights under the ice. The Doctor tells the pie man that he knows he cheated on the coin toss, asking him how he did it; it's a very impressive trick. However, the man takes offence. The Doctor attempts to diffuse the tension by stating he's a skilled con man too, saying he could steal anything in the shop without him knowing it.

Bill and the Doctor are promptly thrown out, with the Doctor revealing he managed to steal some more pie. Concerned about the lights, Bill asks the Doctor if there's any side-effects to time travel. He jokes that seeing lights under the ice is one, annoying Bill; if he saw them before, why didn't he bring it up? The Doctor tells her that he knew they'd get to work eventually, but decided they should enjoy the fair first.

They are approached a girl named Kitty, who asks for help looking for her lost dog. Bill finds her adorable, willing to help. The Doctor looks at the leash Kitty is holding, asking her to describe the dog; a boy in a large red hat sneaks up on the Doctor. Kitty tells him that she's looking for a small brown dog. Smiling, the Doctor congratulates Kitty on her con, knowing she was lying because the leash belongs to a big white dog. He quickly grabs the boy's hand as he attempts to pickpocket his sonic screwdriver from behind, only for Kitty to kick his leg and knock him down. Kitty and the boy flee, while Bill helps the Doctor up.

Hiding in an unoccupied tent, the boy, Spider, wonders what the sonic screwdriver is. Kitty doesn't know, but guesses it could be worth a lot of money. Having lost sight of them, Bill wonders why the Doctor is so upset they stole his "magic wand"; he firmly tells her that it's a sonic screwdriver. She asks how its a sonic screwdriver; the Doctor tells her it's sonic because it makes noise and a screwdriver in a broad sense. They turn around, finding Kitty and Spider running off to pawn the sonic screwdriver. The kids run past "Thin Ice" warning sign, with Kitty making it back to shore. However, Spider becomes distracted by the green lights, despite Kitty yelling at him. Spider is sucked under the ice 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 screwdriver; he examines it to make sure no damage has been suffered. A shocked Bill demands the Doctor save the boy; the Doctor says he can't. The Doctor apologizes to Kitty for not being able to save Spider, warning that the danger is not over. He immediately deduces there are more kids living rough, asking Kitty to take him to them; Kitty thinks its because he wants to take them to the magistrate and turns to run away. The Doctor quickly grabs her arm, stating they only want to help; Kitty points out Bill is gone. When the Doctor turns to look, Kitty gets free of his grip and runs into the fog. The Doctor ponders quickly which girl to go after, deciding he must find Bill first.

The Doctor finds Bill in a secluded spot, telling her to get used asking how he finds her; he asks what's wrong. Bill says she's never seen anyone die before; the Doctor reminds of their last adventure, where the dead were used as fertilizer. Bill states its different because they were already dead. "Morally and practically, that is not a useful distinction, unlearn it." Bill yells at him to stop telling her what to think; he jokes back that as her tutor, he's supposed to. Angered by his indifference towards the death of that boy, Bill demands to know how many people he's seen die; he's lost count. Further angered, Bill asks if he's ever killed, because she sees a look in his eye sometimes. The Doctor starts to defend the action, but Bill adamantly tells him to answer the question. With a deep sigh, the Doctor admits he has killed, but didn't keep count. 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. They need to intervene before the kids living rough end up on the menu. Kitty overhears them talking and asks what they mean.

Seeing the Doctor means well, Kitty takes the Doctor and Bill to meet the rest of the urchin gang - including the girl who handed Bill the flyer. The Doctor produces pies he stole from his top hat, winning the urchins trust after Kitty vouches for him. As night falls, the Doctor reads a book to the urchins by a warm fire. Nearby, Kitty puts provisions in a cabinet. Bill tells her that the Doctor can help them if they help in return. Kitty wonders what Bill does besides yell at the Doctor; Bill says that they were arguing and she has moved on from what she was angry about. Done with the story, the Doctor asks if the children have seen anything strange lately. Per Kitty's prompting the little girl reveals a man with a tattoo of a ship on his hand paid them to pass out fliers for the fair.

Deciding the lead is boring and too broad, the Doctor decides that he and Bill need to follow the only other one they have. Following the Doctor out of the urchins' home, Bill asks him what he means. The Doctor leads Bill along the river, telling her that what they know is that lights under the ice have been "eating" people. However, they don't know why. The logical solution is to "get eaten". Finding some diving suits, the Doctor and Bill don them as preparation of being pulled under by the lights. The Doctor tells her that she doesn't need to worry about them being defenseless. Bill asks if he has magic aliens powers; the Doctor gives an annoyed look, prompting Bill to ask if that was a rude question.

