The Doctor,under his old alias of John Smith along with Rose posing as a lunch lady are undercover at the Daffy Hill school,where ever since the new administrator Mr.Finch and his "handpicked" staff have arrived,the levels of average academic scores for students have been raised uncomfortably high . The lunch staff insist the children are eating food served in special oil that the new staff all avoid,but seem to be making the children smarter. The doctor has suspected alien intervention but so has someone else from his past on Earth:non other than Sarah Jane Smith herself,who has decided to do some investigating of her own at the same school. Upon realizing she is in company with the regenerated doctor and discovering him now in the company of Rose,a rivalry develops between them until they discover a group of bat like creatures hibernating in Finch's office. Sarah than asks the doctor to help repair the now battered K-9 Mark III who,upon reactivation reveals the the oil is actually that used by the Krillitane-a conquering alien race who represent mongrel genetics of the different species they've invaded.
Upon returning to the school the next day the doctor,
Sarah,Rose and Mickey find that the schools computers have been locked. What they do not yet know is that Finch,actually named Brother Kassar,and his compatriots have been actually been using the computers at the school to teach the children through audio/visual association. When the computers are re-activated the doctor finds the program used to teach the children and discovers it's a program designed to decode the Skasis Paradigm,a theory of complete relativity of all things that would allow the Krillitane control over everything in the universe. After failing to persuade the doctor and Sarah into joining him in his species cause to reshape the material universe,Finch is chided by the doctor for using the minds of children for his own goals and ends up using K-9 to destroy the school and the Krillitane staff after the children have all successfully escape. After asking Sarah-Jane Smith to resume her journey's with him and Rose,the doctor is declined by Sarah but taken up on the offer by Mickey. The doctor leaves Sarah with a brand new K-9 Mark IV for her as a parting gift.
Overall this is a wonderful and telling story from the tenth doctor. The plot of the Krillitane,alien eugenics coming to Earth to use the souls of children to do their bidding,is definitely a telling metaphor for the often repressive educational system in England and around the world. The thrust of the story is really the doctor re-uniting with Sarah-Jane Smith. While at first envious Rose,assuming Sarah and the doctor shared the same unrequited romantic relationship she did with him,ended up even sharing a bit of a laugh at some of the doctor eccentric habits he had while they traveled with him. While this provides the comedy in the story,the drama is enhanced by a number of character interactions. Mickey's insistence on joining Rose and the doctor in the TARDIS is helped along by realizing his computer abilities make him the human equivalent of K-9. The doctor informs him that,while he did develop feelings for Sarah-Jane his reason for leaving his many companions behind is due to his compassion for not seeing much shorter lived humans die while he lived on. Brother Kasser nearly succeeds in tempting the doctor into taking his side by insisting the the Skasis Paradigm could revive his people. Again it's Sarah's compassion for the doctor that helps him,and her,realize their true purpose even when they part ways.