The headmaster of Deffry Vale school, Mr Finch (Anthony Stewart Head) is initiating several projects to improve the students’ performance. He is providing free lunches with special chips. The Doctor discovers that the oil on the chips is indeed improving the students’ performance, but it is so hazardous that the kitchen staff must wear hazmat suits.
Journalist Sarah Jane Smith (Elizabeth Sladen) is investigating. She was the Doctor’s companion during his third and fourth incarnation. She meets the Doctor (David Tennant), Rose (Billie Piper), and Mickey (Noel Smith), and immediately dislikes Rose. After all, the Doctor basically turned in his companion for a younger, sexier model. She shows them the robot dog K9 (voice of John Leeson) in the boot of her car. K9 analyzes the chip of oil and identifies it as Krillitane in origin. Rose, who did not know anything about previous companions, confronts the Doctor about why he leaves his companions behind and never mentions them. He does it because he can’t stand to watch them age and die.
The next day, they investigate the school again. The Doctor confronts Mr Finch, who admits that he and several staff members are Krillitanes, who absorb the desirable attributes of the species that they conquer. He tries to recruit the Doctor and fails. Mickey and K9 are waiting in Sarah Jane’s car on surveillance duty. The Doctor, Sarah Jane, and Rose find that the school computers, with the students’ help, are trying to solve the Skasis Paradigm, which is a theory of life, the universe, and everything. Mr Finch tries to recruit the Doctor again, but he and his companions escape.
A student who has not eaten the chips alerts Mickey. He crashes the car through the school door and unplugs the computers, allowing the students to escape as well. The Doctor leads the Krillitanes to the kitchen, whereupon K9 detonates the chip containers, killing the Krillitanes and sacrificing his own existence. Sarah Jane decides not to travel in the TARDIS, but to get on with her life. She encourages Rose to stay with the Doctor and is given a new K9.
At first, Elizabeth Sladen declined to play Sarah Jane again, until she realized how important she was in the story. Afterwards, she was offered a spinoff series, The Sarah Jane Adventures, which debuted in 2006, with K9. She played Sarah Jane until her death too young, in 2011 at 65. Anthony Stewart Head was a well-known character actor, who played Rupert Giles, the Slayer’s Mentor, on Buffy the Vampire Slayer, and had a role in Ghost Rider: Spirit of Vengeance, as well as many other projects on TV and in movies, including a charming true-story film called A Street Cat Named Bob. He tested for the Eighth Doctor. It is ironic that in this episode he plays a kind of vampire lord. Mr Finch was named after Darwin’s finches, who pretty much inspired the Theory of Natural Selection.
Some changes had to be made in the script of School Reunion because real teachers were found with the same character name, or the name was copyrighted. At one school they filmed there was an asbestos scare, and in one café the public was drunk and disorderly. Elizabeth Sladen warned everyone of the difficulties of working with K9 and was proven right, but the lasers were much better than in the old days. In some shots the original K9 from the 1970s was used. The chips in the school were inspired by Jamie’s School Dinners (2005) in which popular British TV chef Jamie Oliver tried desperately to improve the UK school lunch system. School Reunion was a great hit with fans and critics alike and was nominated for a Hugo, which the next episode won. Probably the best thing in the story is Sarah Jane and Rose comparing notes on the Doctor.