[Outside the Headmaster's office]

(A man comes down the stairs of Deffry Vale High School and walks to the Headmaster's office. A girl is sitting on a chair nearby.)

FINCH: What do you want?

NINA: The nurse sent me, sir. I was in English and I got a headache.

FINCH: Then don't bother me, go home.

NINA: I can't.

FINCH: Why, is your mother at work?

NINA: I live in Ambrose Hall. The children's home.

FINCH: No parents. No one to miss you. I see why the nurse sent you. You poor child. Poor thin, child. Come inside.

(He opens the door and the girl walks through.)

FINCH: It's nearly time for lunch.

(The door closes behind them. There is a screech and flapping of wings, and a scream.)

[Physics laboratory]

(Up on the first floor, there is a change of class. The teacher enters, wearing a brown suit and white sneakers.)

DOCTOR: Good morning, class. Are we sitting comfortably?

(The Doctor writes on the board.)

DOCTOR: So, physics. Physics, eh? Physics. Physics. Physics! Physics. Physics, physics, physics, physics, physics, physics, physics. I hope one of you is getting all this down. Okay let's see what you know. Two identical strips of nylon are charged with static electricity and hung from a string so they can swing freely. What would happen if they were brought near each other?

(A young boy with spectacles puts his hand up.)

DOCTOR: Yes, er, what's your name?

MILO: Milo.

DOCTOR: Milo! Off you go.

MILO: They'd repel each other because they have the same charge.

DOCTOR: Correctamundo! A word I have never used before and hopefully never will again. Question two. I coil up a thin piece of microwire and place it in a glass of water. Then I turn on the electricity and measure to see if the water's temperature is affected. My question is this. How do I measure the electrical power going into the coil?

(Just one hand goes up. Everyone else looks totally bored.)

DOCTOR: Someone else. No? Okay, Milo, go for it.

MILO: Measure the current and PDs in an ammeter and a voltmeter.

DOCTOR: Two to Milo. Right then, Milo, tell me this. True or false. The greater the dampening of the system, the quicker it loses energy to its surroundings.

MILO: False.

DOCTOR: What is non-coding DNA?

MILO: DNA that doesn't code for a protein.

DOCTOR: Sixty five thousand nine hundred and eighty three times five?

MILO: Three hundred and twenty nine thousand nine hundred and fifteen.

DOCTOR: How do you travel faster than light?

MILO: By opening a quantum tunnel with an FTL factor of thirty six point seven recurring.

(The Doctor's jaw drops.)

[Canteen]

(Chips are still on the menu here. A young blonde slops mash into the Doctor's tray. He walks away with a grin. A little later she goes over to wipe his table.)

ROSE: Two days.

DOCTOR: Sorry, could you just? There's a bit of gravy. No, no, just, just there.

ROSE: Two days, we've been here.

DOCTOR: Blame your boyfriend. He's the one who put us onto this. And he was right. Boy in class this morning, got a knowledge way beyond planet Earth.

ROSE: You eating those chips?

DOCTOR: Yeah, they're a bit different.

ROSE: I think they're gorgeous. Wish I had school dinners like this.

DOCTOR: It's very well behaved, this place.

ROSE: Mmm.

DOCTOR: I thought there'd be happy slapping hoodies. Happy slapping hoodies with ASBOs. Happy slapping hoodies with ASBOs and ringtones. Huh? Huh? Oh, yeah. Don't tell me I don't fit in.

(The head dinner lady comes over.)

JACKSON: You are not permitted to leave your station during a sitting.

ROSE: I was just talking to this teacher.

DOCTOR: Hello.

ROSE: He doesn't like the chips.

JACKSON: The menu has been specifically designed by the headmaster to improve concentration and performance. Now, get back to work.

ROSE: See? This is me. Dinner lady.

DOCTOR: I'll have the crumble.

ROSE: I'm so going to kill you.

(A dark teacher walks over to a girl with a pony tail.)

WAGNER: Melissa. You'll be joining my class for the next period. Milo's failed me, so it's time we moved you up to the top class. Kenny, not eating the chips?

(Tubby Kenny is eating a home prepared lunch from a Tupperware box.)

KENNY: I'm not allowed.

WAGNER: Luke. Extra class. Now.

(Luke and Melissa follow Mister Wagner out. Headmaster Finch stands on a balcony overlooking the canteen and watches it all.)

