Sarah's last line in the episode to K9 is "We've got work to do." This was also the last line of the original series run, at the conclusion of the Seventh Doctor story Survival.
When the new K-9 MKIV is revealed from behind the TARDIS it differs slightly from its predecessor, most notably the ears, which the Doctor simplified by removing the "antennae" from the center of each ear.
From the moment we first see K-9 until Mickey drives Sarah's car into the school the panel on K-9's right side is missing. K-9 comes into the gym to fight the Krillitanes and the missing panel is now in place. How did it get there?
When Mickey is talking to K-9 in Sarah's car look closely at K-9's face. The small cup that is on the tip of the probe is missing.
When Sarah Jane notes that the Doctor has regenerated, he says that he has done so "half-a-dozen times". The Doctor has regenerated five times, not six, since he and Sarah last met in The Five Doctors, when he appeared in his fifth incarnation (played by Peter Davison). Of course, the Doctor spent very little time with her in this incarnation, and may have overlooked the meeting.
When the kitchen staff are wheeling the barrel of oil into the kitchen they are concerned and very worried about spilling any of it. When it does spill(of course) one of the staff gets some oil on her and is screaming in pain. If the oil is so dangerous to them why was the cap on the barrel off during the move?
When everyone is under attack in the gym K-9 comes to the rescue. The last time he was seen he the in the back of Sarah's car. Since nobody lifted him out of the car how was he able to get to the gym?
Why did Sarah Jane have K-9 in the back of her car? He doesn't work anymore, she states that she couldn't get him fixed and she wasn't bringing K-9 to the Doctor for repairs as she didn't even know he was the Doctor until she was already at the school.
In both this episode and The Hand of Fear (Sarah Jane's final story in the original series), Sarah Jane's exit scene features her having a conversation with a dog. In The Hand of Fear after saying good-bye to the Doctor, Sarah shares some words with a golden retriever she finds sitting on the sidewalk before walking home. In School Reunion after saying good-bye to the Doctor, she discovers and has a chat with K-9 before departing.
Sarah-Jane and Rose's catfight was inspired by Sex in the City, and in the scene where they laugh at the doctor, James Hawes had painted a surprise moustache on David Tennant's face.
K-9 MK III's apparent 'death' adds him to the list of dead companions, following on from Katarina, Sara Kingdom (both in The Daleks' Master Plan), Adric (Earthshock), Kamelion (Planet of Fire) and Captain Jack (The Parting of the Ways). The latter, however, was returned to life by Rose when possessed by the time vortex.
Though by the end of this episode there have been four versions of K9 in the Who story, in the real world, the first 3 versions, at least in the old series, were physically the same prop. First seen in 1977 in The Invisible Enemy, K9 Mark I, voiced by John Leeson, left with Leela in The Invasion of Time in 1978, only for The Doctor to immediately reveal a box with K9 MII written on it under the TARDIS console. This was done because the K9 prop had been fitted with new motors and producers were worried viewers might notice a difference. In season 17 K9 developed laryngitis in Destiny of the Daleks and was voiced (uncredited) by Roy Skelton (Dalek voice). Sidelined for 2 stories K9 resumed the rest of season 3 with David Brierley providing the voice for the rest of the season. At the beginning of season 18 in The Leisure Hive K9 explodes after chasing a ball into the sea and the repaired version sounds like John Leeson again, until he left with Romana in E-Space in Warriors Gate. K9 MkIII was sent to Sarah Jane Smith at some point unknown in the old series, for the standalone story K9 and Company, also voiced by John Leeson. K9 MkIII's first appearence in Doctor Who was in this episode, as was K9 MkIV.
Mickey drives Sarah-Jane's car through the doors to get inside. However, when the children are escaping the school, the doors are intact, and there is no sign of the car. The kids couldn't have escaped another way because the school was locked down.
Mickey is accepted as one of the TARDIS crew this episode. Sarah-Jane tells the Doctor that he needs at least one Smith on board, but that it won't be her.
K-9 Mark III is destroyed this episode, but is rebuilt by the Doctor as K-9 Mark IV.
In this episode, Mickey says to Rose that he can't get pictures of the UFO's because the website keeps locking him out. On the screen, it says "Torchwood: Access Denied".
