The Pilot (TV story)

 was the first episode of series 10 of Doctor Who.

According to Steven Moffat in a video introduction, "Series 10 sort of begins the show again. [The Pilot] introduces everything you need to know about Doctor Who, and tips you into the universe". Indeed, through Bill's eyes, new viewers are introduced again to the character of the Doctor, his TARDIS, his enemies the Daleks, and his versatile tool, the sonic screwdriver, much like they did through Ian and Barbara in the original 1963 introduction, "An Unearthly Child", and Rose Tyler in Series 1's 2005 "pilot", Rose. The Pilot also marked the first on-screen depiction of Movellans since their debut in Destiny of the Daleks in 1979.

The episode introduces St Luke's University, where the Doctor apparently has been lecturing for over fifty years and begins a plot thread surrounding what he and Nardole are hiding in a vault beneath the campus.

Synopsis
The Twelfth Doctor — now living and teaching at St Luke's University on Earth — convinces dinner lady Bill Potts to be his private student. The Time Lord and his companion Nardole soon discover that their new friend has unwittingly made a deal with a prospective girlfriend that threatens their safety in a way that even the TARDIS can't outrun.

Plot
The episode opens with an empty university professor's office; nobody is in. The sound of mechanical whirring is heard for a while before the door opens and Nardole leads Bill Potts in; he gestures for her to take a seat at the professor's desk, with his arm whirring. A bolt falls off his arm, but he discretely kicks it away before Bill can notice. Smiling, Nardole leaves without a word, leaving Bill to wait for the professor to arrive.

Left to her thoughts, Bill looks around with room with nothing but the sound ticking clocks to keep her company. She notices the TARDIS in a corner, with an "Out of Order" sign hanging on it. Bill's attention is next drawn to the professor's desk, where there's a mug filled with the Doctor's old sonic screwdrivers. She reaches out to examine one but is startled when the silence is broken by the sound of an electric guitar playing the opening bars to Beethoven's Fifth Symphony. She coughs loudly to make her presence known, and the Doctor sticks his head around the door. He tells her to wait a moment. After that, Bill listens to him clumsily put away the guitar and turn off his amp.

The Doctor enters the room and takes his seat; he asks Bill for her name. He wonders why she's been attending his lectures as he knows she is not a student at the university. She admits that she actually works at the university canteen, but attempts to deny sneaking into the Doctor's lectures. He counters that he has seen her and asks again why she keeps turning up. After fumbling over a story about a girl she fancied in the canteen, Bill confesses that she loves his lectures and always wanted to attend the university. She then challenges him, asking why the man has taken an interest in her as "plenty of people come to [his] lectures that aren't supposed to". He explains that he has noticed her because she smiles when she doesn't understand something, whilst the majority frown.

Bill asks around about the Doctor, saying that it's rumoured he's lectured for over 70 years. The Doctor thinks she's asking about his age, but Bill says that she wants to know what subject he's supposed to be lecturing on. It seems he has permission to talk about whatever he wishes; on one occasion, he was supposed to give a lecture on quantum physics but talked about poetry. He tells her that they're basically the same thing because of all the rhymes. The Doctor offers to be Bill's personal tutor, and sort out her enrolment with the university, on the condition that she comes to his office at 6pm every weekday, even if someone is dying and that if she gets anything less than a First on every assignment, the tutoring will cease. As she leaves, she asks what she should call him, as everyone calls him the Doctor. He tells her it's what he's called. Bill is sure this cannot be his real name, asking "Doctor what?" The Doctor just smiles.

Bill goes through her daily routine: serving chips, attending the Doctor's lectures and tutoring sessions, and returning home to her foster mother, Moira. Bill explains that the Doctor is kind of her foster tutor, as Moira goes over Bill's assignments, where she has scored at least 87% in each subject the Doctor has taught her, even very obscure topics. Moira is worried that the Doctor might be some kind of pervert, warning Bill to be careful of men. Annoyed, Bill mutters under her breath she's not interested in men.