On the ice, the Doctor explains the lights are smart, choosing isolated targets. However, once he walks a few meters away, lights begins spinning around Bill below the ice. She yells to get the Doctor's attention; however, her voiced is heavily muffled by the primitive diving suit. At wits end, Bill throws her lantern at the Doctor's head; it bounces off, making the Doctor turn around just as Bill falls through. The Doctor jumps through the ice before it reseals, making a human sized hole. Down below, Bill is safe; she and the Doctor 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 head back to the surface.

On the surface, the pie man is fishing. The Doctor and Bill climb out of the hole in the ice he cut. Taking off their helmets, the Doctor notices him; the pie man flees. The Doctor yells for him to wait, as he loves his con. Bill thinks about the creature's moans, telling the Doctor she could feel the creature's voice in her bones; it sounded miserable down there. The Doctor walks over to the fish basket the pie man left behind, picking up an odd angler fish; he questions what they get out of feeding the creature as they are not carnivores. Bill is sickened to learn the pie she ate was made out of those fish; she loved that fish pie.

Deciding the pie man might have answers, they head to the docks, finding him hiding. The Doctor earnestly asking for the pie man to teach him the coin trick. However, Bill reminds him that they have more pressing matters. The Doctor asks if the man's seen someone with a tattoo of a ship, but the pie man is silent. Wondering if the man's against tattoos, the Doctor states that he is as well; "I think that we are bonding." Bill points out the flaw in the question; asking if he's seen someone with the tattoo of ship near the river. Realizing that this will get them nowhere, Bill asks if the pie man has heard of anything odd going on near the Thames. The pie man reveals that the dredgers have been working very hard; they're also heavily guarded. Why guard men dredging mud out of the river?

Day time.

The Doctor and Bill observe the dredgers hauling buckets of gunk out of the Thames. Bill wonders how they are going to get past the guards, prompting the Doctor to flash the psychic paper; Bill reads off that it says he works for the palace. The Doctor notes that he hasn't heard that one in a while. They walk down to the operation, being brought to the foreman, who is intimated by the psychic paper's lie; he asks if Lord Sutcliffe knows they are here. Muttering the question to himself and Bill, the Doctor says Lord Sutcliffe insisted upon them coming for an inspection. As the foreman walks off to check on the workers, Bill inspects a brick of the gunk, wondering why they're investigating over a mile away from the creature; the Doctor tells her that the creature's head is back by the fair. They're on the other end of it now, meaning that it's not mud that the dredgers are hauling out of the Thames; it's the creature's waste matter. Bill drops the brick in disgust, quickly wiping her hand on her coat.

Rejoining the foreman, the Doctor inquires if the men can be trusted and that it's been ensured that no-one follows the muck being transported to Hampton; however, the foreman corrects him, explaining the carts go to the steel mill. The Doctor quickly covers the blunder explaining "Hampton" is code for the steel mill; "we have to speak in code, otherwise anyone can walk in here and get you blabbing away like a fool." The foreman agrees with the idea of using code as precaution; Bill laughs to herself. Noting the foreman is a man of intelligence, the Doctor goads him into revealing what he knows about the "muck". The foreman tells them that the waste of the creature is more efficient than coal, burning a thousand times longer and the fire from it is hotter than can be measured. The Doctor half-jokes that the foreman won't be having his job much longer at the rate he's going, making the man think he's going to get a promotion. The foreman also reveals that it's rumored to burn underwater, astonishing Bill.

They visit the manor home of the employer Lord Sutcliffe. Bill wonders why, to which the Doctor explains that that waste can be used for interstellar fuel, so they need to figure out if Sutcliffe is an alien hiding on Earth or a human who's taking advantage of the creature. The Doctor insists he does the talking as they must charm Sutcliffe into revealing the information they need; Bill is likely to become angered due to the creature eating the children. Under the guise of "Doctor Disco of the Fairford Club", the Doctor meets Sutcliffe who instantly makes sexist, classist but mainly racist, remarks towards Bill even calling her a "creature" and demanding she shows "respect in the presence of [her] betters". Bill quietly seethes with anger, with a look of disgust on her face. The Doctor taps Sutcliffe on the shoulder, prompting him to turn around for a punch to the face. As Sutcliffe falls down, the Doctor states Sutcliffe is very clearly human. Bill agrees that if Sutcliffe was an alien "then that was pretty convincing racism."

