Smile (TV story)

 was the second episode of series 10 of Doctor Who.

The episode marks the fourth time in the BBC Wales series that the Doctor took his companion into the future for their first post-introductory adventure with him, after Rose Tyler, Amy Pond and Clara Oswald. This was the technically the first of these to take place on an alien planet, though The Rings of Akhaten (TV story) was set on a planetoid orbiting a fully fledged planet.

In Doctor Who Extra, Steven Moffat and Peter Capaldi explain that they don't really understand Emoji. Thus they had some trouble understanding texts from Jenna Coleman, who used Emoji frequently in her texts to them.

Synopsis
After shaking off Nardole, The Doctor asks where Bill Potts wishes to travel in time. She asks to see the future. Complying, he takes her to see one of Earth's first space colonies, where it's been stated the inhabitants have cracked the secret of perpetual happiness. However, they soon discover that the cause of this "happiness" has a very deadly punishment for not following along.

Plot
Bill has just entered the TARDIS, asking the Doctor what she should do as a passenger. The Doctor tells her that she should know as she's already been on a trip. Bill counters this by stating it's a proper trip this time, asking if there are seat-belts. She sits in a seat, noting that the layout the TARDIS is completely wrong; the seats are too far away from the controls. She wonders if the Doctor can stretch his arms like Mr. Fantastic of the Fantastic Four. The Doctor explains he stands when piloting the TARDIS. Bill mocks him for never thinking to bring the chairs next to the controls.

Bill then asks where the steering wheel is. The Doctor tells her that they have to negotiate with the TARDIS, as it will not always take them where they wish; often it's the "still point" between where one wishes to go, and where they are needed. Hearing a knock, Bill wonders who could be knocking on the TARDIS doors. The Doctor mumbles "mum" and opens the door.

Nardole enters, reminding the Doctor that he made an oath and cannot leave the Earth unless a emergency happens. The Doctor tells Nardole that he's over 2000 years old and sometimes likes to take his TARDIS downstairs instead of walking. Nardole asks why Bill is inside; the Doctor states it's because she isn't anywhere else. The Doctor kindly asks Nardole to make tea. Nadrole agrees and leaves them; he states if Bill wants tea, she have to make her own.

Thinking that the Doctor has changed his mind again, Bill asks if they are heading back to his office, to which the Doctor states will happen, after they've had some adventures; "between here and my office is everything that ever happened or ever will." The Doctor tells Bill to make her choice: past or future. Bill states the future, because she wants to see if it's happy. The Doctor complies, throwing the brake lever and the TARDIS takes off.

In a future human colony planet, a colonist, named Kezzia, is warned not to come back to the city as they are having a problem with their microbots, the Vardy; however, she ignores the warning, as she talks to a robot with an emoji-like screen. The woman on the radio, named Goodthing, is smiling painfully as Kezzia asks why she's doing it. Smiling still, she explains that their mother and a few other people are dead. Kezzia is horrified to see her smiling at that, prompting another robot to display a crying emoji. The robot's screen turns to a emoji with skulls for eyes and bared teeth; it grabs Kezzia as a section of the roof transforms into a swarm of microbots. It envelops Kezzia and reduces her to bones as they scream. The robot turns to Goodthing, still displaying the emoji, as she pleads with it that she smiling, the microbots chase after her as she screams, while the screen turns back to a happy emoji.

The TARDIS materialises there later, with Bill asking which direction Earth is in. The Doctor tells her any direction, as space is curved; he asks if she's wanting to go back. Bill states she isn't thinking of anything; her mind is "overloading" from time travelling to an alien planet in the future. As they walk towards the city, Bill notes the TARDIS hasn't cloaked itself as something else. The Doctor reminds her that it's stuck; Bill notes that the Doctor must like it enough to have never bother to fix the broken cloaking device. Annoyed, he tells her to let the subject go.

