"School Reunion" is the third episode in the second series of the British science fiction television series Doctor Who. It first aired on 29 April 2006. The episode's narrative takes place some time after the events of "The Christmas Invasion". In the episode's TARDISODE, Mickey Smith alerts the Doctor and Rose Tyler to a comprehensive school where strange things are happening, such as UFO sightings. At the school, the Doctor encounters his former companion Sarah Jane Smith and the robotic dog, K9 Mark III, he sent her, who likewise were interested by the current events. Together, they discover the force behind the events, a race known as the Krillitanes, and work to prevent them gaining control of the universe.

The use of the Doctor's previous companions, in particular Sarah Jane and K9, was first proposed in 2003 to the BBC. After the episode was produced, Elisabeth Sladen was approached by the BBC to star in a spin-off, The Sarah Jane Adventures, which also included K9 in several stories. The episode was positively reviewed, with an Appreciation Index of 85% ("Excellent").

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.

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