[Kitchen]

(Rose is drying up when other dinner ladies wheel a large cooking oil drum through. It has lots of strange symbols on the side. They are wearing breathing masks and heavy duty protective gauntlets.)

JACKSON: Careful. Keep it steady. Don't spill a drop. I said, keep it steady. Careful. That's it. Easy now. Steady.

(Rose's phone rings.)

JACKSON: Right. Second barrel. Quickly now!

ROSE: What you got?

MICKEY [OC]: Confirmation.

[Library]

(Mickey is using the internet somewhere with books in the background.)

MICKEY: I just got into army records. Three months ago, massive UFO activity.

[Kitchen]

MICKEY [OC]: They logged over forty sightings. Lights in the sky, all of that.

[Library]

MICKEY: I can't get any photos, because then it gets all classified and secret. Keeps locking me out.

(With the message - Torchwood Access Denied)

ROSE [OC]: Tell you what, though.

[Kitchen]

ROSE: Three months ago, turns out all the kitchen staff were replaced. And this lot are weird.

[Library]

MICKEY: See? There's definitely something going on.

[Kitchen]

MICKEY [OC]: I was right to call you home.

ROSE: I thought maybe you called me home just, well, just to call me home.

MICKEY: Do you think I'd just invent an emergency?

[Kitchen]

ROSE: You could've done.

MICKEY [OC]: That's the last thing I'd do.

JACKSON: Watch it!

[Library]

MICKEY: Because every time I see you

[Kitchen]

MICKEY [OC]: An emergency just gets in the way.

(The next barrel of oil topples over. One of the women gets splashed and starts screaming.)

ROSE: I've got to go.

JACKSON: Get her up, get her up!

[Library]

MICKEY: Rose, what is it?

[Kitchen]

(The injured woman is hustled into the office and the blinds are pulled down. Rose redials.)

JACKSON: What're you doing?

ROSE: Calling an ambulance.

JACKSON: No need. She's quite all right.

(There is a whumph! like a sudden fire, and a scream.)

JACKSON: It's fine. She does that.

(The woman goes back into the smoke filled office. Rose sees that the spilt oil has eaten through metal.)

[IT classroom]

WAGNER: I'd like you all to put your headphones on now, please. Now, children, the things you will see.

(Symbols and equations scroll down on the slaved computers and the children start typing faster and faster.)

[Staircase]

(Finch is talking to a not quite so young woman investigative reporter, who hasn't changed her hairstyle in thirty years.)

FINCH: Our work here. My improvements aren't confined to the classroom. Oh, no, no, no, no, no. We've introduced a new policy. School dinners are absolutely free, but compulsory. Do try the chips.

SARAH: Oh, I'd love to. Thank you. And it's got to be said, the transformation you've brought about is amazing. I mean, maybe you're working the children a little bit too hard now and then, but I think good results, they're more important than anything.

FINCH: Exactly. You're a woman of vision, Miss Smith.

SARAH: Oh, I can see everything, Mister Finch. Quite clearly.

[Staff room]

PARSONS: Yesterday, I had a twelve year old girl give me the exact height of the Walls of Troy in cubits.

DOCTOR: And, it's ever since the new headmaster arrived?

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.

DOCTOR: How's that weird?

PARSONS: She never played. Said the ticket was posted through her door at midnight.

DOCTOR: Hmm. The world is very strange.

(The Headmaster enters.)

FINCH: Excuse me, colleagues. A moment of your time. May I introduce Miss Sarah Jane Smith. Miss Smith is a journalist who's writing a profile about me for the Sunday Times. I thought it might be useful for her to get a view from the trenches, so to speak. Don't spare my blushes.

(Finch leaves.)

SARAH: Hello.

DOCTOR: Oh, I should think so.

SARAH: And, you are?

DOCTOR: Hm? Er, Smith. John Smith.

SARAH: John Smith. I used to have a friend who sometimes went by that name.

DOCTOR: Well, it's a very common name.

SARAH: He was a very uncommon man. Nice to meet you.

DOCTOR: Nice to meet you. Yes, very nice. More than nice. Brilliant.

SARAH: Er, so, er, have you worked here long?

DOCTOR: No. Er, it's only my second day.