They walk into the city, where the Doctor explains that this place was able to crack the mystery of perpetual happiness. Bill then questions the Doctor about what Nardole said. The Doctor explains that as a result of something that happened a long time ago, he swore an oath to watch the vault beneath the university. Bill points out that he isn't right now. However, the Doctor counters that he IS guarding it, as his time machine can take him back to right when he left. Bill questions the Doctor as to whether or not he's considered the possibility of him getting stuck or lost away from the university. The Doctor tells Bill that he's thought of it, and has decided to not dwell on things that haven't happened yet. Bill catches sight of the Vardy, asking if they're alien birds. The Doctor tells her they are microbots, and that they are likely conducting a security check on the two of them. Bill is disappointed with the robots; the Doctor tells her not to say such things, as it's rude. Bill sarcastically asks if she hurt their feelings. Feeling pain in their ears, Bill wonders what happened. The Doctor shouts they've been fitted with some kind of communication device; bizarrely his voice comes out her ear. Bill notes how wonderful this is; she'll never need her mobile phone again. However, she then becomes concerned about being able to turn off the device when using the loo. The Doctor dismisses her worry, stating if they can be given a communication device without any fuss, "there's probably an app for that."

They come across a robot, which Bill is impressed by. However, the Doctor tells her it's not a robot, but instead a interface device to interpret for the Vardy. Bill wonders if it speaks; the Doctor says it depends on what factor of human language has survived into this era. The robot's face smiles, with two smiling faces for eyes. Bill laughs that it speaks emoji; the Doctor is less than impressed. The Emojibot gives them badges that reflect their moods in the form of emojis, which only other people can see.

Placing the badges on, both the Doctor and Bill are surprised that they instantly go to their backs. The Doctor presumes that this is so that seeing one's own emotion on the badge doesn't change it, which would have caused a "feedback loop". Bill questions this, as what if the wearer fancies someone; the Doctor laughs, "Well, I guess you'd have to maintain eye contact with them, then." Bill thinks the idea is brilliant.. They head further inside, while Bill's badge shows her happy, the Doctor's shows him as puzzled. Since the interface speaks in emoji, the Doctor postulates that the entire city was developed by teenagers.

The Emojibot serves cubes of gelatinous blue food; which Bill notes smell like fish. The Doctor says he is not fond of fish, except for conventionalising; thus eating less evolved fish would lead to an awkward conversation. Bill wonders if the food is safe for them to eat, as she doesn't know if humans are meant to be living here; the Doctor points out that the cutlery is human and only humans speak in emoji. Bill begins eating her cube of food. Seemingly responding to the Doctor's puzzlement, a swarm of Vardy wait outside the building. The Doctor notices there is no livestock, to which Bill is pleased to know that humanity no longer needs to eat animals; instead, this algae-like substance. The Doctor notes he met an emperor made of algae once, piquing Bill's curiosity; the Doctor adds the emperor fancied him.

The Doctor reiterates that this place is ready for humans to live in, as the entire place is ready for someone to move right in, but there is no-one around. Bill compares it the student union, where a place to live is prepared before anyone moves in. Seeing Bill is on to something, the Doctor comes to the conclusion they've just arrived too early; the colonists sent the Vardy ahead of them to prepare the planet for them. The Doctor goes on to note that if the atmosphere wasn't human-friendly, the Vardy would fix it for them; even better, the robots could build a home for the colonists to live in before they got there.

However, Bill notes that the Doctor has two helpings, wondering if there's food sexism in the future; "is this bloke utopia?" The Doctor explains that it's likely because the Vardy have mistaken him for two different people; as the Vardy work for humans, they do not understand the concept of a person having two hearts and thus think the Doctor is two people. Bill is surprised, wondering why the Doctor has two hearts; he retorts "why one?" Finishing her meal, an amused Bill wonders if the Doctor has really high blood pressure as a result of having two hearts. He confirms this.

