School Reunion (TV story)

School Reunion was the third episode of series 2 of Doctor Who.

It continued from the prelude established in Tardisode 3, and saw the start of Mickey Smith's travels in the TARDIS. This episode was most notable for marking the return of original series companions Sarah Jane Smith and K9, both having last appeared on-screen in 1983's The Five Doctors. With Sarah Jane's inclusion in this story, it was established the current series was not a reboot, but rather a continuation of the Original Series. K9 Mark III was destroyed and replaced by a newer model at the end of the episode. Months after this episode aired, rumours of a spin-off series with Sarah were confirmed by the BBC and The Sarah Jane Adventures premiered on BBC One on 1 January 2007.

This story is the first to bring up the impact travelling with the Doctor as a companion has on one's life, even providing insight on the Doctor's perspective, notably why he never usually came back for a companion.

Synopsis
Deffry Vale High School is haunted by strange, bat-like creatures at night. When the Tenth Doctor investigates, he finds an old friend, Sarah Jane Smith, already working undercover.

Plot
At Deffry Vale High School, the headmaster, Mr Finch, notices a student waiting outside his office. She has a headache, and cannot simply go home because she lives in an orphanage. Mr Finch offers his sympathies that there is no-one to miss her and invites her into his office; it is nearly time for lunch. The door closes, then there is a flap of wings, and the girl screams.

"John Smith" and Rose have gone undercover in the school. He teaches a physics class. He asks a few simple questions. Only an intelligent student called Milo can answer. The Doctor advances to higher levels of knowledge, finally asking how to travel faster than light. Milo answers every question without missing a beat.

Meanwhile, Rose is working undercover in the cafeteria. At lunch, she complains to the Doctor about the last two days. He mentions that it was Mickey who alerted them to something odd going on and rightly so. Everyone at the school is well-behaved and there is something odd about the chips. Rose eats a few, saying she enjoys them. The school menu has been designed by Mr Finch himself to improve concentration and performance. Another teacher, Mr Wagner, approaches Melissa, one of the students. He tells her Milo has failed him, so she is being moved to the top class. He also summons another student, Luke, but not Kenny, who is not allowed to eat the chips. The Doctor observes all this. He looks up and sees Mr Finch gazing down on the cafeteria floor, watching everything.

In the kitchen, Rose watches the other kitchen staff, all wearing gloves and face masks, bringing in a large barrel. One of the dinner ladies warns them not to spill a drop. Mickey calls Rose on her mobile phone, telling her about the massive UFO activity he has discovered around the area. However, his investigations are being blocked by something called Torchwood. Rose, in turn, tells him the kitchen staff were all replaced three months ago with new personnel. As they speak, the barrel slips, spilling something on one of the staff, who starts to burn. The rest usher her into a side room. Rose starts to phone for an ambulance, but is told not to worry, she is all right, even as Rose hears screams and lots of smoke billows out of the side room. Rose glances down at the barrel, which is leaking a golden, oily substance.

In the maths classroom, Mr Wagner tells the children at their computers to put on their headphones. The screens flicker on. The monitors display a green, rotating cube with rapidly scrolling, alien-looking symbols and seemingly random text on the right. The children type on their keyboards with incredible speed. Clearly, something is very wrong.

Mr Finch escorts a journalist, Sarah Jane Smith, around the school. Sarah has been assigned to write a profile on him. Finch explains that one of the policy changes he has made is free — but compulsory — school dinners. In the staff room, the Doctor is speaking to Mr Parsons, head of History, who tells him of the extraordinary knowledge of his students since Finch became Headmaster. One of his students has given him the exact height of the walls of Troy in cubits. Also, the day after Finch arrived, seven teachers came down with flu, and were replaced by strange new ones. The one the Doctor replaced hadn't had the flu. She had resigned when she won the lottery, despite the fact that she never played, claiming the ticket was posted through her door at midnight. Mr Finch brings Sarah into the staff room. When the Doctor sees her, he smiles delightedly at unexpectedly seeing his old friend, but introduces himself as John Smith. Sarah remarks that she once knew a man who went by that name. When she learns "Smith" is a new teacher, she asks if he has noticed anything odd. She has lost none of her inquisitive nature, and the Doctor is elated, although he does not reveal his real identity to her.