SARAH: Oh, you're new, then. So, what do you think of the school? I mean, this new curriculum? So many children getting ill. Doesn't that strike you as odd?

DOCTOR: You don't sound like someone just doing a profile.

SARAH: Well, no harm in a little investigation while I'm here.

DOCTOR: No. Good for you.

(Sarah moves away from the scary grinning Doctor.)

DOCTOR: Good for you. Oh, good for you, Sarah Jane Smith.

[IT classroom]

(The end of day bell rings. Kenny walks into the now dark room to investigate a strange sort of eating sound. A nasty large set of teeth snap at him from behind a row of computer screens, then the teacher stands up.)

WAGNER: This isn't your classroom, Kenny. Now run along.

[Corridor]

(After dark, Sarah Jane Smith breaks into the school.)

ROSE: Oh, it's weird seeing school at night. It just feels wrong. When I was a kid, I used to think all the teachers slept in school.

DOCTOR: All right, team. Oh, I hate people who say team. Er, gang. Er, comrades. Anyway, Rose, go to the kitchen. Get a sample of that oil. Mickey, the new staff are all Maths teachers. Go and check out the Maths department. I'm going to look in Finch's office. Be back here in ten minutes.

(The Doctor leaves.)

ROSE: You going to be all right?

MICKEY: Me? Please. Infiltration and investigation? I'm an expert at this.

(Mickey leaves, then comes back.)

MICKEY: Where's the Maths department?

ROSE: Down there, turn left, through the fire doors, on the right.

MICKEY: Thank you.

(Sarah Jane chases something flying in an upper corridor. Rose gets the oil sample as instructed, then looks up as something screeches overhead.)

[Corridor]

(Sarah Jane opens a storeroom door to discover the Tardis parked inside.)

DOCTOR: Hello, Sarah Jane.

SARAH: It's you. Oh, Doctor Oh, my God, it's you, isn't it. You've regenerated.

DOCTOR: Yeah. Half a dozen times since we last met.

SARAH: You look incredible.

DOCTOR: So do you.

SARAH: Huh. I got old. What are you doing here?

DOCTOR: Well, UFO sighting, school gets record results. I couldn't resist. What about you?

SARAH: The same. I thought you'd died. I waited for you and you didn't come back, and I thought you must have died.

DOCTOR: I lived. Everyone else died.

SARAH: What do you mean?

DOCTOR: Everyone died, Sarah.

SARAH: I can't believe it's you.

(Mickey screams.)

SARAH: Okay, now I can!

(Rose runs up.)

ROSE: Did you hear that? Who's she?

DOCTOR: Rose, Sarah Jane. Sarah Jane, Rose.

SARAH: Hi. Nice to meet you. You can tell you're getting older. Your assistants are getting younger.

ROSE: I'm not his assistant.

SARAH: No? Get you, tiger.

[Classroom]

MICKEY: Sorry! Sorry, it was only me. You told me to investigate, so I started looking through some of these cupboards and all of these fell on me.

ROSE: Oh, my God, they're rats. Dozens of rats. Vacuum packed rats.

DOCTOR: And you decided to scream.

MICKEY: It took me by surprise!

DOCTOR: Like a little girl?

MICKEY: It was dark! I was covered in rats!

DOCTOR: Nine, maybe ten years old. I'm seeing pigtails, frilly skirt.

ROSE: Hello, can we focus? Does anyone notice anything strange about this? Rats in school?

SARAH: Well, obviously they use them in Biology lessons. They dissect them. Or maybe you haven't reached that bit yet. How old are you?

ROSE: Excuse me, no one dissects rats in school anymore. They haven't done that for years. Where are you from, the dark ages?

DOCTOR: Anyway, moving on. Everything started when Mister Finch arrived. We should go and check his office.

[Corridor]

ROSE: I don't mean to be rude or anything, but who exactly are you?

SARAH: Sarah Jane Smith. I used to travel with the Doctor.

ROSE: Oh. Well, he's never mentioned you.

DOCTOR: Oh, I must've done. Sarah Jane. Mention her all the time.

ROSE: Hold on. Sorry. Never.

SARAH: What, not even once? He didn't mention me even once?

MICKEY: Ho, ho, mate. The missus and the ex. Welcome to every man's worst nightmare.

[Outside the Headmaster's office]

(The Doctor uses the sonic screwdriver on the lock.)