The Emojibot escorts the Doctor and Bill to a greenhouse outside, where more Emojibots pollinate plants and an automated system sprays calcium-based fertiliser onto the plants. The Doctor finds a discarded necklace, keeping it hidden from Bill. Bill wonders what they should do: leave before the colonists show up, or put the kettle on. The Doctor walks past her into the next room, where Bill notices a rosemary plant. Bill smells it, remembering there's one outside the student union. She thanks the Doctor for bringing her; she's smelling the scents of home several years into the future.

The Doctor asks if Bill believed his theory about there haven't been any humans here yet; Bill confirms she did. He then asks if he convinced himself; Bill looks at his badge and tells the Doctor that he didn't. The Doctor then goes on the explain that he is worried because there should be a set-up team of humans commanding the Vardy. However, no-one is here. Noticing the Doctor's badge has a thinking face, Bill asks what's on his mind, and he replies, suspicious that he is thinking about "a magic haddock". As they have no answers, the Doctor decides they should put two questions together: where is the set-up team and what is the source of this mineral fertiliser?

Pulling out his sonic screwdriver, the Doctor opens the machine pumping the fertiliser; it has grinding gears pulverising calcium. He then opens the bottom, revealing several skulls. The Doctor tells Bill that despite how this appears, the humans haven't been dead long. He has come to the conclusion that the Vardy killed the set-up team and feed them into the fertiliser. Bill wonders why, to which the Doctor says they'd better not stick around to ask them. They turn to find an Emojibot with a crying face. The Doctor tells it that they were admiring the garden and are leaving now. They run out the room, as the teary-eyed Emojibot changes to show the same emoji that it showed when the women were killed. As they run, Bill notes that if the Emojibot is chasing them, it's being awfully slow. The Doctor explains to Bill that there's a reason for something chasing you to be slow, when it's targets are running; the chaser doesn't need to be fast.

In the centre of a corridor, the Doctor and Bill find themselves surrounded by Emojibots that can sense their fear. The city is full of them, so the robots will catch you eventually; thus making a chase pointless, as they can maneuvre targets into a trap. The Doctor wonders why they're being attacked NOW, when they've been there a long time. Realising something, the Doctor tells Bill to smile using her whole face; she wonders why as the Doctor checks her badge. Seeing Bill's badge change to a smile, the Doctor tells her that smiling has a psychological effect on one's mood. He further explains the robots' programming has caused them to make these mistakes; the Vardy were programmed to make people happy by ensuring their environment was comfortable, but the programming has evolved and gotten jumbled up somehow. The Vardy now seem to think that they need to kill people when they become unhappy.

The Doctor tells Bill they should give the Emojibots what they want, and smile. The Doctor starts walking down the exit hall, telling Bill to not even bother following him unless she is smiling. They slowly walk down the hall. The Doctor tells the robots that they've enjoyed their stay. Bill tells him that next time, they should go to Aberdeen or Wiltshire. Agreeing with her, the Doctor and Bill move past the Emojibots, now bearing annoyed faces, but still poised to attack.

They run outside, where an Emojibot grabs Bill's arm. Bill screams in terror, as the Doctor tells her to smile. However, Bill is too scared to fake happiness. Instead, the Doctor sonics the robot's arm, breaking it and freeing Bill. This signals a swarm of Vardies to seemingly emerge from the building's structure itself. They continue running, with the Doctor sure the Vardy will not be concerned with them once they are out of the city.