As the Doctor wanders the halls in a slight nostalgic haze, Kenny goes into the Maths room. He is shocked to glimpse a bat-like creature under one of the desks. It transforms rapidly into Mr Wagner. Wagner tells Kenny to leave and the boy beats a hasty retreat.

That night, Sarah Jane Smith breaks into the school to explore, even as the Doctor, Rose and Mickey do the same. The Doctor sends Mickey to the maths room and Rose to get a sample of the oil, while he checks the headmaster's office. There are sounds of flapping and the occasional shriek. Winged shadows flit across walls.

Sarah, trying to break into Mr Finch's office, notices she is being watched. She enters a storeroom — where she finds the TARDIS. Stunned at seeing the ship again, she backs out of the room into the gym, where "John Smith", now wearing his long coat, is waiting and says, "Hello, Sarah Jane." Sarah manages to say, "It's you...Doctor! Oh, my God, it's you, isn't it? You've regenerated..."

The Doctor replies that he has done so "half a dozen times since we last met." She says he looks "incredible", and he says she does too, but she dismisses it as being true for herself and says she has grown old. She then reveals she had waited for him and thought he had died, and in response, the Doctor speaks to her of the Time War for the first time: "I lived. Everybody else died." Sarah Jane is still in disbelief until they hear a piercing scream and run towards the sound; now she knows this is the Doctor.

They meet Rose along the way. The Doctor introduces the two and Sarah comments on Rose's youth. The scream turns out to be Mickey, who opened a cupboard, only to be covered in shrink-wrapped rats. Sarah suggests the rats are for dissection, suggesting cattily that maybe Rose isn't old enough to have gotten to that point in school, but Rose retorts that rat dissection hasn't been performed in schools for years, making a snide dig at Sarah's age. As they head for Finch's office, and Sarah and Rose begin to bicker, Mickey laughs and tells the Doctor, "The missus and the ex — welcome to every man's worst nightmare!"

The Doctor suggests that the rats may be food for something. When they enter Finch's office, they find what that "something" is; large, bat-like creatures hanging from the ceiling, asleep. They back out hurriedly, but as the door shuts, one of the creatures wakes and shrieks. Outside, the Doctor tells the others that when Finch arrived at the school he brought twelve staff members with him, so they've got thirteen giant bats to deal with. He is then about to head back in despite the danger so he can use the TARDIS to analyse the oil sample Rose procured. Sarah tells the Doctor she may have something that can help him. To the Doctor's excitement, in her car boot is an inactive and rusty K9 Mark III, with one of his side panels missing. Sarah explains that one day it had just stopped working and she could not repair its advanced technology.

Not knowing that they are being trailed by Mr Finch and another bat-creature, they go in Sarah's car to a nearby café, where the Doctor repairs K9. Mickey teases Rose about her jealousy, while Sarah asks the Doctor if she did something wrong since he never came back for her after his visit home. In an intense moment, the Doctor tries to brush it off, saying that she was getting on with her life. Sarah replies that he was her life. The hardest thing was adjusting back to mundane life after all she had seen. She asks him why he could not have come back. The Doctor looks dour and does not reply.

K9 comes to life and recognises the Doctor, who smears some of the oil sample on its eye sensor. K9 determines it is Krillitane oil. The creatures are Krillitanes, a composite species who take the best physical parts of other species they conquer. The Doctor did not recognise them because they had looked different when he had last seen them; basically humans with Giraffe necks. He realises that they are doing something to the children.

As they leave the café, Rose is struggling to come to terms with the realisation that she's not the first person the Doctor travelled with. She tells the Doctor how troubled she is that he's never mentioned Sarah despite the two once being as close as they are now, and that she feels like she's seeing her own future and will end up just being left behind like all his other companions. The Doctor promises he will not just leave her, explaining that he did not go back for Sarah because it would have been too hard: "I don't age. I regenerate. But humans decay, you wither and you die. You can spend the rest of your life with me. But I can't spend the rest of mine with you. I have to live on, alone. That's the curse of the Time Lords."