DOCTOR: Maybe those rats were food.

ROSE: Food for what?

[Headmaster's office]

DOCTOR: Rose, you know you used to think all the teachers slept in the school? Well, they do.

(Giant bats are hanging from the ceiling.)

MICKEY: No way!

(Mickey runs. The others follow more sedately. One wakes when the Doctor shuts the door.)

[School yard]

MICKEY: I am not going back in there. No way.

ROSE: Those were teachers.

DOCTOR: When Finch arrived, he brought with him seven new teachers, four dinner ladies and a nurse. Thirteen. Thirteen big bat people. Come on.

MICKEY: Come on? You've got to be kidding!

DOCTOR: I need the Tardis. I've got to analyse that oil from the kitchen.

SARAH: I might be able to help you there. I've got something to show you.

[Car park]

(In the boot of Sarah's car is - )

DOCTOR: K9! Rose Tyler, Mickey Smith, allow me to introduce K9. well, K9 Mark Three to be precise.

ROSE: Why does he look so disco?

DOCTOR: Oi! Listen, in the year five thousand, this was cutting edge. What's happened to him?

SARAH: Oh, one day, he just, nothing.

DOCTOR: Well, didn't you try and get him repaired?

SARAH: Well, it's not like getting parts for a Mini Metro, Beside, the technology inside him could rewrite human science. I couldn't show him to anyone.

DOCTOR: Ooh, what's the nasty lady done to you, eh?

(K9 is getting rusty round the edges. Something is watching them from midair.)

ROSE: Look, no offence, but could you two just stop petting for a minute? Never mind the tin dog. We're busy.

(A giant alien bat creature flies across the face of the full moon.)

[Coffee shop]

(Rose and Mickey are at the counter while the Doctor and Sarah Jane have put the defunct K9 on a table. The musak is Love will tear us apart by Joy Division.)

MICKEY: You see, what's impressive is that it's been nearly an hour since we met her and I still haven't said I told you so.

ROSE: I'm not listening to this.

MICKEY: Although, I have prepared a little I was right dance that I can show you later.

WOMAN: Two quid, love.

(Rose pays for a portion of chips.)

MICKEY: All this time you've been giving it, he's different, when the truth is, he's just like any other bloke.

ROSE: You don't know what you're talking about.

MICKEY: Maybe not. But if I were you I'd go easy on the chips.

[Street]

(Finch is standing on the roof of the building opposite the coffee shop, watching the Doctor working on K9.)

FINCH: Come to me. Come to me.

(A giant bat flies over to him.)

[Coffee shop]

SARAH: I thought of you on Christmas Day. This Christmas just gone? Great big spaceship overhead. I thought, oh yeah, bet he's up there.

DOCTOR: Right on top of it, yeah.

SARAH: And Rose?

DOCTOR: She was there too.

SARAH: Did I do something wrong, because you never came back for me. You just dumped me.

DOCTOR: I told you. I was called back home and in those days humans weren't allowed.

SARAH: I waited for you. I missed you.

DOCTOR: Oh, you didn't need me. You were getting on with your life.

SARAH: You were my life. You know what the most difficult thing was? Coping with what happens next, or with 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?

DOCTOR: All those things you saw, do you want me to apologise for that?

SARAH: No, but we get a taste of that splendour and then we have to go back.

DOCTOR: Look at you, you're investigating. You found that school. You're doing what we always did.

SARAH: You could have come back.

DOCTOR: I couldn't.

SARAH: Why not?

(The Doctor keeps working on K9.)

SARAH: It wasn't Croydon. Where you dropped me off, that wasn't Croydon.

DOCTOR: Where was it?

SARAH: Aberdeen.

DOCTOR: Right. That's next to Croydon, isn't it?

(K9 comes back to life.)

DOCTOR: Oh, hey. Now we're in business.

K9: Master.

DOCTOR: He recognises me.

K9: Affirmative.

DOCTOR: Rose, give us the oil.

ROSE: I wouldn't touch it, though. That dinner lady got all scorched.

DOCTOR: I'm no dinner lady. And I don't often say that.

(The Doctor smears a sample on to K9's probe.)

DOCTOR: Here we go. Come on, boy. Here we go.

K9: Oil. Ex ex ex extract. Ana ana analysing.

MICKEY: Listen to him, man. That's a voice.

