A news report on a couple looking for their missing son, Ashley, brings a surprise for Sarah Jane (Elisabeth Sladen) when she discovers that the missing boy looks exactly like Luke (Tommy Knight). And when Mr Smith (Alexander Armstrong) confirms that Luke is a genetic match with Ashley, Sarah Jane sends him back to live with his 'parents'. With no knowledge of his former life, Luke finds it hard to fit in and, when he tries to escape, he makes a startling discovery about who his parents really are. Sarah Jane, meanwhile, makes some investigations into the Pharos Para-Science Institute – a research unit that has been conducting experiments using alien technology. Floella Benjamin guest stars as Professor Rivers.

Maria (Yasmine Paige) and her dad, Alan (Joseph Millson), dig deeper into the mystery of Luke's (Tommy Knight) new parents and discover that Luke has actually been kidnapped by the Slitheen. There are more shocks when Sarah Jane (Elisabeth Sladen) and Maria find out that Mr Smith (Alexander Armstrong), Sarah Jane's super computer, is one of the bad guys. With Clyde (David Anthony) nowhere to be seen, Luke in the clutches of aliens and no Mr Smith to help, things are not looking good for Sarah Jane and Maria. But just what do the Slitheen want with Luke? And who, or what, exactly, is the Xylok?