This is the fourth reference to Torchwood, The first being in Bad Wolf, the second in The Christmas Invasion, and the third as Queen Victoria sets up "The Torchwood Institute" in Tooth and Claw.
When Rose and Sarah Jane discuss what they've seen with the Doctor, lots of previous episodes are referenced. These include Aliens Of London and World War Three (Slitheen in Downing Street), The Parting Of The Ways, (Emperor Dalek), The Empty Child and The Doctor Dances (gas-mask zombies), Tooth And Claw (real-life werewolf), Pyramids Of Mars (mummies), Robot and The Sontaran Experiment (robots), The Android Invasion (lots of robots), Genesis Of The Daleks (daleks), Invasion Of The Dinosaurs (real-life dinosaurs) and Terror Of The Zygons (the Loch Ness monster).
The Doctor: Sorry I didn't get a chance to ask. You haven't... There's hasn't been anyone... You know...
Sarah Jane Smith: Well... there was this one guy. I travelled with him for a while. But he was a tough act to follow. (sadly) Good-bye, Doctor.
The Doctor: Oh, it's not good-bye-
Sarah Jane Smith: (tearing up) Please, say it. Please. Say it. This time. Say it.
The Doctor: Good-bye... My Sarah Jane.
Mr. Finch: Do try the chips.
Rose: Oh my God they're rats, dozens of rats, vacuum packed rats.
The Doctor: And you decided to scream.
Mickey: It took me by surprise.
The Doctor: Like a little girl.
Mickey: It was dark, I was covered in rats.
The Doctor: Nine, maybe ten years old, I'm seeing pigtails, frilly skirt...
Sarah Jane Smith: He replaced you with a brand new model.
Sarah Jane Smith: Yup, he does that.
Mr. Parsons: Finch arrived three months ago, next day half the staff got flu. Finch replaced them with that lot. Except for the teacher you replaced, and that was just plain weird, her winning the lottery like that.
The Doctor: How's that weird?
Mr. Parsons: She never played. Said the ticket was posted through her door at midnight.
The Doctor: (Looking shifty) Hmm. The world is very strange.
The Doctor: H-alright team. Oh, I couldn't say team.. Um.. gang.. Um.. com-rades... uh... anyways...
The Doctor: Correctomundo! A word.. I have never used before and hopefully, never will again.
Mr Finch: (to fellow Krillitanes) Forget the shooty dog-thing!
Rose: What do I do? Do I stay with him?
Sarah Jane Smith: Yes. Some things are worth getting your heart broken for.
Mr Finch: The paradigm gives us power but you could give us wisdom. Become a God at my side. Imagine what you could do. Think of the civilisations you could save. Perganon. Acinta. Your own people, Doctor, standing tall. The Time Lords. Reborn.
Sarah Jane Smith: Doctor, don't listen to him!
Mr Finch: And you could be with him throughout eternity. Young, fresh. Never wither. Never age. Never die. Their lives are so fleeting. So many goodbyes. How lonely you must be, Doctor.
Mr. Finch: And what of the Time Lords? I always thought of you as such a pompous race. Ancient, dusty senators, so frightened of change and chaos. And of course, they're all but extinct. Only you. The last.
The Doctor: This plan of yours. What is it?
Mr. Finch: You don't know?
The Doctor: That's why I'm asking.
Mr. Finch: Well, show me how clever you are. Work it out.
The Doctor: If I don't like it, then it will stop.
Mr. Finch: Fascinating. Your people were peaceful to the point of indolence. You seem to be something new. Would you declare war on us, Doctor?
The Doctor: I'm so old now. I used to have so much mercy. You get one warning. That was it.
Rose: I've been to the year 5 billion, right, but this... no, this is really seeing the future. You just leave us behind. Is that what you're going to do to me?
The Doctor: No. Not to you.
Rose: But Sarah Jane... you were that close to her once and now... you never even mention her. Why not?
The Doctor: I don't age. I regenerate. But humans decay. You wither and you die. Imagine watching that happen to someone you...
Rose: What, Doctor?
The Doctor: 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.