The Doctor gives a lecture on the concept of the passage of time; in truth, time doesn't pass, because life is a great magician that only allows people to live one day at a time. As people remember living yesterday, they hope they'll live another day tomorrow. He further explains that each day is like a photo, a frozen moment, which, like a movie, when played in quick succession, comes alive. The Doctor tells his audience to imagine all time happening at once. Every moment from birth to death, all the triumph and heartbreak of their lives comprising an entire city; it's the best place a person can ever hope to be. Writing on the chalkboard, the Doctor calls this Time and Relative Dimension in Space, going on to say "it means life". That night, when out at the student bar with her friends, Bill's eye's meet those of another girl across the room. As she heads to the bar, Bill bumps into her and notices that the girl has a gold pattern like a star in her eye.

The next morning, Bill sees the Doctor and Nardole suspiciously hurrying through the university quad and decides to follow them out of curiosity. The pair descend into a hidden vault under the university, and Bill is surprised to find that the security door opens for her. She overhears snatches of their conversation but accidentally kicks over some clutter on the floor and fearing detection, hurriedly makes her way out again. Outside, she notices Heather, whom she recognises as the girl from the bar, sitting on a bench looking distraught. Bill asks if she is okay and enquires about the star in her eye, but Heather brushes this off as a "defect in the iris." Sensing that Heather is troubled, Bill asks if she is "freaking out about something." Heather hesitates, before asking Bill to come with her and see something; Bill accepts and they set off across campus. On the way, Heather says that she hates the university but doesn't know why, remarking "Everywhere I go, I just want to leave."

The pair arrive at a concrete service yard behind one of the university buildings, entering through a gap in the security fence. Heather points out a mysterious puddle on the concrete that ought not to be there, as it hasn't rained in a week. When Bill finds this unremarkable, Heather asks her to look at her reflection in the puddle and tell her what's wrong with it. Bill notices that her face does appear wrong in the puddle's reflection, but can't figure out how. Whilst Bill ponders the unsettling reflection, Heather is overcome by her desire to leave and walks off, though not without regret. Bill is left crestfallen and leaves too. When the yard is empty again, a voice issuing from the dark liquid announces that the "pilot is located".

At Christmas time, Bill celebrates with the Doctor, giving him a rug as a present; though the Doctor admits that he hasn't gotten her anything. They discuss travelling, Bill saying that she can sense that the Doctor travels around, even though he denies it. She says "With some people you can smell the wind in their clothes." This is what she presumes her birth mother would have said, revealing that she died when Bill was young. Bill tells the Doctor that she apparently looks like her mother but doesn't know this as there are no photographs of her. She wonders whether photos really help after loved ones are gone, causing the Doctor to look wistfully at the photographs of his wife River and granddaughter Susan on his desk. At home, Bill's foster mother Moira shows her a newly discovered box containing photos of Bill's birth mother. Bill weeps upon discovering that she looks just like her mother. She notices the reflection in a mirror in one of the photos, in which the Doctor can be seen holding the camera.

Months later, Bill walks past the service yard again and is delighted to find Heather, who is once more staring into the puddle. Bill calls out and Heather smiles, though her demeanour is noticeably different from the last time they met; there is a knowing look in her eyes. She asks if Bill ever worked out what was wrong with her reflection in the puddle. When Bill answers that she did not, Heather invites Bill to come round and offers to show her. Remembering her hasty departure when they last met, Bill asks Heather to promise not to go without her. Heather promises, but when Bill has run around the building and through the gap in the fence, her friend is nowhere to be seen. Assuming Heather to have walked out on her again, Bill is slightly offended, and leaves; not seeing Heather's screaming face beneath the surface of the puddle. As she is pulled down into the liquid, a voice announces "Pilot confirmed. Pilot engaged."

Worried about Heather and her fascination with the strange puddle, Bill consults the Doctor. She tells him about Heather's eye and the inexplicable reflection in the puddle. On hearing this, the Doctor runs out of his office to investigate, and Bill follows. Bill begins rambling on about Heather, wondering if something has taken over her mind like in the science fiction shows she's seen on Netflix. When Bill presents an idea, which is that an alien lizard is in Heather's head, a confused Doctor states "you see a girl with a star in her eye and your first thought is that she has a lizard in her brain?" The Doctor eventually figures out that the reflection looks wrong because it is not a reflection at all. He explains that when looking at your reflection you never see your face the right way round; it looks wrong in the puddle because it looks right.