Bill and the Doctor are quickly captured by Sutcliffe's minions with a bruised Sutcliffe revealing the creature has been a family secret for generations. The Doctor deduces Sutcliffe funded the current fair to draw in more people for the creature to digest; the man holding the Doctor has a tattoo of a ship on his hand, meaning he's the one who hired the urchins to hand out fliers. Sutcliffe explains the creature is responsible for freezing the river, which allows the Frost Fairs to take place. The Doctor says he preferred it when he thought Sutcliffe was an alien; it would explain the lack of humanity. Sutcliffe states its no different from losing lives in a coal mine.

Sutcliffe plans on blowing up the ice and sinking numerous victims to feed the creature. Bill is confused, as everyone will see it; however, Sutcliffe states the people would be too stupid to notice the creature as they will stop to mourn the ice "breaking". He claims to be moving the world forward by using this new fuel, citing that the lives of those eaten by the creature were not important. However, the Doctor tells him real value is putting an unimportant life above his own. Sutcliffe notes that the speech would have moved anyone with the slightest bit of compassion; he then adds "not your lucky day, is it?"

The Doctor and Bill are shoved into a carriage by Sutcliffe, who plans on storing them at the fair until he can detonate the ice; not wishing to risk the duo having warned others to stay away, he plans on detonating at noon after he brings in an elephant to draw in a crowd. Sitting in the carriage, Bill mocks the Doctor for telling her that passion fights and logic wins; the Doctor tells her not to be smug, as that belongs to him. She asks if the Doctor really is 2000 years old, prompting him to ask why. With an impressed smile, Bill asks if it takes that long to be able to make a righteous speech like the Doctor did. If so, its worth the wait for getting that old. The Doctor smiles, happy Bill has seen his better side.

Once at the fair, the Doctor and Bill are tied to a pole near the explosives in a tent. Bill thinks she can save them by screaming for help; however, the Doctor tells her not to bother. Ignoring the Doctor, Bill screams for help at the top of her lungs; the Doctor only waits for her to stop with annoyed look. Bill's screams are being drowned out by the cheers of the patrons watching the performances. Once done yelling, Bill asks the Doctor what they should do. He asks her to help him get his sonic screwdriver to cut through the ropes they're in. This also attracts the angler fish. Their guard rushes in and the Doctor hands him the sonic causing the fish to become attracted to him. The Doctor tells him to toss the sonic back, having already untied himself; the man is sucked below by the fish.

Free, the Doctor tells Bill that they have two choices: let the creature suffer for centuries, or free it. Bill points out that it could kill everyone if let go. She adds they already have the answer; the Doctor tells her only idiots have the answers. The Doctor tells her that it's her choice; he works for the human race, and will only do something that could drastically change the future of their planet only if he's ordered to do so by a human. Remembering the creature's moans of agony, Bill tells the Doctor to free it. With that, the Doctor tells her that he will, telling her to get everyone off the ice beforehand.

Elsewhere, Kitty is attempting to pickpocket someone until her arm is grabbed by another person. To her relief, it's Bill; she tells Kitty to get the urchins to spread word that the ice is thawing to scare people back to safety. The urchins spread word of the ice breaking and someone falling in, quickly getting everyone to leave. Sutcliffe sees everyone running away, and attempts to dissuade the people from leaving. Realising that he needs to hurry, Sutcliffe uses the detonator repeatedly, but nothing happens. However, that's far from the truth; below, each attempt to detonate instead breaks the chains holding the creature in place; the Doctor switched the explosives' positions.

Aggravated by the lack of success, Sutcliffe goes back to the stockpile to see what's wrong. Entering the tent, he sees some of the barrels are missing and the sonic screwdriver hanging on a wire, jamming the signal. He attempts to hold it, but is startled by the sudden sound of the cracking of the ice. He goes outside to see the creature's fin swimming by; the force of it going by cracks the ice and Sutcliffe falls in, never to be seen again.

Bill manages to avoid falling in, thanks to the Doctor; he pulls her up to the docks. As he pulls the wire holding the sonic screwdriver up, both of them are splashed by the creature as it swims away and avoiding harming anyone. Bill wonders where it will go now; the Doctor believes the fish will end up somewhere cold and try to avoid the human race for the rest of its life. However, she then wonders if they just doomed Greenland. As this is possible, the Doctor promises to check on Greenland in its future to see if the fish lives there now.

Later, Kitty arrives outside Sutcliffe's home with the urchins, telling the children to behave and not steal anything. Bill lets them in, leading them to the dining hall. In the hall, the Doctor is scraping a name off Sutcliffe's will, asking a boy for his name; it's Perry. When Kitty asks why, Bill states that Sutcliffe's "long lost heir" couldn't be Kitty.