Sarah Jane Smith: You were my life. Do you know what the most difficult thing was? Coping with what happens next. Or what doesn't happen next. You took me to the furthest reaches of the galaxy, you showed me supernovas, intergalactic battles and then you just dropped me back on Earth. How could anything compare to that?
(after Rose and Sarah Jane meet)
Mickey: (to the Doctor) Ah, mate, the missus and the ex. Welcome to every man's worst nightmare.
Sarah Jane Smith: (after meeting Rose; to the Doctor) You can tell you're getting older, your assistants are getting younger.
Rose: I'm not his assistant.
Sarah Jane Smith: No? (to the Doctor) Get you, tiger.
Rose: Nevermind the tin dog. We're busy.
Girl: Did you have something to do with it?
Kenny: Yeah, I did.
Girl: Oh my god. (shouts to everyone) Kenny blew up the school! It was Kenny!
Sarah Jane Smith: No. The Universe has to move forward. Pain and loss, they define us as much as happiness or love. Whether it's a world or a relationship. Everything has its time and everything ends.
Mr. Finch: Little dog with a nasty bite. Not so powerful now, are you?
(K-9 shoots a laser at a barrel, and it explodes)
Mr.Finch: You bad dog.
Sarah Jane Smith: I had no problem with space stuff. I saw things you wouldn't believe!
Rose: Try me.
Sarah Jane Smith: Mummies.
Rose: I've met ghosts.
Sarah Jane Smith: Robots. Lots of robots.
Rose: Slitheen, in Downing Street.
Sarah Jane Smith: Daleks!
Rose: Met the Emperor!
Sarah Jane Smith: Anti-matter monsters!
Rose: Gas masked zombies!
Sarah Jane Smith: Real. Living. Dinosaurs!
Rose: Real. Living. Werewolf!
Sarah Jane Smith: The Loch Ness Monster!
Rose: Why does it looked so..disco?
The Doctor: Oi! Listen, in the year 5000 this was cutting edge!
The Doctor: (seeing K9) K9! Rose Tyler, Mickey Smith. Allow me to introduce K9. Well, K9 Mk.3 to be precise.
Sarah Jane Smith: Hello.
The Doctor: Oh, I should think so.
Sarah Jane Smith: And you are?
The Doctor: uh.Ar..Smith..John Smith.
Mickey: So what's the deal with the tin dog?
Sarah Jane Smith: The Doctor likes travelling with an entourage. Sometimes they're human, sometimes they're aliens and sometimes they're tin dogs. (Mickey grins) What about you? Where do you fit in the picture?
Mickey: Me? I'm their man in a van. I'm their technical support. I'm...(realises) Oh my god. I'm the tin dog!
Turkey: July 25, 2010 on CNBC-e
During a run through of the scene where Mickey is in the car and switches K9 on, Noel Clarke broke one of K9's ears and it had to be fixed before the final scene.
In the commentary for this episode, Helen Raynor mentions that Finch's first name, not stated in the episode, is Hector. However on the Deffry Vale School tie-in website his name is Lucas. The reason for the name change was that it was learned that there was a real teacher by the name of Hector Finch.
The appearance of K-9 Mark IV at the story's end was not included in the early drafts of the script. The introduction of a rebuilt version of the character was requested by K-9's co-creator, Bob Baker.
A working title for this episode was Old Friends.
This episode was nominated for a 2007 Hugo Award in the "Best Dramatic Presentation, Short Form" Category, but lost out to another Doctor Who episode: The Girl in the Fireplace.
The Krillitane's bat-like appearance coincides with Buffy star Anthony Stewart Head's appearance. Featuring Buffy actors in episodes with a vampiric motif is becoming a common theme in the 21st century (such as Amber Benson in Supernatural's "Bloodlust" and James Marsters in Smallville's "Thirst"). Also Mercedes McNab as the Vampire Lucy in Supernatural's "Fresh Blood".
The music playing in the café when Mickey is teasing Rose about The Doctor and Sarah Jane is Love will tear us apart by Joy Division.
On the Deffry Vale website it states that 'Mr Smith' previously taught Physics in Shoreditch, the location of Coal Hill School from the very first Doctor Who episode, An Unearthly Child. Presumably this is a cover story, as none of the Doctor's incarnations were ever seen on screen being employed, or even teaching there.