Mr Finch hears the phrase "Time Lords" and sends the other Krillitanes to swoop over and scare them.

The next day they all return to the school. The Doctor sends Rose and Sarah to discover what is inside the computers, and Mickey to stay in the car with K9 as surveillance — a task Mickey compares to being "sent to the back of the class with the safety scissors and glitter". The Doctor himself is going to have a word with Mr Finch. They confront each other at the swimming pool. Finch confirms he is a Krillitane named Brother Lassar and the wings are a recent addition to their form. What the Doctor sees as human is just a morphic illusion. Surprised to see a Time Lord, Lassar calls them a race of pompous, dusty senators, afraid of change and chaos and now all but extinct. He can sense that the Doctor is different but still refuses to reveal his plans, challenging him to work it out. Lassar says that they are not enemies. The Doctor quietly replies that he had much more mercy when he was younger. This is their only warning. Lassar smugly promises that the next time they meet, the Doctor will join with him.

Working on the computers, Sarah and Rose argue about who has had more experience time travelling, yelling the names of the different monsters they have met. Sarah settles it when she mentions she met the Loch Ness Monster, but they soon realise the argument is pointless and bond by comparing notes on the Doctor (Sarah is very amused to hear from Rose that the Doctor still strokes parts of the TARDIS), bursting into laughter when he enters and confusing him.

Lassar tells the other Krillitanes that they are moving to the final phase. The school will be sealed and they will become gods. Even though it is break time, the intercom calls all pupils to class and the staff to the staff room. All the pupils appear strangely happy that the break has ended early, except Kenny, who hesitates, but eventually follows the others inside. The Krillitanes begin by devouring the rest of the staff.

In the maths room, the Doctor finds the computers fixed with a deadlock seal which the sonic screwdriver cannot breach. Lassar seals all of the school's exits while Mr Wagner activates the computer program which the children begin working on again. Kenny cannot get out of the school, but he attracts Mickey's attention. Mickey tries to reactivate K9 but he doesn't respond so Mickey hits it in frustration which makes it come to life. Mickey asks K9 if it has some way to get through the locked doors in the school and it reminds him they are in a car. Mickey is initially confused by this answer but then realises what the dog means and he shouts for Kenny to hold on.

Rose, Sarah and the Doctor watch the symbols flash on a large screen. The Doctor works out that the Krillitanes are trying to solve the Skasis Paradigm. He explains that the Paradigm is the God-Maker, the Universal Theory; whoever solves it can control the building blocks of the universe — all of time, space and matter. The Krillitanes are using the children as a giant processing device and are boosting their intelligence using the oil. Rose points out that oil is on the chips which she has been eating. The Doctor then gives her a complex sum and she instantly answers it correctly, much to her shock. Sarah asks why the Krillitanes are using children instead of adults and the Doctor replies that the God-Maker needs imagination to be solved and adds "they're not just using the children's brains to break the code. They're using their souls." Lassar appears at this point, asking the Doctor to join them, tempting the Doctor with the ability to change the universe, to save everyone, even restore the Time Lords. He offers Sarah and Rose the chance to travel with the Doctor forever, never growing old. The Doctor appears tempted, but Sarah tells him that pain and loss define them as much as happiness or love. Everything has its time and everything ends, whether a world or a relationship. The Doctor picks up a chair and hurls it at the screen, smashing it. He tells Rose and Sarah to get out.

Mickey crashes the car through the front doors of the school. He and Kenny rush to the pupils. Lassar shrieks, summoning the other Krillitanes, who transform into their bat-forms. Mickey and Kenny meet up with the others and run into the cafeteria, pursued by the bat creatures. Lassar tells them he wants the Doctor alive, but to eat the others.

