School Reunion ( TV story ) School Reunion was the third episode of the second series of BBC Wales " 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 . 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 computer s 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 cubit s . 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 store room — 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 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 rat s . 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 thirteen 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 says they have to go back in 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 why he never came back for her . 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 Krillitane s , 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 asks the Doctor if Sarah is her future , that she will be left behind like all his other companion s . The Doctor tells her 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 Lord s . " Mr Finch hears the phrase " Time Lords " and sends the other Krillitanes to swoop over them , although they do not harm anyone . 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 computer s , 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 god s . 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 reactivates K9 , asking it if it has some way to get in the school . K9 reminds him they are in a car . 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 and space . The Krillitanes are boosting the children ' s intelligence with the oil , using them and their imaginations as a giant processing device . 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 . After Mickey voices his frustration at being reminded they are in a car , he suddenly understands what K9 is saying and 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 unplugs the computers and gets the children out of the school . 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 . 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 someday , if 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 . . .