As Wagner recruits students for his advanced class, he mentions that Milo has failed him. According to the commentary, a scene cut from the broadcast showed Milo falling over on his desk, his brain being shorted out, explaining this scene.
When the newly repaired K9 appears at the end of the episode, the producers used the original K9 model from the Tom Baker episodes, which they tracked down and borrowed from its owner.
The success of Elisabeth Sladen's appearance in this episode led to the commissioning of The Sarah Jane Adventures, a spin-off show featuring the intrepid reporter.
The final viewing figure for the BBC One airing of this episode was 8.31 million.
K-9 was not given to Sarah Jane in the regular run of the series but, instead, in the rejected pilot for a spin-off series, K-9 And Company (1981). K-9 made his debut in The Invisible Enemy (1977), a year after Sarah had left the TARDIS crew. Sarah and K-9's first appearance alongside the Doctor was The Five Doctors (1983), a special anniversary story that fell between seasons 20 and 21 but is not considered to be a part of either. They did not appear again until School Reunion (2006).
Sarah Jane mentions that the Doctor has "regenerated". She accompanied the Doctor in his third incarnation (played by Jon Pertwee), was present when he regenerated into his fourth (played by Tom Baker) and met the first, second and fifth (Richard Hurndall, Patrick Troughton and Peter Davison). This is the first evidence on screen that the new series started with a ninth Doctor, rather than a rebooted character, since no regeneration scene was shown. It was originally intended to hint at his previous lives in The End of the World with a DNA scan showing nine separate strands, but this scan never made it on screen in the episode. It can be seen in artist concept drawings on the BBC website.
Anthony Head (The Headmaster) once auditioned for the part of the Eighth Doctor, but didn't get it.
This episode features the first appearance of characters from the original series. Sarah Jane Smith (Elisabeth Sladen) and K-9 (John Leeson) were last seen in The Five Doctors.
Mickey: Me? I'm their Man in Havana.
Our Man In Havana is a novel by Graham Greene about an unassuming vacuum cleaner salesman in Havana with an expensive daughter, who is recruited by a British spy to collect information of political or military importance in Havana and pass it along to British intelligence.
The Doctor: Good morning, class. Are we sitting comfortably?
This is derived from the catchphrase, "Are you sitting comfortably? Then I'll begin," popularised by the 1950-1982 BBC Radio children's series 'Listen with Mother', which began every episode with those words. It was later used again by The Wire in 'The Idiot's Lantern'.
Sarah Jane: Everything has its time. And everything ends.
It seems that Sarah Jane and The Doctor still think alike as her speech to stop him falling under Finch's spell echoes the sentiments he expressed to Rose about Lady Cassandra in 'The End of the World', when he said 'Everything has its time and everything dies.'
Sarah Jane: Well, it's not like getting parts for a mini-metro!
The Mini Metro is the model of car that Sarah Jane Smith drove when last seen in the Doctor Who spin off K-9 and Company.
The Doctor: Correct-a-mundo. A word I have never used before and, hopefully, never will again.
Correct-a-mundo was a catchword used by the character Arthur 'Fonzie' Fonzarelli on the US television series 'Happy Days'.
When Rose and Sarah Jane are arguing over who saw more than the other, during their respective stints with the Doctor, the latter one-ups the former with a shout of:
"Loch Ness Monster!"
This alludes to a 4th Doctor story arc called "Terror of the Zygons," in which Nessie was a giant reptile genetically engineered by some alien octopus men (whose talent for mimicry had evolved from colour-alteration to shapeshifting), stranded beneath the lake.
It was later suggested, however (during the 6th Doctor/Colin Baker era), that Nessie might actually be an alien dictator-in-exile named Borad of Karfel! Apparently, some experiment went wrong, and he was mutated into a half-lizard humanoid, before his eventual overthrow.
The Doctor: Oh, I should think so.
Sarah: And you are?
The Doctor: uh.Ar..Smith..John Smith.
This is The Doctor deliberately alluding on screen to an alias he used regularly in his time as the third Doctor, including the point at which he met Sarah for the first time. Even though Sarah recognises the name and ties it back to The Doctor, the reason for the allusion is lost on her at this point. She doesn't realise he is The Doctor until she encounters the TARDIS.