As the Krillitanes attack, a laser shoots one of them down. K9 appears in the doorway, blaster at the ready. The Doctor tells K9 to hold them off while they retreat. However, K9's battery is failing and Lassar tells the Krillitanes to ignore "the shooty dog thing" and get the others. In the physics lab, the Doctor realises the solution is the oil. The Krillitanes have changed their physiology so often that even their own oil is toxic to them now. The Krillitanes start bashing down the door. The Doctor tells Mickey to get the children unplugged and evacuated. Kenny triggers the fire alarm, the high-pitched sound hurting the bat-like ears of the Krillitanes and stunning them long enough to get past to the kitchens. However, this doesn't work for long. Lassar simply punches through the wall and rips out the power lead to the alarm.

Mickey runs into the computer room and yells for the children to leave but none of them respond. After a few moments he unplugs the computers which snaps the children out of their trances and they begin to leave. In the kitchens, the Doctor finds the barrels of oil are deadlock sealed. He gets the others out. He and K9 stay behind. K9 tells the Doctor that the barrels will not withstand a direct hit from his laser, but as his batteries are weak, he has to remain. The Doctor protests, knowing that K9 will be caught in the explosion, but the dog replies there is no alternative. Sadly, the Doctor bids his old friend goodbye, calls him a good dog and exits the building. The Doctor takes Sarah's hand and while she asks about K9, drags her away from the school.

Brother Lassar and his brethren enter the kitchen in human form, searching for the Doctor. Lassar mocks K9 when he sees him, but K9 shoots a barrel, spilling the toxic oil over the aliens. Lassar snarls to K9, "You bad dog"; the dog replies, "Affirmative." The explosion takes out a large chunk of the school. The pupils cheer the school's destruction and hail Kenny as the hero who did it. Sarah weeps over K9's sacrifice as the Doctor comforts her.

Later, Sarah enters the TARDIS in a park. The Doctor suggests that Sarah join them, but Sarah declines, saying it is time she found a life of her own. Mickey asks if he may join them in the TARDIS; he wants more than just to be their 'tin dog' and is ready to see the universe. Sarah says they need a Smith aboard the TARDIS, and the Doctor agrees, although Rose does not look pleased.

Before she goes, Sarah admits that she wants to stay. When Rose asks her what to do, she reassures her that some things are worth getting your heart broken for. She adds that if someday, Rose needs to, she should find her. Outside, Sarah thanks the Doctor for her time with him. The Doctor asks if there has been anyone special. Sarah tells him that there was one man whom she travelled with for a while, but he was a tough act to follow. She asks him to say goodbye this time — and he complies, saying, "Goodbye...my Sarah Jane!" He smiles at her and they hug tightly, with Sarah looking to be on the verge of tears. Sarah then watches the TARDIS disappear, but as it does so, a brand new K9 is revealed. K9 explains that the Doctor rebuilt and improved him. Happily, Sarah orders K9 home. They have work to do...

Cast

 * The Doctor - David Tennant
 * Rose Tyler - Billie Piper
 * Mickey Smith - Noel Clarke
 * Mr Finch - Anthony Head
 * Sarah Jane Smith - Elisabeth Sladen
 * Mr Parsons - Rod Arthur
 * Mr Wagner - Eugene Washington
 * Nina - Heather Cameron
 * Kenny - Joe Pickley
 * Luke - Benjamin Smith
 * Milo - Clem Tibber
 * Melissa - Lucinda Dryzek
 * Dinner Lady - Caroline Berry
 * Voice of K9 - John Leeson

British locations

 * Nina is from the children's home, Ambrose Hill.
 * Sarah Jane tells the Doctor that he left her behind in Aberdeen, Scotland, not South Croydon.

Science

 * Non-coding DNA is DNA that doesn't code for a protein.
 * Sarah Jane suggests the vacuum-packed rats are used in biology lessons. Rose indicates that the practice of dissecting rats has been out of date for a long time.
 * Finch says the Krillitanes appearing as a human is a simple morphic illusion.
 * The Skasis Paradigm is a supremely complex algorithmic sequence that if cracked, would allow one the key to manipulate time, space, and universal matter.

Technology

 * The Doctor's sonic screwdriver cannot open deadlock seals.

