The Lost Boy ( TV story ) The Lost Boy was the sixth and final story of series 1 of " The Sarah Jane Adventures " . It was comprised of episodes 9 & 10 of the regular series and saw the reappearance of the Slitheen family . Additionally , K9 Mark IV returned briefly in the second episode . During this adventure , Mr Smith , Sarah ' s thus far loyal supercomputer , had his alien origin unveiled , along with the unsettling discovery of his true colours . Up to this point he was operating under a friendly pretext , but the facade dropped to reveal a malicious alien underneath the indiscernible guise . Sarah had him subjected to a computer virus to erase the part of his memory that drove him to hostility . After this , his disposition became genuinely benevolent . Synopsis . A missing child turns out to be , both visually and genetically , Luke . Sarah is forced to hand him over to the " parents " . Heartbroken , Sarah rejects Maria and Clyde , and with Maria ' s dad threatening to sell the house to keep his daughter out of danger , it seems the gang ' s adventures have come to an end , whilst Luke discovers his new " parents " are not all they seem — an old enemy has returned , and this time , they are in league with a member of Sarah ' s faithful team in a plot to bring the moon crashing to Earth . . . Plot . Part 1 . Having discovered what Sarah , Maria , Clyde and Luke do together , Alan Jackson threatens to move house again , only to have a change of heart when he sees how upset Maria is . His condition for not moving away , however , is that he is kept up to date on their battles against aliens . A BBC news report on a family searching for their son , Ashley , missing for five months and last seen boarding the Bubble Shock bus , brings a shock . Ashley looks exactly like Luke . Mr Smith compares their DNA , and confirms that Luke and Ashley are genetic matches . Apparently , Luke was not " grown " by the Bane , but was in fact a kidnapped boy whose memory was wiped . His lack of a navel is explained as the result of its removal by the Bane , who are eggborn and find navels offensive . Chrissie Jackson calls the police and reports Sarah as a child abductor . As Clyde arrives wanting answers , Sarah is arrested and the police put Luke into the charge of Ashley ' s parents , Jay and Heidi . Sarah is detained by the police until UNIT intervene and have her released . Depressed , Sarah decides she was wrong to involve children and tells Maria to stay away from her , suggesting that she and her father move away . Mr Smith suggests she take on a case to take her mind off things . She visits the Pharos Institute , a research centre where alien technology is being used to conduct experiments into telekinesis . Here she meets the annoying child prodigy Nathan Goss . That night , Luke ' s new parents watch television and switch to a channel that glows green . They announce to the Xylok that they have the boy in their possession . Luke ' s new parents are very unpleasant . They keep Luke locked up in his room at all times . Clyde Langer cuts school to visit Luke , but Heidi does not let them see each other . She claims that her son is a keen skateboard er , which worries Clyde ; he knows Luke has a poor sense of balance and is a dreadful skateboarder . He decides to bring a photo of Jay and Heidi to Mr Smith . Luke becomes frantic upon seeing Clyde leave the house . He tries desperately to escape . Mr Smith , meanwhile , sends Sarah back to the lab to steal one of their headsets , which he needs to work out Nathan ' s plans . Back at Luke ' s new home , his " parents " are meeting with Nathan , just as Luke is breaking out of his room . He fails to escape , but finds his parents are actually Slitheen . Luke asks how they are not fat , as Slitheen wearing skin suit s are known to be , and they explain they are using improved compression field technology . Nathan Goss is Korst Gogg Thek Lutiven - Day Slitheen , a child Slitheen Luke met before , now in a new disguise and looking for revenge . Clyde gets to the house only seconds after Sarah has left . He takes the photo to Mr Smith , asking if the photo can be checked to see if it is a fake . Mr . Smith tells Clyde it is a fake , and he knows . . . because Mr . Smith faked it . Clyde is confused , as Mr Smith " is " the Xylok who has been behind everything . Noting that Clyde is not a part of his plan , he fires a bolt of energy at the boy , causing him to digitise and vanish from the real world , and laughs fiendishly . Part 2 . Clyde wakes to find himself inside Mr Smith . Sarah Jane Smith returns from the Pharos Institute , having stolen a telekinetic headset . Elsewhere , Maria and Alan go to Ashley ' s house , where they find a skinsuit . Maria realises the Slitheen have Luke . Alan gets a message on his computer from Clyde , who states Mr Smith has turned evil . The Jacksons and Sarah confront Mr Smith and are nearly killed as it fires a bolt of energy from a built - in gun . The Slitheen take Luke to the Pharos Institute , where they test his abilities . Luke overcomes the Slitheen and escapes as Maria , Alan and Sarah arrive . Working together , Sarah ' s gang and the Slitheen discover Mr Smith ' s intentions . Alan adapts a computer virus to destroy Mr Smith . Using the Slitheen teleporter , Sarah arrives in her attic , where Mr Smith is using Luke to awaken other Xylok from the Earth ' s core . He plans to crash the Moon into the planet , cracking it open like an egg and freeing the other Xylok . Clyde returns when Mr Smith states he is merciful , but Mr Smith then threatens the humans with its gun . Sarah unlocks a safe and calls on K9 . The robots battle and Sarah inputs the virus , which makes Mr Smith forget its purpose . As the Moon grows closer to the Earth , Sarah tells Mr Smith it has a new purpose : to safeguard planet Earth . The Moon returns to its original position , K9 goes back to the safe to guard the black hole and the Slitheen return to Raxacoricofallapatorius . As Sarah , Maria , Luke , Clyde , Alan and Chrissie watch the ship leave , Sarah reflects that she never thought she could be part of a family . Story notes . Filming locations . " to be added "