Back in 2017, Nardole enters the Doctor's office at St Luke's University. Placing the tea on the Doctor's desk, he notices that the Doctor is still in period garb. Nardole reminds the Doctor that he promised not to go off-world; the Doctor sarcastically asks if it looks like he's wearing off-world clothes. He tells him that he's returned to his office as promised in his "tea-drinking" clothes. Bill exits the TARDIS, still in her period garb, revealing that the Doctor has broken his oath. 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 out before hurrying away looking very worried.

Cast

 * The Doctor - Peter Capaldi
 * Bill - Pearl Mackie
 * Nardole - Matt Lucas
 * Sutcliffe - Nicholas Burns
 * Kitty - Asiatu Koroma
 * Pie-Man - Peter Singh
 * Overseer - Simon Ludders
 * Dowell - Tomi May
 * Spider - Austin Taylor
 * Dottie - Ellie Shenker
 * Harriet - Kishaina Thiruselvan
 * Perry - Badger Skelton

Culture

 * The Doctor refers to the whitewashing in history when Bill mentions that there were more black people present than shown in movies, specifically mentioning Jesus Christ as an example.
 * A frost fair is being held at the frozen River Thames. The frost fair has many activities and performances, notably an elephant, wrestling, sword swallowing, skittles, cartwheel performers, magicians, ring toss, M. G. Clark Printing Co., souvenir shops, book shops, strongmen, pubs, The Nelson Arms, The City of Moscow and "try your luck" stalls.
 * A map for 19th century London is displayed on the TARDIS scanner.
 * The Doctor reads Struwwelpeter out loud for the street urchins.
 * The Doctor shows Bill the British Newspaper Archives when trying to find information about the frost fair-incident.

Biology

 * Bill refers to the melanin in her skin.

Technology

 * Spider steals the Doctor's sonic screwdriver, which Bill calls a "magic wand". When pressed by Bill, the Doctor admits it's a screwdriver in "a very broad sense", and that it's sonic because it "makes a noise".
 * A detonator is wired up to explosive barrels.
 * The Doctor's sonic screwdriver works underwater.
 * The Doctor and Bill wear diving suits.

Science

 * Bill thinks they have travelled to a parallel world.
 * The Doctor says only real wrestling is in zero gravity with tentacles and magic spells.
 * In Sutcliffe's mansion, the Doctor plays with an Orrery.

Currency

 * The frost fair costs only sixpence to take part.

Food and beverages

 * A bypasser accidentally drops a basket of fruits and vegetables on the Thames.
 * A seller is selling chestnuts just above the stairs to the fair.
 * Food stalls at the fair include: Shish kebab, shish squid, shish chicken, ox cheek, Lapland Mutton, sheep heart, fish pie, meat pie, chicken pie, Gingerbread and Gin stalls.
 * The Doctor steals fish pies from a con-artist.
 * Nardole has made tea and adds coffee to give it flavour.
 * Sutcliffe has a bowl of nectarines, pears, clementines, grapes and apples.
 * At Sutcliffe's mansion, the street urchins are given a traditional Christmas dinner, consisting of turkey, salad, potatoes, gravy, broccoli, red cabbage, a Christmas pudding and glasses of milk.

Species

 * Lure fish were swimming in the River Thames, luring people out on the thin areas of the ice with their green light bulbs and trap them underneath for the serpent to feed on.
 * The Doctor mentions fireflies and glow worms.
 * The excrement from the sea serpent is used as fuel by Sutcliffe's people.

People

 * The Doctor jokes with Bill about an imaginary person called "Pete" who stepped on a butterfly and got erased from history.
 * The Doctor likes good con artists.
 * Watermen are in charge of keeping order at the frost fair.
 * Bill says she was a skittles champion two years in a row.
 * Sutcliffe is racist and sexist.
 * Sutcliffe has hired dredgers.
 * Sutcliffe is low in iron.
 * The workplace has a dredger-in-chief overlooking the work.
 * Sutcliffe has a manservant.
 * Sutcliffe greets the Doctor as "Doctor Disco, from the Fairford Club".
 * The Doctor refer to the sea serpent as "Tiny", "the loch-less monster" and the "not-so-little mermaid".

Locations

 * The TARDIS lands on the Blackfriars Bridge next to New Lime Wharf. The fair stretches from Blackfriars to London Bridge.
 * The Doctor and Bill are heading towards Freezeland Street.
 * Kitty worries that the Doctor will take her and the other urchins to the Magistrate.
 * Sutcliffe has a workhouse upriver where his men work with the serpent's excrement.
 * The Doctor questions whether the chief dredger checked that no one followed the transport carts all the way to Hampton. He says he checks all the way to the steel mill.
 * Bill worries the serpent will eat people off the South Bank and half of London and that they've doomed Greenland after freeing it.
 * According to the psychic paper, the Doctor works for the Palace. The Doctor says he hasn't heard that one in a while.

