"School Reunion" is the third episode in the second series of the British science fiction television series Doctor Who. It first aired on BBC One on 29 April 2006. 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.