Bill then notices that the badge on her jacket is on the wrong side in the puddle. The Doctor explains that it was easier for Heather to spot because the star in her eye makes her face asymmetrical. Bill questions how it behaves like a reflection, leading the Doctor to conclude "It's not reflecting you, it's mimicking you. There's something in the water pretending to be you." He scoops up a sample in a test tube and examines the pattern of scorch marks on the concrete nearby. Sensing danger, the Doctor sends Bill home to keep her safe; he tells her it's just a freak optical trick to shut Bill up. As she heads off, the voice from the puddle declares "Passenger selected. Pursuit engaged" and the puddle begins to move.

Back at her flat, Bill hears the shower and guesses Moira is in. However, she then gets a call from Moira, who she knows is seeing her ex-boyfriend Neville again because the mobile number belongs to him. Bill asks if Moira allowed anyone to stay at the flat, to which Moira denies; when she asks why Bill passes it off as a random concern and hangs up. She grabs an umbrella as a blunt weapon and enters the bathroom to find no-one is in there. She looks in the drain to find Heather's starry eye star. Realising it's not a freak optical trick like the Doctor dismissed it as, Bill runs out of the flat to see if he's still at the university.

Outside St Luke's University, Bill is confronted by Heather. When Bill tries to speak to her, she only mimics Bill's words. As Bill steps closer, she sees that liquid is constantly pouring over Heather's body, dripping off her hair and hands; her face is pale and expressionless. Bill is horrified, exclaiming "You're dead?" Heather suddenly glides towards her, but Bill escapes and rushes into the Doctor's office, where he is examining a sample of the puddle. Though Bill bars the door, the puddle gushes under it and reforms into Heather; she tells the Doctor he was wrong about what the puddle was.

The Doctor convinces Bill to enter his blue box, taking the sign off as they do. Bill protests that the box can't save them since the doors looked wooden and they had windows, only to turn around as the console lights up and the Doctor reveals the TARDIS. Bill asks if it's a knock-through, which the Doctor confirms is partially correct. Amazed by the sight of the console room, Bill compares it to a "really posh kitchen", before wondering why the doors to the TARDIS aren't as fancy; she wonders if he ran out of money building it. Annoyed, the Doctor begins pondering what to do; on the monitor, Heather hasn't moved. Bill asks him if there's a toilet in the TARDIS, as she needs to go after having that fright. He gives her directions on where it is. On the way down, Bill runs into Nardole; he asks if the Doctor wants Bill removed. The Doctor tells him she needs the toilet; Nardole tells Bill to wait a bit.

Suddenly the TARDIS shakes. Nardole asks what's happening, to which the Doctor explains is an attack. On the monitor, Heather is shown attacking the TARDIS to get in. The Doctor proceeds to take them down to the vault, where Bill realises that the police box in the corner of the Doctor's office is his spaceship, which is bigger on the inside. The Doctor and Bill infer that the puddle creature is not here for whatever is hidden inside the vault and is instead there to chase them. Even as they reach this conclusion, however, liquid trickles down the steps and Heather reappears. She issues a piercing scream as the Doctor, Bill and Nardole escape in the TARDIS.

The Doctor decides to give Heather a challenge, taking the TARDIS away from the university. He exits the TARDIS into daylight, confusing Bill; they are now at a dock. Bill asks if they've gone to the future, prompting the Doctor to tell her that they haven't; they've just moved to the other side of the world, where it's daytime. The Doctor moves out of Bill's sight to reveal the Sydney opera house; they're in Australia. Bill begins panicking and runs into a restroom in a nearby establishment.

The Doctor follows after her, asking if Bill is fine. Bill asks the Doctor if he's from space; he tells her that he's not: "Nobody's from space. I'm from a planet like everybody else." When asked if he's from Earth, the Doctor tells her "not particularly". Calmed down some, Bill then states that TARDIS doesn't make sense then, as it only works in English; if he's an alien, why would he name his spaceship in English instead of his native language? The Doctor replies that most people don't bring this up. Heather soon catches them up, forming out of droplets on a mirror, so the trio escape in the TARDIS.