SARAH: Careful. That's my dog.

K9: Confirmation of analysis. Substance is Krillitane Oil.

DOCTOR: They're Krillitanes.

ROSE: Is that bad?

DOCTOR: Very. Think of how bad things could possibly be, and add another suitcase full of bad.

SARAH: And what are Krillitanes?

DOCTOR: They're a composite race. Just like your culture is a mixture of traditions from all sorts of countries, people you've invaded or have been invaded by. You've got bits of Viking, bits of France, bits of whatever. The Krillitanes are the same. An amalgam of the races they've conquered. But they take physical aspects as well. They cherry pick the best bits from the people they destroy. That's why I didn't recognise them. The last time I saw Krillitanes, they looked just like us except they had really long necks.

ROSE: What're they doing here?

DOCTOR: It's the children. They're doing something to the children.

[Outside the coffee shop]

(They put K9 back in the boot of Sarah's car.)

MICKEY: So what's the deal with the tin dog?

SARAH: The Doctor likes travelling with an entourage. Sometimes they're humans, sometimes they're aliens, and sometimes they're tin dogs. What about you? Where do you fit in the picture?

MICKEY: Me? I'm their Man in Havana. I'm the technical support. I'm. Oh, my God. I'm the tin dog.

[Street]

FINCH: On my command.

(Rose and the Doctor come out of the coffee shop.)

ROSE: How many of us have there been travelling with you?

DOCTOR: Does it matter?

ROSE: Yeah, it does, if I'm just the latest in a long line.

DOCTOR: As opposed to what?

ROSE: I thought you and me were. I obviously got it wrong. I've been to the year five billion, right, but this? Now this is really seeing the future. You just leave us behind. Is that what you're going to do to me?

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?

DOCTOR: I don't age. I regenerate. But humans decay. You wither and you die. Imagine watching that happen to someone who you

ROSE: What, Doctor?

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.

FINCH: Time Lord.

(The Doctor looks up. The giant bat swoops down. They all duck and it flies off.)

SARAH: Was that a Krillitane?

ROSE: But it didn't even touch her. It just flew off. What did it do that for?

[School yard]

(Next morning, the pupils are arriving at the school.)

DOCTOR: Rose and Sarah, you go to the Maths room. Crack open those computers, I need to see the hardware inside. Here, you might need this.

(Rose holds out her hand, but the Doctor gives the screwdriver to Sarah.)

DOCTOR: Mickey, surveillance. I want you outside.

MICKEY: Just stand outside?

SARAH: Here, take these you can keep K9 company.

(Sarah throws Mickey her car keys.)

DOCTOR: Don't forget to leave the window open a crack.

MICKEY: But he's metal!

DOCTOR: I didn't mean for him.

ROSE: What're you going to do?

DOCTOR: It's time I had a word with Mister Finch.

[Swimming pool]

(They stand at opposite ends of the pool.)

DOCTOR: Who are you?

FINCH: My name is Brother Lassa. And you?

DOCTOR: The Doctor. Since when did Krillitanes have wings?

FINCH: It's been our form for nearly ten generations now. Our ancestors invaded Bessan. The people there had some rather lovely wings. They made a million widows in one day. Just imagine.

DOCTOR: And now you're shaped human.

FINCH: A personal favourite, that's all.

DOCTOR: And the others?

FINCH: My brothers remain bat form. What you see is a simple morphic illusion. Scratch the surface and the true Krillitane lies beneath. 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.

DOCTOR: This plan of yours. What is it?

FINCH: You don't know.

DOCTOR: That's why I'm asking.

FINCH: Well, show me how clever you are. Work it out.

DOCTOR: If I don't like it, then it will stop.

FINCH: Fascinating. Your people were peaceful to the point of indolence. You seem to be something new. Would you declare war on us, Doctor?

DOCTOR: I'm so old now. I used to have so much mercy. You get one warning. That was it.

FINCH: But we're not even enemies. Soon you will embrace us. The next time we meet, you will join with me. I promise you.

[Sarah's car]

MICKEY: Surveillance. If you ask me, it's just another way of saying go sit at the back of the class with the safety scissors and glitter. That'd be me talking to a metal dog, then.

[IT classroom]

(Sarah isn't making progress with the school computers.)

SARAH: It's not working.