Diseases and illnesses

 * Nina was suffering from a headache.
 * After Mr Finch arrived at the school, half the staff seemingly got flu.

Mathematics

 * Mr Parsons tells the Doctor that he had a student who told him the height of the walls of Troy in cubits.

Newspapers

 * Sarah Jane claims to be writing an article on Mr Finch for The Sunday Times.

Individuals

 * The Doctor thought the students would be happy-slapping hoodies with ASBOs and ringtones.
 * The teacher the Doctor replaced "apparently" won the lottery.
 * As a derogatory comment on her age, Rose suggests Sarah Jane is from the Dark Ages.
 * Rose talks about her friend Shareen.
 * Rose considers K9's appearance to be "so disco", implying she thinks K9 is a dated relic from the seventies. The Doctor bristles at the remark and tells her "In the year 5000 this was cutting edge".

Vehicles

 * Rose phones for an ambulance.
 * Mickey drives a car through the school doors (after K9 repeatedly tells him, "We are in a car").
 * Sarah Jane says getting parts to fix K9 isn't like getting parts for a Mini Metro.

Planets

 * The Krillitanes invaded Bessan.

Civilisations

 * Mr Finch mentions Perganon, Ascinta and the Time Lords as civilisations the Doctor could save if the Skasis Paradigm were to be cracked.

Story notes

 * This episode notably confirms the fact that the revival series was a direct continuation of the series's original 1963-1989 run and not a reboot, a topic which was then under minimal debate in fandom.
 * This is the first appearance of a classic series companion in the revival series.
 * This episode marks the first time the revived series clearly recognised on screen that incarnations prior to the Ninth Doctor existed. There had earlier been hints, such as the Ninth Doctor's comment in Rose about his ears, his reference to kissing complete strangers in The Long Game and his reference to the Cyberman head in Dalek as "an old enemy". This episode stops short of explicitly calling the current Doctor the Tenth, hinting only that "half a dozen" regenerations had occurred since he and Sarah Jane last met, in keeping with the fact that it was the Fourth Doctor who left Sarah Jane on Earth. However, The Five Doctors indicates the last Doctor that saw Sarah Jane was actually the Fifth Doctor, though the later inclusion of the War Doctor amends this error.
 * This does not take into account Sarah's meetings with the Seventh and Eighth Doctors in the BBC novels (Bullet Time and Interference- Book One and Book Two respectively), but these can be reconciled on the assumption that later events erased these meetings from history from Sarah's perspective.
 * Mickey Smith joins the Doctor on his travels at the end of the episode.
 * According to an interview with the writer Toby Whithouse in Doctor Who Magazine #367, this episode had the working title Black Ops. The original storyline submission for the episode involved the Doctor meeting Sarah Jane while investigating events at an army base near an isolated village. The change of location to a school was suggested by Russell T Davies. Whithouse later clarified in the DVD commentary that the original specification for the script, given to an unnamed writer, had been for a school; the writer had instead submitted an episode set in an army base and admitted that writing for Doctor Who was possibly; "not for him", so Whithouse had to write a script set in the school in a hurry.
 * The K9 prop used at the end of the episode is one of the original props from the 1970s.
 * Finch's unusual turn-of-phrase "Forget the shooty dog thing" (referring to K9) strongly echoes the dialogue patterns used extensively on Joss Whedon's television series Buffy the Vampire Slayer. This is perhaps an intentional nod, as the actor who plays Finch, Anthony Head, played Rupert Giles on that series.