Once back at the TARDIS, the Doctor tells Bill to stay in inside while he returns to the city and follows a "childish impulse to blow it up." Bill is flabbergasted, as she thought they were leaving. Surprised Bill cannot see the bigger picture, the Doctor explains "that city is a living death trap; we can't leave it with it's mouth open." As there is likely a colony ship coming, they have to ensure that the Vardy do not claim anymore lives due to the error in their programming. The Doctor tells Bill to watch some movies in the TARDIS. Bill wonders why he can't call a "helpline" to deal with the problem instead, the Doctor runs off, telling her not to look at his browser history. Seeing the Doctor run, Bill heads back outside the TARDIS, reading the sign on the door: "Advice and assistance obtainable immediately"

In the city, the Doctor convinces an Emojibot that he is happy, but his happy-faced badge changes to show a light bulb shortly afterwards. He hears someone breathing on his communication device; thinking it's a survivor, he asks the person to join him. To his annoyance, it's Bill, who asks if he's Scottish because of his accent; the Doctor says he isn't, just cross. Bill then wonders if there's a "space Scotland" too. The Doctor notes Scotland demands independence from every planet they land on.

Bill tells the Doctor her deduction; he doesn't call a helpline because he's it. He gives advice and assistance. The Doctor tells her that is just a coincidence; he just happened by this house of horrors, and decided to muck in. He tells Bill to look at the wall, going on to explain that the Vardy are what makes up the city. Bill genuinely smiles at the absurdity. However, the Doctor believes that the centre of the city houses the spaceship in which the colonists first arrived, comparing it to the Vikings, who lived in their ships until they built new homes.

They follow a staircase downwards, finding a damaged ships's hull; the Doctor is glad to have found a real wall. Finding the entrance to ship, Bill tries to pull the door open to no avail. Humorously, the Doctor presses a button that opens the door inward. "Not even locked; they were expecting to live in peace" the Doctor sadly notes. He then suggests locking it behind them, which alerts all Emojibots in the city, whose smiley emojis now incorporate exclamation marks for eyes.

Inside the spaceship, the Doctor comments on the difference between design of the ship interior and the city outside, each designed by "wet brains"; the humans and "dry brains"; the Vardies. They come across a map labelling where they are, and the Doctor plans to walk to the engine room in the middle of the ship. He instructs Bill to stay back to guide him through the map, which displays the Doctor's current position. On his way, the Doctor finds all sorts of furniture and artefacts, including the bust of Nefertiti.

Bill asks how the Doctor is allowed to blow up the spaceship without consequences, but he responds that it is a "moral imperative" to destroy the "murder machine" that surrounds them. Bill notices an unexpected empty space surrounding the engine room, but the tannoy of the ship distracts her. Outside, the Emojibots' faces change to skulls as the ship, Erehwon, begins to wake up, and as the Doctor descends into the engine room. Bill realises she could have followed the Doctor the entire time if she had photographed the map. The Doctor reveals that he had already memorised the map, as Bill protests against his leaving her "out of trouble". Bill follows the Doctor's path, but she stumbles upon a room containing a recently deceased old woman, with a book placed at her feet. Bill finds that the book contains digital images of human history on Earth, continuing past her time and through an apocalyptic war. Bill asks the Doctor these colonists were the last of humanity.

The Doctor explains that humans evacuated Earth following this conflict, and that this colony must be one of those evacuative ships. He meanwhile fiddles with the engine room controls, trying to reroute the engine's flow into the calorimeter. However, as he tries to do so, the calorimeter begins to rise to its peak, which spells doom for the Doctor. Bill proceeds to the engine room, but to her shock, finds a boy ,who asks, "Are we there yet?" Meanwhile, an Emojibot closes in on the Doctor, who is still waiting for Bill to help him hold the calorimeter tightly while he jams it shut with a spanner. The Doctor sees its reflection in a dial, and smashes the Emojibot off the catwalk with the spanner, nearly dragged down himself, only to run into Bill and the confused boy, who asks where everyone else is.

The Doctor is puzzled by this, until he and Bill find that the empty space around the engine room is where the remaining human population is cryogenically stored. Shocked, the Doctor tells Bill the colonists were already there, waiting to sheparded into the new utopia; the Doctor feels disgusted with himself for nearly killing them all. One of the stored humans, Steadfast, wakes up. The Doctor assertively instructs him to stay inside the ship as the other humans begin to unfreeze behind him. As they walk away, the Doctor explains to Bill that he doesn't have a plan yet, but he acted like he had one to make sure idiots didn't cause more trouble by trying to think up their own plans.