ROSE: Give it to me.

SARAH: Used to work first time in my day.

ROSE: Well, things were a lot simpler back then.

SARAH: Rose, can I give you a bit of advice?

ROSE: I've got a feeling you're about to.

SARAH: I know how intense a relationship with the Doctor can be, and I don't want you to feel I'm intruding.

ROSE: I don't feel threatened by you, if that's what you mean.

SARAH: Right. Good. Because I'm not interested in picking up where we left off.

ROSE: No? With the big sad eyes and the robot dog? What else were you doing last night?

SARAH: I was just saying how hard it was adjusting to life back on Earth.

ROSE: The thing is, when you two met they'd only just got rid of rationing. No wonder all that space stuff was a bit too much for you.

SARAH: I had no problem with space stuff. I saw things you wouldn't believe.

ROSE: Try me.

SARAH: Mummies.

ROSE: I've met ghosts.

SARAH: Robots. Lots of robots.

ROSE: Slitheen, in Downing Street.

SARAH: Daleks!

ROSE: Met the Emperor.

SARAH: Anti-matter monsters.

ROSE: Gas masked zombies.

SARAH: Real living dinosaurs.

ROSE: Real living werewolf.

SARAH: The Loch Ness Monster!

ROSE: Seriously? Listen to us. It's like me and my mate Shireen. The only time we fell out was over a man, and we're arguing over the Doctor. With you, did he do that thing where he'd explain something at like, ninety miles per hour, and you'd go, what? and he'd look at you like you'd just dribbled on your shirt?

SARAH: All the time. Does he still stroke bits of the Tardis?

ROSE: Yeah! Yeah, he does. I'm like, do you two want to be alone?

(The sisters have bonded and laugh. The Doctor enters.)

DOCTOR: How's it going?

(They keep laughing.)

DOCTOR: What? Listen, I need to find out what's programmed inside these.

(Hysteria is setting in.)

DOCTOR: What? Stop it!

[Headmaster's office]

(Finch opens the door on the dinner ladies and teachers.)

FINCH: Brothers, we must initiate the final phase. Get the children inside and seal the school. Our time has come, my brothers. Today we shall become Gods.

[School yard]

(Games of football and netball are interrupted by a klaxon.)

TANNOY: All pupils to class immediately. And would all members of staff congregate in the staff room.

MELISSA: Breaktime's finished early. Isn't that fantastic?

(Kenny is the last to go inside.)

[IT classroom]

(Rose turns children away at the door.)

ROSE: No, no. This classroom's out of bounds. You've all got to go to the South Hall. Off you go. South Hall!

[Staff room]

PARSONS: What is it now, Mister Finch?

FINCH: A slight change in the timetable. We're having an early lunch.

(Wagner shuts the door on us. Flaps and screams come from inside.)

[IT classroom]

(The Doctor does his old trick of looping wires around his neck and shoulders as he tries to get inside the CPU.)

DOCTOR: I can't shift it.

SARAH: I thought the sonic screwdriver could open anything!

DOCTOR: Anything except a deadlock seal. There's got to be something inside here. What're they teaching those kids?

(Kenny doesn't go into a classroom.)

[Headmaster's office]

FINCH: Close the school.

(Alien symbols come up on his computer screen, then Security Override. He burps. The external doors all slam shut. In another computer room, the children are typing quickly again.)

[IT classroom]

(The symbols are all on these screens too.)

SARAH: You wanted the programme? There it is.

DOCTOR: Some sort of code.

(It starts to resolve itself. Kenny runs along the corridors, looking into the classrooms. All the children are working at computers. He runs downstairs to the main entrance, and tries to open the doors. Mickey spots him.)

DOCTOR: No. No, that can't be.

[Main entrance]

KENNY: They've taken them all!

MICKEY: What?

KENNY: They've taken all the children!

[Sarah's car]

(Mickey pushes the buttons on K9's back.)

MICKEY: Come on, I need some help.

(He hits K9's head, and it starts up.)

K9: System restarting. All primary drives functioning.

MICKEY: You're working! Okay, no time to explain. we need to get inside the school. Do you have like, I don't know, a lock picking device?

K9: We are in a car.

MICKEY: Maybe a drill attachment?

K9: We are in a car.

MICKEY: Fat lot of good you are.

K9: We are in a car.

