The episode's narrative takes place in England some time after the events of the 2005 episode "The Christmas Invasion", and involves the Tenth Doctor (David Tennant) reuniting with his former travelling companion Sarah Jane Smith (Elisabeth Sladen), whom the Fourth Doctor left behind in the 1976 serial The Hand of Fear.
In the episode, the alien race the Krillitanes, disguising themselves as school faculty, use the minds of children to solve a theory of everything that would allow them to control time and space.
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.
This episode's mention of the Series wide arc, "Torchwood", comes when Mickey attempts to investigate the school on his computer and is blocked when the website reads "Classified.
Mr Finch's successes arouse the attention of investigative journalist Sarah Jane Smith, a former companion of the Doctor.
With the Doctor's help, Sarah Jane and Rose discover that the school computers—bolstered by the students' enhanced intelligence—are part of a Krillitane effort to solve the "Skasis Paradigm", a theory of everything.
It was Davies' full intention for Sarah Jane to be used for this, and while Sladen originally declined a request, thinking her role would not be important, she changed her mind when she realised she would be the focal point of the adventure.
[1] After production of the episode was finished, Sladen was approached about a full spin-off series, The Sarah Jane Adventures, which was formally announced on 14 September 2006.
[2] The episode went through several changes in production: working titles included "Old Friends" and "Black Ops", the latter being set in an army base.