Inside the TARDIS, the Doctor noted it took Heather mere minutes to catch up to them; this time, he's going to give her a real challenge. Bill wonders where they're going, to which the Doctor explains: the other side of the universe, 23 million years in the future. Bill is amazed to be on an alien planet, which has a yellow sky; she wonders what it's made of. The Doctor jokes "lemon drops". Bill then asks the Doctor if he thinks the possessed Heather is evil; he explains that evil is based on perspective. Some things in the universe are not trying to cause harm, but are just hungry; "hunger looks like evil from the wrong end of the cutlery." He then jokingly asks if Bill thinks her food loves her back. Nardole asks the Doctor why he thinks the creature is hostile, to which the Doctor explains is because most things in the universe that they're encountered have turned out to be so.

The Doctor hypothesises that the puddle is a type of sentient oil or perhaps a small part of a shape-shifting "liquid spaceship" which had lain dormant until it saw something it needed. On the Doctor's prompting Bill remembers Heather's desire to leave, explaining why the puddle chose her; it needed a pilot, someone who wanted to fly away. When Nardole asks why the creature wants Bill, the Doctor begins to suspect that it may be the result of the mutual crush she and Heather had on one another, asking: "What, in the end, are any of us looking for? We're looking for someone who's looking for us." As the Doctor says this, Bill looks into a pool of liquid she has found. Heather begins to rise to the surface, smiling at Bill, who smiles back. But then her smile fades and Heather grabs Bill, trying to pull her down. The Doctor and Nardole free Bill and they retreat to the TARDIS, as liquid gushes out of the pool and begins to reform.

The Doctor's last test of Heather's abilities is basic sterilisation. They are going to run her through "the deadliest fire in the universe"; Nardole likes the wording. However, the Doctor adds that there's only one way to run Heather through it, and that's to run through themselves first. Nardole notices the one flaw in that plan. However, he then notices when the TARDIS is going on the monitor; he begs the Doctor to not take them there. The Doctor hands him the Fourth Doctor's sonic screwdriver, telling him to run interference. The TARDIS lands and Bill is curious as to why they've landed; the Doctor explains that they've arrived in a battle in the middle of a war. He's got "old friends" here. They exit the TARDIS in one direction, while Nardole goes in the other.

As they walk through the wreckage of the area, Bill wonders if this is the future again. The Doctor tells her that this is the past (on a different planet). She wonders if they're safe, to which the Doctor says is up to Nardole; not really, Nardole tends to blunder a bit. Moments later, Movellans rush in and are blasted by Dalek fire. The Doctor and Bill notice Heather has appeared and he leads Bill into another corridor. They are met by a lone Dalek; the Doctor tells Bill that it's the deadliest fire in the universe. It demands to know who they are. The Doctor holds out his sonic screwdriver and demands it be scanned. Identified as an enemy, the Doctor smiles and pulls Bill away as the Dalek fires. The blast hits Heather, who appeared behind them; however, it passes through her. The Dalek fires in vain, as Heather repeats its "exterminate" cries in an annoyed tone.

Elsewhere, Nardole is sealing off the area but is caught by a Dalek and fired upon. Nardole manages to escape, yelling for the Doctor. The Doctor and Bill attempt to get back to the TARDIS, but are halted by a shot fired by a Dalek. It says "exterminate" in a garbled voice; the Doctor knows something is off as he has TONS of experience with Daleks. He gets close and notices something off with its eyestalk. He tells Bill to look at it as well. Heather's eye is in it; it's not a real Dalek, just her. Nardole arrives, saying he just sealed the Daleks out of the area; he then notices the fake Dalek, freezing in fear until the Doctor tells him its Heather. The Dalek leaks fluid and dissolves into a puddle. The puddle then reforms as Heather.

