Doctor: David Tennant (Tenth Doctor)
Companions: Billie Piper (Rose Tyler)
Noel Clarke (Mickey Smith)
Elisabeth Sladen - Sarah Jane Smith
Anthony Head – Mr Finch
Rod Arthur – Mr Parsons
Eugene Washington – Mr Wagner
Heather Cameron – Nina
Joe Pickley – Kenny
Benjamin Smith – Luke
Clem Tibber – Milo
Lucinda Dryzek – Melissa
Caroline Berry – Dinner Lady
John Leeson – Voice of K9 Mk. III
Directed by James Hawes
Written by Toby Whithouse
Script editor Helen Raynor
Produced by Phil Collinson
Executive producer(s) Russell T Davies
Incidental music composer Murray Gold
Production code 2.3
Series Series 2
Length 45 minutes
Originally broadcast 29 April 2006
Mr. Finch, the headmaster of Deffry Vale School, has been changing the school to improve the students' performance; his changes include free lunches with special chips. The Doctor, under the alias "John Smith", is undercover as a science teacher in the school, and his companion, Rose Tyler, is working undercover in the school's cafeteria. The Doctor is surprised by the good behavior of the students and intrigued by the uncommon intelligence of one of his physics students. Both the Doctor and Rose relate the mysterious events around the school to the chips: Rose observes that the chip oil has an adverse effect on the other kitchen staff, who must use hazmat suits to handle the oil, while the Doctor notes that the chips themselves are making children more intelligent.
Mr. Finch's successes have aroused media attention — in particular, the attention of investigative journalist Sarah Jane Smith. While at first the Doctor feigns ignorance (but is barely able to restrain his pleasure at seeing her again), when Sarah Jane discovers the TARDIS during the night, he reveals his identity to her. Sarah Jane meets Rose and Mickey, with an immediate rivalry sparking between the two women. As the group searches the school, they find thirteen bat-like creatures asleep in Finch's office. Returning to Sarah Jane's car, the journalist reveals a battered, inactive K9 Mark III, who—once repaired—identifies the chip oil as Krillitane oil. Rose, unaware that the Doctor had even had past companions, confronts him; he replies sadly that, while she can spend the rest of her life with him, he cannot spend the rest of his life with her, so he must remain aloof.
The following day, the group returns to the school to investigate further. The Doctor confronts Mr. Finch, who confirms that he and the others are Krillitanes — a composite species that takes desirable attributes of the species they conquer. He attempts to subvert the Doctor, without success. Meanwhile, Sarah Jane and Rose, now getting on better than at their first meeting, try to unlock the programming on the school's computers; Mickey and K9 remain in Sarah Jane's car for surveillance. With the Doctor's help, they discover that the computers — bolstered by the students' enhanced intelligence — are part of a Krillitane effort to solve the "Skasis Paradigm", a theory of everything. As the paradigm is nearly solved, which would allow the Krillitanes full control over time and space, they seal the school. Mr. Finch propositions the Doctor a second time, tempting him with the ability to save the Time Lords and override human mortality. With Sarah Jane's counseling, the Doctor refuses, and they run from Mr. Finch. After Kenny (a student who had not eaten the chips) alerts Mickey to the students' plight, Mickey crashes Sarah Jane's car through the school's doors and unplugs the computers, allowing the children to flee. The Doctor leads the Krillitanes to the kitchen. Upon their arrival, K9 detonates the chip oil container, saturating the Krillitanes. The Krillitanes suddenly explode, destroying the school and K9.
At episode's end, Rose, Sarah Jane, Mickey, and the Doctor are standing in the TARDIS. Sarah Jane declines a second chance of travelling in the TARDIS, having finally decided to move on with her life and stop waiting for the Doctor. Mickey says he would like to join the TARDIS crew, a suggestion of which Sarah Jane approves. She then asks Rose to stay with the Doctor, and to find her one day if she needs to. Sarah Jane then gets a chance to part properly with the Doctor, who leaves her with a brand new K9 as a parting gift.