Story notes

 * Steven Moffat joked in an interview that the intro to EastEnders was always about a giant snake, a reference towards the view of the River Thames from above, which makes it look like a serpent.
 * Spider is one of the few child characters to be killed on-screen throughout the history of the program.
 * For shots involving the Doctor falling below the ice in his diving gear were created by The Model Unit using a miniature 1/3 scale puppet inspired by a similar effect in 's film . To create Capaldi's head, Stephen Mansfield was invited back to the production to sculpt the feature. Mansfield had previously worked on the show during the era of Sylvester McCoy, making such creatures as the Destroyer and the Haemovores. The members of the team who worked on the effect, as listed on the Facebook page of the Model Unit, are:
 * Model Unit Supervisor: Mike Tucker
 * Senior VFX Technician / Sculptor: Alan 'Rocky' Marshall
 * Senior VFX Technician: Colin Mapson
 * Portrait Sculptor: Stephen Mansfield
 * Director: Bill Anderson
 * Producer: Nikki Wilson
 * Line Producer: Steffan Morris
 * Underwater DoP: Rob Franklin
 * Grip: Sam Reeves
 * Gaffer: Mark Hutchings
 * Best Boy: Andy Gardiner
 * Kitty warning Spider about how they'll be transported if caught is a reference to how England would transport convicts to Australia contemporaneously.

Ratings

 * 3.76m (UK overnight figures)
 * 5.61m (UK final)

Filming locations

 * Kings Weston House, Bristol - Sutcliffe Manor

Production errors
to be added

Continuity

 * The Doctor and Bill find themselves in the middle of the frozen River Thames with an elephant crossing their path. (TV: Smile)
 * The Doctor previously visited the 1814 Frost Fair with River Song. They brought Stevie Wonder with them to perform. (TV: A Good Man Goes to War) He also mentions having been there before, "a few times". (AUDIO: Frostfire; PROSE: The Frozen)
 * The Doctor gives Bill a similar set of directions to the TARDIS wardrobe that the Ninth Doctor gave Rose Tyler. (TV: The Unquiet Dead)
 * In Regency London, Bill voices her fear that she will be enslaved due to the colour of her skin. Martha Jones voiced the same concern when she visited London of 1599 with the Tenth Doctor. (TV: The Shakespeare Code)
 * When thinking they are in the present day of 2017, Bill speculates that they have travelled to a parallel world. (TV: Inferno, Rise of the Cybermen)
 * Bill is concerned about the butterfly effect, also exactly like Martha in her first visit to London's past. (TV: The Shakespeare Code)
 * The Doctor reminds Bill of her recent experience with death when they visited a garden full of dead people. He confirms this was only a few hours ago. (TV: Smile)
 * The Doctor reminds Bill that he is her tutor. (TV: The Pilot)
 * The Doctor once again notes humanity's habit of overlooking and forgetting the bizarre. (TV: Remembrance of the Daleks, In the Forest of the Night)
 * Nardole reminds the Doctor of his oath and that he said he wouldn't be going off-world. (TV: The Pilot, Smile)
 * Nardole has made tea for the Doctor. (TV: Smile)
 * Nardole mentions being reassembled. (TV: The Husbands of River Song, The Return of Doctor Mysterio)
 * The Doctor once more states that he's 2,000 years old. (TV: Deep Breath, Smile)
 * The Doctor mentions how sometimes the options available are limited, and begins to say something about choices, alluding to the explanation he provided to Clara. (TV: Mummy on the Orient Express)
 * Bill questions if the Doctor cares about people dying; Psi had similarly questioned the Doctor's detached nature. (TV: Time Heist)
 * Sutcliffe refers to the Doctor as "Doctor Disco". (TV: The Zygon Invasion)
 * The Doctor and Bill return to the Doctor's office. Bill remarks that it looks the exact same, from the cup of previous sonics, wooden raven and family photographs to the books, the busts and the placing of the chairs. (TV: The Pilot)
 * Bill uses search-wise.net to look for the serpent incident. Rose Tyler used the same web to look up the Ninth Doctor. (TV: Rose)
 * Nardole checks on the Vault. (TV: The Pilot)
 * The Doctor once again defers to his human companion's judgement as to whether to allow a creature to live or die. (TV: The Beast Below, Kill the Moon)

DVD releases
This episode was released as part of the Series 10 DVD box set on 13 November 2017.

Blu-ray releases
to be added

Digital releases
to be added