The Doctor infers that the creature doesn't mean to harm Bill, as it didn't use the Dalek's weaponry. He challenges Heather, demanding she leave Bill and fly away; asking "why won't you just go?" At this, Bill suddenly remembers that the last thing Heather said was a promise not to leave without her. "Never underestimate a crush" the Doctor notes, as they realise that Heather is inviting Bill to fly with her. The Doctor tells Bill to release Heather, so Bill sadly absolves her of the promise, telling her "I really liked you." However, Heather reaches out her hand to Bill, who takes it, in spite of the Doctor's protests. As their hands clasp a sphere of water surrounds them and Bill sees a stunning vision of planets and stars interspersed with Heather's memories of their mutual attraction.

The Doctor tells Bill to resist, still fearing the possibility that it is a trick. Bill sadly says goodbye to Heather, to which Heather sadly responds "Goodbye, Bill": the first thing she has said since her transformation that is not mimicry, indicating that Heather remains herself and is not possessed. Heather departs leaving Bill shaken and upset. The Doctor brusquely returns to his ship while Nardole comments on the former's seemingly ignorant behaviour. Bill comments that the tears she sheds don't feel like her own.

Back in the university office, Bill seems to regret her decision not to leave with Heather, asking the Doctor if they will see her again and looking hopefully at the TARDIS. The Doctor says that she must forget about seeing Heather or travelling in the TARDIS and tries to wipe her memories of doing so, explaining that he must remain in disguise, unknown to everyone owing to a promise he made. Bill retorts, asking how he would feel if someone tried to wipe his memories. The Doctor tells Bill to leave immediately without another question before he changes his mind. He looks at the picture of River, telling it to shut up, then the same for Susan's picture. Even the TARDIS briefly hums causing the Doctor to confess he "can't keep doing this".

Outside, Bill runs down the steps, checks that she can remember Heather, and smiles. She finds the Doctor leaning on his TARDIS on the university courtyard. He tells her that it's a big universe, but maybe one day they will find Heather. When Bill asks what caused him to change his mind, the Doctor responds "Time and Relative Dimension in Space. It means what the hell." Bill excitedly runs into the TARDIS, followed by the Doctor, who shuts the door behind them.

Cast

 * The Doctor - Peter Capaldi
 * Bill - Pearl Mackie
 * Nardole - Matt Lucas
 * Moira - Jennifer Hennessy
 * Heather - Stephanie Hyam
 * Voice of the Daleks - Nicholas Briggs

Science

 * The Doctor is Bill's tutor on physics and astrophysics, among other subjects.
 * The Doctor says quantum physics and poetry are the same thing.
 * Bill's graded paper on The Cosmic Far Ultraviolet Background mentions blazars, which are members of the galactic nuclei and quasars, polarisation, the Pittsburg Conference on BL Lac Objects.
 * Bill's graded paper on quantum statistics of light mentions the atom, electrons, protons, trees, balls, automobiles.
 * Bill's graded paper on Laser cooling of ions: atomic clocks and quantum jumps mentions ions and a few words from the Lord.
 * The Doctor lectures on Time And Relative Dimension In Space, saying it means life and so references past days and future days, present days, little moments, big moments and hypotheses them all happening at once.
 * On the blackboard used by the Doctor, the squeeze theorem was used on a calculus demonstration.

Technology

 * The Doctor has, in his office, several sonic screwdrivers, of various designs from his past.

TARDIS

 * The Doctor tells Bill that "TARDIS" stands for "Time and Relative Dimension in Space". Bill points out that this acronym would only ever work in English.
 * Bill compares the TARDIS control room design to a kitchen.
 * She also asks where the toilet is, just before Nardole enters and implies he has just been there.
 * While she discovers the TARDIS is bigger on the inside, Bill at first thinks it's a knock-through behind his office, then that it's a lift.
 * The TARDIS has a macaroon dispenser.

Cultures

 * Bill is a fan of science fiction and recognises many of its tropes in her experiences with the Doctor, including mind wipes.
 * Bill watched a thing on Netflix about lizards in people's brains, controlling them.
 * Moira has a Crock-Pot Cookery cookbook in the kitchen.

Species

 * The Dalek doesn't recognise the Twelfth Doctor until it scans the sonic screwdriver, as according to its own knowledge the Doctor has brown curly hair and is wearing a long scarf.

