The Sarah Jane Adventures: The Lost Boy

A tearful couple appear on television to appeal for the return of their lost son, Ashley. According to Mr. Smith's analysis of the boys' DNA, 'Ashley' and Luke are the same person and, after Chrissie Jackson has called the police, a reluctant Luke is returned 'home'. He is locked in his room by the couple, who are, in fact Slitheen, as is another boy, a child genius called Nathan, though Luke knew him previously as Carl. The Slitheen get in touch with 'the Xylok', for instructions, but the Xylok turns out to be Mr. Smith, who has turned rogue and plans to destroy the Earth by releasing others of the Xylok race.