MICKEY: Wait a second. We're in a car. (to Kenny) Get back!

[IT classroom]

DOCTOR: The Skasis Paradigm. They're trying to crack the Skasis Paradigm.

SARAH: The Skasis what?

DOCTOR: The God maker. The universal theory. Crack that equation and you've got control of the building blocks of the universe. Time and space and matter, yours to control.

ROSE: What, and the kids are like a giant computer?

DOCTOR: Yes. And their learning power is being accelerated by the oil. That oil from the kitchens, it works as a, as a conducting agent. Makes the kids cleverer.

ROSE: But that oil's on the chips. I've been eating them.

DOCTOR: What's fifty nine times thirty five?

ROSE: Two thousand and sixty five. Oh, my God.

SARAH: But why use children? Can't they use adults?

DOCTOR: No, it's got to be children. The God maker needs imagination to crack it. They're not just using the children's brains to break the code, they're using their souls.

(The Headmaster has entered the room.)

FINCH: Let the lesson begin. Think of it, Doctor. With the Paradigm solved, reality becomes clay in our hands. We can shape the universe and improve it.

DOCTOR: Oh yeah? The whole of creation with the face of Mister Finch? Call me old fashioned, but I like things as they are.

FINCH: You act like such a radical, and yet all you want to do is preserve the old order? Think of the changes that could be made if this power was used for good.

DOCTOR: What, by someone like you?

FINCH: No, someone like you. 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, Assinta. Your own people, Doctor, standing tall. The Time Lords reborn.

SARAH: Doctor, don't listen to him.

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. Join us.

DOCTOR: I could save everyone.

FINCH: Yes.

DOCTOR: I could stop the war.

SARAH: 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.

(The Doctor throws a chair at the big screen, smashing it.)

DOCTOR: Out!

[School entrance]

(Mickey drives Sarah's car through the glass doors.)

MICKEY: Come on!

[Staircase]

(Finch summons his brother. They throw off their disguises in the corridors and fly to him. Mickey and Kenny meet the Doctor, Sarah Jane and Rose at the bottom of the staircase.)

MICKEY: What is going on?

(The Krillitanes are coming.)

[Canteen]

(Finch walks in followed by the bats.)

KENNY: Are they my teachers?

DOCTOR: Yeah. Sorry.

FINCH: We need the Doctor alive. As for the others? You can feast.

(The Krillitanes swoop. They hide under the tables. Suddenly a laser beam fells one of the bats. Finch is furious.)

SARAH: K9!

K9: Suggest you engage running mode, mistress.

DOCTOR: Come on!

(Everyone runs.)

DOCTOR: K9, hold them back!

K9: Affirmative, master. Maximum defence mode.

(The Doctor seals the doors.)

K9: Power supply failing.

FINCH: Forget the shooty dog thing.

K9: Power supply failing.

[Physics laboratory]

DOCTOR: It's the oil. Krillitane life forms can't handle the oil. That's it! They've changed their physiology so often, even their own oil is toxic to them. How much was there in the kitchens?

ROSE: Barrels of it.

(The Krillitanes are battering at the door.)

DOCTOR: Okay, we need to get to the kitchens. Mickey.

MICKEY: What now, hold the coats?

DOCTOR: Get all the children unplugged and out of the school. Now then, bats, bats, bats. How do we fight bats?

(Kenny sets off the fire alarm. The noise hurts the Krillitane's ears. The humans escape.)

[Corridor]

(Finch manages to punch through the wall and pull out the alarm wires. The noise stops.)

FINCH: Get after them.

[Outside the canteen]

K9: Master.

DOCTOR: Come on, boy. Good boy.

[Classroom]

MICKEY: Okay, listen everyone. We've got to get out of here.

(The children don't hear him, or notice when he waves his hand in front of their face.)

[Kitchen]

(The Doctor tries his sonic screwdriver on the barrels of oil.)

DOCTOR: They've been deadlock sealed. Finch must've done that. I can't open them.

K9: The vats would not withstand a direct hit from my laser, but my batteries are failing.

DOCTOR: Right. Everyone out the back door. K9, stay with me.

[Classroom]

(Mickey follows the computer power leads back to - a single wall switch. Click, fizz, and the screens go blank.)

MICKEY: Everyone get out. Now!

(The children get up.)

MICKEY: Come on, move! Let's go! Let's go!