Food and beverages

 * Bill serves chips.
 * The Doctor drinks water during his lectures.
 * The Doctor mentions vegan wraps.
 * The Doctor jokes that the air and sky on the alien planet is made of lemon drops.
 * The Doctor rhetorically asks Bill if she thinks her bacon sandwich loves her back.

Locations

 * While running away from the sentient oil puddle, the Doctor, Nardole and Bill visit Sydney, with a view of the Opera House. They also visit a planet on the other end of the universe, and a war zone in the Dalek-Movellan War.

St Luke's University

 * The Doctor has been lecturing at St Luke's University in Bristol for apparently 50 years.
 * Bill serves chips there, and Heather is a student.
 * The Doctor's office contains the TARDIS, photographs of River Song and Susan Foreman, Rembrandt van Rijn's Self Portrait with Two Circles, black-and-white prints of Joshua Reynolds' Portrait of Emma Hart and George Romney's Lady Hamilton in a Straw Hat, and busts of Ludwig van Beethoven and William Shakespeare. He also has an old-fashioned radio, a record of His Master's Voice and a gramophone.
 * The Doctor and Nardole are keeping something hidden in an underground vault beneath the university.

People

 * Nabeela is a member of staff at St Luke's University.
 * Bill Potts' mother died when Bill was a baby.
 * Neville is Moira's ex-boyfriend.
 * Barry is Moira's friend.

Music

 * A choir is singing "Jingle Bells" during Christmas.

Story notes

 * A working title for this episode was A Star In Her Eye.
 * Before filming started, a two-minute promo teaser introducing Bill known as Friend From the Future was broadcast in April 2016. Parts of this promo were adapted into the episode during the encounter in the Dalek war zone. (DWM 511)
 * This marked the first time that a BBC Wales episode of Doctor Who opened without any music or dialogue for the first few minutes, leaving only the sound of a clock in the background.
 * The song "Love Will Tear Us Apart" by Joy Division plays during the bar scene when Bill and Heather first meet.
 * Bill mentioning people being possessed by lizards in the brains is a reference to the Stargate franchise.
 * During the scene in which the TARDIS travels to the Dalek-Movellan War Zone, a short scene from TV: The Doctor's Wife is reused in which the TARDIS leaves through the bubble universe rift for the House planetoid.
 * As described by Caroline Lie, many if not all of the extras who attend the Doctor's lecture were employees from Doctor Who Experience in Cardiff. She can be seen, in fact, in multiple places, often near her friends, including Joe (TARDISArchives). Both previously participated in Classmates, a YouTube series consisting of fan reactions to episodes of Class.
 * During the scene when the Doctor is about to wipe Bill's memory and she tells him to imagine if someone did it to him, "Clara's Theme" plays. This is a reference to the events of TV: Hell Bent.
 * "The Sad Man with A Box" plays in the scene in which Bill is introduced to the TARDIS. A happier, more fairy-tale rendition of this same tune, "The Mad Man with a Box", played during the scene in The Eleventh Hour when Amy walked into the TARDIS and saw the interior for the first time.
 * The Doctor uses a chalkboard with the same white roundel frame in his university classroom as Miss Quill does in her classroom at Coal Hill Academy in the spin-off Class.
 * The ringtone for Bill's mobile phone is the same as the one for Martha Jones' phone, which was later kept onboard the TARDIS by the Tenth Doctor.
 * When Nardole shows Bill the Doctor's office, his arm makes a mechanical sound and a bolt falls from it, implying that his body is robotic, or at least partially cybernetic.
 * According to Doctor Who Magazine #512, two scenes were filmed but edited out before broadcast. One features Bill asking Nardole about the Doctor; the other was an alternate edit of the club sequence where Bill meets Heather for the first time. As originally filmed, Bill was to be shown watching the Doctor, on stage, playing Clara's Theme. The only remnant of this in the final episode is a brief glimpse of the neck of the Doctor's guitar and his left hand.
 * This episode includes both a "Next Time" trailer for the following episode and a "Coming Soon" trailer for the remainder of the season. Making this the first episode that contains both a next time trailer and a coming soon trailer.