Bill takes the Doctor to the old woman's corpse, and they try to figure out her role in what happened here. Using the tablet-like logbook, Bill and the Doctor see the woman was the first person to die, then a few followed after her and then everyone else. Unlike the rest of the set up team, this woman is a corpse instead of a pile of bones; the Doctor states she died of natural causes, but all the other deaths weren't natural. Bill guesses it could have been a virus, that went viral. The Doctor then has an epiphany: grief!

The Doctor tells a confused Bill grief is caused all these deaths. This woman was the first person to die here, and before that, everything was happy. The Vardy were never programmed to understand humans need to grieve, so they thought grief was a threat to happiness; thus any human who felt grief was killed. And so on and so on until all of the set up team was killed by the Vardy. The Doctor again mentions the magic haddock, and produces the discarded necklace, which contains a hologram of the boy waving happily. Bill says that they will find the owner of the necklace when she wakes up, but the Doctor tells her that he found it in the greenhouse, belonging to one of the dead advance colonists.

Meanwhile, the boy has left the ship and exited into the city. An Emojibot offers a mood badge to the boy, who promptly places it on. He looks for his mother, asking the Emojibot if it knows, before walking off. His mood changes to a puzzled emoji. At the same time, the Emojibot gives an annoyed face.

Inside the ship, the Doctor and Bill explain the Vardy are malfunctioning to the colonists. The Doctor then adds the colonists used the Vardy to make their lives easier, but like every slave race, they started having ideas of their own; now they think they have to kill people when they're too unhappy to live. The colonists arm themselves with laser guns to fight the Vardies. Bill tells Steadfast that he needs to listen to the rest of the explanation; he bluntly says he did listen, now knowing that the Vardy are killing people. The Doctor quickly runs up to Steadfast, explaining "The Vardy want to help you; killing you is just a side-effect." The colonists take off to destroy the Vardy; the Doctor and Bill run after the colonists to prevent another mass slaughter.

In the city, everyone finds the boy flanked by two Emojibots. The boy asks where his mother is, beginning to cry. Bill tries silencing him into smiling, telling him to admire his new home; however, the boy doesn't listen and cries. The Emojibots grab the boy, ready to kill him; however, Steadfast fires upon the Emojibots. One is damaged and the second's screen begins flashing between two new images: skull and gun. The Doctor realises that the symbols mean "Rage" and "Revenge". The rest of the colonists begin to fire, as some Vardies break off from the ceiling to attack, and kill a man.

Instead of caring about the attack, the Doctor begins thinking about how the Vardy are reacting; they are registering as under attack, meaning they're "alive" He runs to a broken Emojibot, using the sonic screwdriver on its wiring. The Doctor tells Bill there's a reason he never loses at chess; he has a secret move. Whenever he's about to lose, he kicks over the board and ends the game. Hoping what he's about to doesn't hurt, the Doctor finishes messing with the Emojibot, creating a blinding, disorienting flash throughout the city.

The Doctor tells the story of the fisherman and the magic haddock. A fisherman caught the fish and it offered three wishes in exchange for it's life. The fisherman wished for his son to return from war and 100 gold pieces, but the haddock thinks in a different way, instead sending the son's corpse home from the war, along with 100 gold pieces commending his service. The fisherman then wished that he hadn't wished the first two wishes, pressing the "reset button".

The colonists are now confused as to why the Vardy aren't killing them. The Doctor repeats that he hit the rest button, basically turning the Vardy on and off again; as a result they no longer remember murdering the humans or the fact they are robots belonging to the colonists. "Welcome to your new planet. Meet the Vardy. They are, as of now, the indigenous life-form." The Doctor tells Steadfast that the colonists should try making peace unless they'd rather live like homeless people as the waking colonists have no clue how to work the technology. Walking over to an interface, the Doctor introduces himself and apologises for not recognising the Vardy as an emergent new species. As penance, he is willing to negotiate with them to allow humanity to stay in the city; he asks if the Vardy wish to charge rent.

