The lives of Sarah Jane and her friends are thrown into turmoil when a news report reveals that Luke is not an artificial creation of the Bane after all, but an amnesiac boy named Ashley Stafford. While Sarah Jane faces allegations of child abduction, Maria realises that she may never be allowed to see her mentor again. Mr Smith encourages Sarah Jane to take her mind off things by visiting the Pharos Parascience Institute, where Professor Rivers introduces her to the brilliant but bratty young Nathan Goss. Meanwhile, Maria and Clyde begin to unearth a conspiracy involving an old foe... and one of their closest allies.

When he was planning the first season of The Sarah Jane Adventures, executive producer Russell T Davies originally intended to contribute one of its five stories himself, with the most likely candidate being the finale. However, the 2008 season of Doctor Who presented Davies with a number of time-consuming challenges, drawing his attention away from the spin-off. Instead, it was agreed that Phil Ford -- who had already written Eye Of The Gorgon -- would tackle the year's climactic adventure. Ford was asked to explore the origins of Sarah Jane's supercomputer, Mr Smith, while also bringing the season full circle by reintroducing the Slitheen who had appeared in the premiere serial, Gareth Roberts' Revenge Of The Slitheen.

Ford's story was eventually called The Lost Boy. Having decided to present Mr Smith as a villain who would imprison Clyde in a void, at one point Ford intended to confront him with a tangible manifestation of the Xylok. For his physical form, Mr Smith would have drawn on Clyde's memories of the sinister, dark-suited Agent Smith from the 1999 science-fiction film The Matrix and its sequels. Professor Rivers originally worked at the Beacon Foundation, rather than the Pharos Institute; her first name was intended to be Joan, although it would not be used on-screen.

Floella Benjamin (Professor Rivers) was a well-known personality in children's television

The Lost Boy was paired with Warriors Of Kudlak as the season's third and final recording block, under director Charles Martin. Amongst the guest cast was Floella Benjamin, playing Professor Rivers. Having emigrated from Trinidad as a child, Benjamin grew up to become a well-known personality in children's television, most notably as a presenter on shows such as Play School. She had also acted in programmes including Within These Walls, Angels and Bergerac; in 2001, she was appointed an Officer of the Order of the British Empire.

The first four days of filming for The Lost Boy -- from July 11th to 14th, 2007 -- were spent at the show's usual studio home in Upper Boat, principally for scenes in Sarah Jane's attic. For the action involving K·9, Mat Irvine controlled the original version of the robot dog, as he had on the introductory special Invasion Of The Bane, and John Leeson was again present in studio to provide K·9's voice. Additional material recorded at the same time included the police press conference on the 12th, footage in the interview room on the 13th, and sequences in both Maria's bedroom and Mr Smith's void on the 14th.

The first location shooting for The Lost Boy took place on July 15th at Hensol Castle in Hensol, which served as the exterior of the Pharos Institute. The van used by the Slitheen was ostensibly for “Dickel & Lyons” -- an homage to production designer Tim Dickel and property master Phil Lyons. The 16th was spent in Penarth, first at the “Stafford” residence on Wood Street, followed by Clinton Road for Bannerman Road material.

July 18th and 19th took Martin's team to the Vittac Building, within the Tredegar Business Park in Tredegar, for sequences in the Pharos Institute's telekinesis laboratory and corridors. Then it was back to Clinton Road for more filming from the 20th to the 22nd. Work on the latter day also encompassed recording at Cardiff High School, for material at the park, in the Slitheen van, and at Park Vale Comprehensive -- including a scene originally planned for the playground, which was shifted to the canteen due to inclement weather. Additional Wood Street footage was captured on June 25th, followed by more work at Clinton Road on the 26th. This left just a few final shots on the attic set at Upper Boat, which were completed on July 2nd and 13th.

The Lost Boy Episode Two capped off the first season of The Sarah Jane Adventures with aplomb on November 19th, as the 659,000 viewers who tuned in marked an all-time high for the CBBC digital channel. This audience eclipsed the record already set two weeks earlier by the concluding installment of Whatever Happened To Sarah Jane?. A few months later, Sarah Jane and Luke appeared in The Stolen Earth / Journey's End, the story which concluded the 2008 season of Doctor Who. By that time, it was already known that The Sarah Jane Adventures would be returning for a second season... but there were changes in store.

• Doctor Who Magazine Special Edition #23, 24th December 2009, “Episodes 1.9/1.10: The Lost Boy” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who Magazine Special Edition #32, Autumn 2012, “Episodes 5.7/5.8: Meet Mr Smith” by Andrew Pixley, Panini UK Ltd.