 * In one scene Rose and Sarah break up with laughter when the Doctor enters the room, after spending several minutes comparing notes on the Doctor. To get a realistic reaction, David Tennant wore a fake moustache (unbeknownst to the actresses) as he entered the scene. This is why Billie Piper points at him in the episode. Their reaction was caught in one take. (DCOM: School Reunion)
 * During the scene in the fish and chip shop when the Doctor and Sarah talk about her leaving him, "Love Will Tear Us Apart" by Joy Division is played in the background.
 * Although this episode marked the return of Elisabeth Sladen as Sarah Jane Smith to Doctor Who proper, Sladen had been reprising the role for some time in other media. Earlier in the same month School Reunion was first broadcast, the final episode of Big Finish Productions' Sarah Jane Smith audio drama series was released, which had been going off-and-on since 2002. Sladen also reprised the role in the 1995 independent film HOMEVID: Downtime, in two early-90s BBC Radio dramas with Jon Pertwee and had also appeared in the 1993 pastiche, Dimensions in Time.
 * Sarah Jane has a car in this episode. By The Sarah Jane Adventures, she has switched to a.
 * Phil Collinson got to voice K9 as some scenes were recorded, including the one set in the chip shop. Only John Leeson's voice was heard in the finished production, however. According to Julie Gardner, it was a moment of supreme joy for the producer.
 * Phil Collinson also took a week's vacation during the filming of this episode, forcing Gardner to take over his duties for a week, something she claimed to have adored. (DCOM: School Reunion) She would later become the effective producer on The End of Time, when credited producer Tracie Simpson had to break away from the Russell T Davies production unit to work with Matt Smith and the Steven Moffat unit. (PCOM: The End of Time, Part Two)
 * Rose complains that the Doctor leaves his companions behind after a while, but in fact, out of thirty-two companions up to this point, only four have been left behind: one (Adam Mitchell) got kicked out for bad behaviour, another one the Doctor was terrified of (Jack Harkness), his own granddaughter, whom he believed to deserve a better life and Sarah Jane Smith. All the rest died (Katarina, Sara Kingdom, Adric and Kamelion), were separated from him due to circumstances beyond his control, had their memories erased or decided to leave on their own. Still other individuals (such as Grace Holloway) were acquainted with the Doctor only for the course of a single adventure but never travelled with him as companions.
 * In Amy's Choice, the Dream Lord mocks the Doctor about how his companions never see him again after they "grow up" and that he never keeps in touch with them.
 * Although not indicated on-screen in this episode nor any episode of The Sarah Jane Adventures, the novelisation of the SJA pilot episode Invasion of the Bane revealed that the Doctor left Sarah Jane a sonic lipstick and other items in a secret compartment within K9. On-screen, the sonic lipstick debuts in the Invasion of the Bane episode, but no explanation is offered as to where it came from. It appeared for the first time on Doctor Who during Journey's End.
 * A slow instrumental of Song for Ten can be heard playing throughout the episode, most notably when the Doctor says goodbye to Sarah Jane.
 * The Doctor tells Sarah Jane he's regenerated six times since they last met. Going by televised stories, this would go back to The Five Doctors. However, as the Tenth Doctor keeps the War Doctor buried in his memories, it is purposely misleading to the viewers that he means The Hand of Fear.
 * Originally the story was entitled "Black Ops" and involved alien activity in a rural village and a nearby army base.