Returning to Earth in the TARDIS, Bill asks the Doctor if the human-Vardy relationship will work. The Doctor replies that it is up to them. Bill asks him if he normally deals with such affairs, acting as an "intergalactic policeman". The Doctor replies that the police box exterior of the TARDIS is a mere coincidence. They land, exactly at the point they left, but when Bill steps out of the TARDIS, it is snowing.

To their shock, an elephant is approaching the TARDIS. The Doctor explains that they are in London, not Bristol, and that they are standing on the frozen Thames. Seeing this, the Doctor realises Bill might have been right about a steering wheel.

Cast

 * The Doctor - Peter Capaldi
 * Bill - Pearl Mackie
 * Nardole - Matt Lucas
 * Kezzia - Kiran L Dadlani
 * Goodthing - Mina Anwar
 * Steadfast - Ralf Little
 * Praiseworthy - Kaizer Akhtar
 * Nate - Kalungi Ssebandeke
 * Emojibot 1 - Kiran Shah
 * Emojibot 2 - Craig Garner

Language

 * The Emojibots speak Emoji. Emoji used by them include:
 * 😃 – smiling face
 * 😊 – happy face
 * 💀 – a skull, a symbol of death
 * 😢 – crying face, one tear
 * 😭 – crying face, two tears
 * 💡 – a light bulb, symbolising a new idea forming
 * 😕 – puzzled face
 * 😮 – shocked face
 * 😯 – interested face
 * 😰 – worried face
 * 😦 – confused face
 * 🤔 – thoughtful face, can also have a drop of sweat
 * 😐 – suspicious face
 * 🤗 – open-armed smiling face
 * 😵 – dead face
 * 🔫 – under attack
 * 😡 – anger
 * 🗝 – key, interacting with locks
 * ❓ – question mark, indicating confusion, can also be a face with a question mark above
 * ❗️ – indicating awareness
 * 👍 – thumbs up, indicating approval
 * 💷 – pound sterling, indicating an interest in monetary gain

Culture

 * The Doctor says that the colonists will come expecting the Garden of Eden.
 * The Vardy identified grief as the enemy of happiness.
 * The Doctor tells the story of "The Magic Haddock".
 * Bill ask whether the Doctor has "stretchy arms" like Mister Fantastic.
 * Bill says Gliese was like a Student Union before the students arrive.
 * Because Bill is only served one jelly while the Doctor is given two, she asks whether they have food sexism even in the future.
 * The Doctor mentions how Vikings used to turn boats upside down and use them as houses.
 * The record Bill looks into shows the entirety of human history.
 * A photo in the archive of the Erehwon shows protestors with placards which read: "Socialist Worker: Kill the poll tax. Tories out". Another reads: "Socialist Worker: Finish off the Tory poll tax. Amnesty now. Stop the bailiffs".

Locations

 * Bill suggests visiting Wiltshire or Aberdeen.
 * The spaceship the Doctor and Bill find on the planet is called the Erehwon. The company that owns it is called United Earth.

Species

 * The Vardy can eat other species, leaving behind bones and skulls.
 * The Vardy form the walls of buildings by interlinking with each other, but are also able to detatch themselves when needed.
 * The Doctor calls the Vardies the "worker bees of the Third Industrial Revolution".
 * The Doctor and Bill encounter an elephant a the frozen river.

People

 * 00125-323458 was the first woman to die at the colony.
 * Two of the earliest people to die at the colony were 00125-35468 I and 00125-35468 II.
 * The Doctor says he knew an emperor made of algae that fancied him.
 * Steadfast is MedTech One.