(The Krillitanes pause to disguise themselves as humans again.)

[Kitchen]

(The Doctor lines up the oil barrels for K9 to shoot at.)

K9: Capacity for only one shot, Master. For maximum impact, I must be stationed directly beside the vat.

DOCTOR: But you'll be trapped inside.

K9: That is correct.

DOCTOR: I can't let you do that.

K9: No alternative possible, Master.

DOCTOR: Goodbye, old friend.

K9: Goodbye, Master.

DOCTOR: You good dog.

K9: Affirmative.

[Outside the kitchen]

(The Doctor seals the door.)

SARAH: Where's K9?

DOCTOR: We need to run.

SARAH: Where is he? What have you done!

[Kitchen]

FINCH: When you find him, eat him if you must, but bring me his brain.

(They find K9.)

FINCH: The little dog with a nasty bite. Not so powerful now, are you?

(K9 fires a sustained burst at the nearest barrel. The Krillitanes get splattered with their own oil.)

[School yard]

MICKEY: Come on, guys! Let's go, let's go! Run!

[Kitchen]

JACKSON: Burning!

FINCH: You bad dog.

K9: Affirmative.

[Outside the school]

(The school goes KaBOOM! The children rejoice as paper rains down on them.)

KENNY: Yes!

MELISSA: Did you have something to do with it?

KENNY: Yeah, I did.

MELISSA: Oh my God. Kenny blew up the school! It was Kenny!

CHILDREN: Yay! Kenny! Kenny! Kenny! Kenny!

DOCTOR: I'm sorry.

SARAH: It's all right. He was just a daft metal dog. It's fine, really.

(Sarah bursts into tears. The Doctor tries to comfort her.)

[Outside the Tardis]

(Which has relocated itself from the destroyed school to Belle Vue Park. Sarah Jane walks up and the Doctor steps out.)

DOCTOR: Cup of tea?

[Tardis]

SARAH: You've redecorated.

DOCTOR: Do you like it?

SARAH: Oh, I, I do. Yeah. I preferred it as it was, but er, yeah. It'll do.

ROSE: I love it.

SARAH: Hey, you what's forty seven times three hundred and sixty nine?

ROSE: No idea. It's gone now. The oil's faded.

SARAH: But you're still clever. More than a match for him.

ROSE: You and me both. Doctor?

DOCTOR: Er, we're about to head off, but you could come with us.

SARAH: No. I can't do this anymore. Besides, I've got a much bigger adventure ahead. Time I stopped waiting for you and found a life of my own.

MICKEY: Can I come? No, not with you, I mean with you. Because I'm not the tin dog, and I want to see what's out there.

SARAH: Oh, go on, Doctor. Sarah Jane Smith, a Mickey Smith. You need a Smith on board.

DOCTOR: Okay then, I could do with a laugh.

MICKEY: Rose, is that okay?

ROSE: (lying) No, great. Why not?

SARAH: Well, I'd better go.

ROSE: What do I do? Do I stay with him?

SARAH: Yes. Some things are worth getting your heart broken for. Find me, if you need to, one day. Find me.

[Outside the Tardis]

SARAH: It's daft, but I haven't ever thanked you for that time. And like I said, I wouldn't have missed it for the world.

DOCTOR: Something to tell the grandkids.

SARAH: Oh, I think it'll be someone else's grandkids now.

DOCTOR: Right. Yes, sorry. I didn't get a chance to ask. You haven't? There hasn't been anyone? You know.

SARAH: Well, there was this one guy. I travelled with him for a while, but he was a tough act to follow. Goodbye, Doctor.

DOCTOR: Oh, it's not goodbye.

SARAH: Do say it. Please. This time. Say it.

DOCTOR: Goodbye, my Sarah Jane.

(He lifts her off her feet in a big hug, then goes back inside the Tardis. Sarah Jane turns her back and walks away as the Tardis starts to dematerialise. At the last moment, she turns back to see -)

SARAH; K9!

K9: Mistress.

SARAH: But you were blown up.

K9: The Master rebuilt me. My systems are much improved with new undetectable hyperlink facilities.

SARAH: Oh, he replaced you with a brand new model.

K9: Affirmative.

SARAH: Yeah, he does that. Come on, you. Home. We've got work to do.

K9: Affirmative.