Ratings

 * 8.31 million viewers (39.8% of television share; UK final)

Filming locations

 * The primary location for the school filming was Duffryn High School in Newport.
 * Fitzalan High School, Leckwith, Cardiff
 * Belle Vue car park and DaVinci's coffee shop, Newport
 * Studio: Unit Q2, Newport

Production errors

 * When Mickey takes the plug out, the children's headphones come off by themselves.
 * At the start of the episode, the door of the headmaster's office is blue and has a window in it. However, when the Doctor and the others return to the school at night, and investigate the headmaster's office, the door is larger and has no window.

Continuity

 * When Sarah and Rose argue they refer to a lot of encounters with aliens. Sarah refers to meeting mummies, (TV: Pyramids of Mars) "lots of robots", (TV: The Time Warrior, Robot, The Sontaran Experiment, Revenge of the Cybermen, The Android Invasion, The Five Doctors) Daleks, (TV: Death to the Daleks, Genesis of the Daleks) anti-matter monsters, (TV: Planet of Evil) dinosaurs (TV: Invasion of the Dinosaurs) and the Loch Ness Monster. (TV: Terror of the Zygons)
 * Rose refers to meeting ghosts, (TV: The Unquiet Dead) Slitheen in Downing Street, (TV: Aliens of London/ World War Three) the Dalek Emperor, (TV: The Parting of the Ways) gas mask zombies (TV: The Empty Child) and a werewolf. (TV: Tooth and Claw)
 * Sarah Jane would later recall Rose's words when faced with the Slitheen at Park Vale Comprehensive School. (TV: Revenge of the Slitheen)
 * The Fourth Doctor left Sarah on Earth, thinking he had dropped her home in South Croydon in London. Sarah quickly realised at the time that the Doctor had "goofed". (TV: The Hand of Fear) She tells him now that he had dropped off in Aberdeen.
 * Sarah Jane discusses her departure from the Fourth Doctor in TV: The Hand of Fear, and how she was only left behind because of his summons to Gallifrey, back when humans were prohibited from entering the planet.
 * Strangely, the discussion seems to ignore the events of The Five Doctors, which marked the latest meeting between Sarah and the Fifth Doctor, though this could be attributed to the large number of overlapping points in the Doctor's timeline.
 * This discussion ignores Sarah's meetings with the Seventh and Eighth Doctors in the novels Bullet Time and Interference- Book One and Book Two, but from a continuity perspective this can be explained as it is suggested in later novels The Ancestor Cell and Sometime Never... that certain parties involved in these events are erased from history.
 * The Doctor tells Sarah that he has regenerated "half a dozen times since we last met"; this statement also seems to exclude these meetings
 * Considering the later revelation of the existence of the War Doctor, the idea that it has been half a dozen regenerations since the Doctor and Sarah met could be a reference to her meeting with the Fifth Doctor, with the Doctor not referencing his encounters in his seventh and eighth incarnations as they were essentially erased from history.
 * At some point after their parting, the Doctor left K9 Mark III for Sarah. (TV: A Girl's Best Friend, The Five Doctors)
 * The Doctor says, "Correct-a-mundo", adding, "A phrase I have never used before, and will hopefully never use again." He once used the phrase, "Ace-a-mundo", and makes an almost identical comment on it. (PROSE: The Feast of the Drowned) Similarly, the newly-regenerated Eleventh Doctor proclaimed "Who 'da man!?" and was met with blank stares, which made him resolve to never use that phrase again. (TV: The Eleventh Hour)
 * Sarah remembered that "Christmas just gone" there was a spaceship "overhead" and she thought of the Doctor that day. The Doctor mentions that he and Rose were "right on top of it". (TV: The Christmas Invasion)
 * The Ninth Doctor had previously mentioned to Rose that he had "travelled with a lot of people". (TV: The Doctor Dances)
 * K9 has been called a tin dog twice before, by the Time Lord Drax (TV: The Armageddon Factor) and Lady Adrasta (TV: The Creature from the Pit)
 * The Doctor tells Sarah that humans previously weren't allowed on Gallifrey. (TV: The War Games, The Invasion of Time)
 * Sarah Jane says that repairing K9 is "not like getting parts for a Mini Metro", a car she previously had. (TV: A Girl's Best Friend)
 * Rose recalls going to the year 5000000000. (TV: The End of the World)
 * Sarah Jane says 'Home' and 'We've got work to do', mirroring the Seventh Doctor and Ace's conversation. (TV: Survival) However, it may have simply been a coincidence, as Sarah is referring to 13 Bannerman Road, whereas Ace was referring to the TARDIS.
 * The Eighth Doctor previously revealed the existence of a K9 Mark IV to Sarah Jane in 1996. (PROSE: Interference - Book Two)
 * Rani Chandra heard about the events in Deffry Vale High School in the press. (WC: Alien File: The Doctor)
 * The Doctor has previously been offered another great power under the guise that it could be used for good, only to turn it down, believing he should not have it. (TV: Colony in Space)

Home video releases

 * This episode was released as a "vanilla" DVD along with Tooth and Claw and The Girl in the Fireplace.
 * It was also released as part of the Series 2 DVD boxed set.
 * On a single disk by itself, it was given away with the purchase of a newspaper, as part of a promotion by The Sun.
 * This was also released with issue 9 of the Doctor Who DVD Files.