Technology

 * The Emojibots are an interface of the Vardy.
 * The Doctor and Bill receive sound implants on arrival at the colony. The Doctor calls the sound implants "thingamabobs".
 * All inhabitants and visitors are given mood badges.
 * Bill's smart phone is also a camera phone, meaning she can take a picture of the map.
 * The colonists await their awakening aboard the Erehwon.
 * The Erehwon has a Fleishman Cold Fusion Engine.
 * The engine has a Kelvin calorimeter.

Anatomy and physiology

 * The Doctor mentions his binary vascular system, and tells Bill it leads to high blood pressure.
 * The Doctor says that smiling psycologically has an effect on the mood.

Botany

 * The colony has rosemaries in the garden, which reminds Bill of home.

Food and beverages

 * The Doctor mentions the colony will have wheat and olive groves.
 * The Doctor and Bill are served squares of algae jelly. According to Bill, it smells of fish.

Science

 * The Doctor explains that because space is curved, the Earth is in any direction you choose to look in.

TARDIS

 * Bill asks why the chairs in the TARDIS are so far from the console.
 * Bill asks whether she has to wear a seatbelt during flight.
 * Bill also asks if there is a steering wheel.
 * Bill questions the Doctor on the police box exterior.
 * The Doctor says the TARDIS has broadband and suggest she watch some movies or something to pass the time.

Story notes

 * The Doctor says that Scotland demands independence every planet they go to. In real world current events, at the time of this episode's release, Scotland was again from the United Kingdom, as a result of Brexit.

Ratings

 * 4.25m (UK overnight)
 * 5.98m (UK final)

Filming locations

 * The City of Arts and Sciences (Ciutat de les Arts i les Ciències), in Valencia, Spain, is the real setting for the colony planet scenes.

Production errors

 * Kezzia states that she is using the Vardy to pollinate the wheat. However, the field used in filming is not a field of wheat, but a field of barley.
 * The field of barley is ready for harvest, long past the time of pollination.

Continuity

 * The Doctor mentions Bill's first trip in the TARDIS taking her across space and time. (TV: The Pilot)
 * The Doctor says that he stole the TARDIS. (TV: Logopolis, The Name of the Doctor)
 * Nardole reminds the Doctor that he can not go off-world because he promised not to. (TV: The Pilot)
 * Bill again calls the Doctor "a penguin with its arse on fire" when she sees him run. (TV: The Pilot)
 * The Doctor states that the TARDIS takes him where they need to go, rather than where he wants to. (TV: The Doctor's Wife)
 * The Doctor states he cheats at chess to avoid losing. (TV: Nightmare in Silver)
 * Bill asks why the TARDIS is a police box. (TV: Boom Town) She mentions that she knows it's a "cloaking device". (TV: Doctor Who, The Pilot)
 * Bill deduces that the Doctor likes the police box sign, because of its message. The Seventh Doctor admitted as much: "I like the shape. And the motto. Call here for help. That's what I do. I let little children sleep safely at night, because I've searched through all the shadows and chased the baddies away." (PROSE: Love and War)
 * The Doctor again avoids long explanations by saying "a thing happened". (TV: The Husbands of River Song)
 * There's a bust of Queen Nefertiti in the spaceship. (TV: Dinosaurs on a Spaceship)
 * The Doctor tells Bill to not look at his browser history. (TV: The Zygon Inversion)
 * Bill mentions that the Doctor couldn't leave her serving chips. (TV: The Pilot)
 * Bill says the Doctor is an awesome tutor. (TV: The Pilot)
 * The Doctor mentions having previously encountered human colony ships, (TV: The Ark, The Beast Below) and stations of colonists in cryogenic suspension. (TV: The Ark in Space)
 * The Doctor had once before visited a human colony in which unhappy people were killed. (TV: The Happiness Patrol)
 * The Doctor mentions Scotland wanting independence everywhere they go, such as when they wanted a separate ship from Starship UK. (TV: The Beast Below)

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