Myths and rumours

 * The episode was to include a "very mild" sex scene between Bill and another character. This rumour circulated widely on Twitter in the months prior to broadcast, allegedly citing an advance copy of the episode and emerged in conjunction with the (later confirmed) rumour that Bill would be gay. There is no indication such a scene was ever actually scripted or filmed.
 * The character of Bill and Heather, who become attracted to each other, are named for William Hartnell and his wife, Heather. Steven Moffat has confirmed that this was a coincidence.

Ratings

 * The overnight ratings on BBC One were 4.64 million, up 60,000 viewers from the previous series opener, The Magician's Apprentice.
 * Consolidated figures were 6.68 million, the highest figures for a standard series episode since the Series 8 finale, Death in Heaven.

Filming locations

 * The Main Building, the Sir Martin Evans Building, and Tower Building of Cardiff University largely fill in for St Luke's University, while the Reardon Smith Theatre is where the Doctor gives his lectures.
 * The Cardiff Metropolitan University (Llandaff Campus)'s Atrium Cafe is where Bill serves chips.
 * The bar scene was shot at Clwb Ifor Bach in Cardiff.
 * Bill runs into the bathroom in "Australia", in World of Boats in Cardiff.
 * CEMEX's Taffs Well quarry, just outside of Cardiff, is the exotic planet the TARDIS team travel to.

Production errors

 * Bill is wearing a completely different shirt when she gets back to the university after Heather leaves.

Continuity

 * The Doctor still has a jeweller's eyeglass. (TV: Heaven Sent)
 * The Doctor has an "Out of Order" sign on the doors of the TARDIS, nearly identical to the one used by his first incarnation. (TV: The War Machines)
 * The Doctor has previously been a teacher at a college (PROSE: Human Nature, TV: Human Nature), as did another one of his very best Time Lord friend (TV: Shada).
 * The Doctor and Bill are wearing paper hats and pull Christmas crackers. (TV: The Christmas Invasion, The Time of the Doctor, Last Christmas)
 * The Doctor has a wooden raven on his desk in his office. (TV: Face the Raven)
 * The Doctor's Rembrandt van Rijn portrait Self Portrait with Two Circles appears in his office. (TV: The Husbands of River Song)
 * The reveal of the TARDIS strongly resembles how the Ninth Doctor revealed it to Rose Tyler, with Bill mirroring Rose's resistance to initially enter, sceptical it is safe. (TV: Rose)
 * The Doctor has previously met sentient liquid beings: the Kar-Charratans on Kar-Charrat (AUDIO: The Genocide Machine) and the Flood on Mars. (TV: The Waters of Mars) He also previously encountered a creature which mimicked living people to steal their form. (TV: Midnight)
 * The Doctor mentions all of time hypothetically happening at once, during one of his lectures on time. He once experienced such an event in his eleventh incarnation. (TV: The Wedding of River Song)
 * In his office, the Doctor has his bust of Beethoven, previously seen in his TARDIS. Similarly, he also plays Beethoven's Fifth Symphony. (TV: Before the Flood)
 * In his office, the Doctor also has a bust of William Shakespeare. (TV: The Chase, The Shakespeare Code)
 * The Doctor tells Bill that being scared is good and rational. He previously told Rupert Pink the same thing. (TV: Listen)
 * Bill writes an essay entitled "Laser cooling of ions: atomic clocks and quantum jumps." The Eighth Doctor encountered an atomic clock on the eve of the 2000th year. (TV: Doctor Who)
 * The Doctor pilots the TARDIS to the Dalek-Movellan War. (TV: Destiny of the Daleks)
 * Near the Vault, the Doctor has a sign belonging to the Mary Celeste. (TV: The Chase)
 * The Doctor had encountered scorch marks left in concrete by a shuttlecraft once before, also on a school campus. (TV: Remembrance of the Daleks)
 * The Doctor attempts a memory wipe using the method he once employed on Donna Noble. (TV: Journey's End) The Doctor relents on wiping Bill's memory when she poses the question of how he would feel if it happened to him. (TV: Hell Bent)

Behind the Scenes

 * Some of the Sonic Screwdrivers seen in the mug on The Doctor's desk appear to be the toy versions produced by Character Options.

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