Maria Jackson has just moved onto Bannerman Road with her recently-divorced father, Alan, when she becomes aware of strange goings-on involving her reclusive neighbour, journalist Sarah Jane Smith. Then things get even weirder when Maria's new friend, Kelsey, invites her on a tour of a factory run by the supercilious Mrs Wormwood, at which the addictive energy drink Bubbleshock is produced. There Maria rescues a strange boy, and finds that she has nowhere to turn for help but to Sarah Jane. Together, they must discover the connection between Bubbleshock, the boy, and an alien plan to conquer the Earth.

Elisabeth Sladen originally played Sarah Jane Smith in Doctor Who from The Time Warrior in 1973 to The Hand Of Fear in 1976. Over the course of three-and-a-half seasons, opposite both Jon Pertwee (the Third Doctor) and Tom Baker (the Fourth Doctor), the character became recognised as one of the programme's archetypal companions. Indeed, in 1981, Sladen was invited to return to Doctor Who and reprise her role to help ease the transition between Baker and Fifth Doctor Peter Davison. She refused that offer, as she felt that playing second banana to the Doctor once again would represent a backwards step in her career. However, Sladen subsequently agreed to star in a spin-off series, K·9 And Company, which was being created in response to the public outcry over the departure of the popular robot dog K·9 from Doctor Who in Warriors' Gate. Unfortunately, K·9 And Company did not survive past its December 1981 pilot episode, A Girl's Best Friend.

In the mid-Eighties, the arrival of daughter Sadie prompted Sladen to put her acting career on the backburner. Nonetheless, she made brief returns to Doctor Who in two anniversary specials -- 1983's The Five Doctors and 1993's Dimensions In Time -- as well as in various spin-off media. Then, in 2005, Doctor Who made its triumphant return to BBC television after a hiatus which had begun in 1989 and had been interrupted only by a 1996 telefilm. For Doctor Who's 2006 season, executive producer Russell T Davies was eager to bring back a TARDIS occupant from the classic series, in an episode which would provide a greater emotional context for the travels of the Doctor's current companion, Rose Tyler. Davies convinced a reluctant Sladen to return as Sarah Jane; the production team considered the resulting adventure, School Reunion, to be a resounding success.

Russell T Davies was approached about developing a Doctor Who spin-off with the Doctor as a teenager

Meanwhile, Children's BBC had recognised the tremendous popularity of the Doctor Who relaunch amongst their demographic. Davies was approached about developing a spin-off series for younger viewers which would feature the Doctor as a teenager on Gallifrey. However, he quickly concluded that such a project would puncture the mystery which surrounded the Doctor's past. Around the end of 2005, he instead recommended that a new programme should be created to showcase Sarah Jane. It would mark a return to children's television for Davies, whose earliest successes had included the youth-oriented fantasy serials Dark Season and Century Falls.

With CBBC confirming their interest in the concept -- dubbed Sarah Jane Investigates -- Davies and his fellow Doctor Who executive producer, Julie Gardner, took Sladen to lunch in January 2006 to discuss the proposal. Ironically, Sladen went into the meeting under the misapprehension that she was being invited to appear in Doctor Who's other spin-off series, the adult-oriented Torchwood, which had been announced a few months earlier. As such, she was very surprised by the offer to take the lead in her own show, but she ultimately agreed to participate. It was planned that Sarah Jane Investigates would launch with an hour-long special for the Christmas 2006 season, followed by a full season of five stories -- each consisting of two half-hour episodes, bridged by a cliffhanger -- during the latter part of 2007.

A key element of Sarah Jane Investigates would be the introduction of several juvenile characters, towards whom Sarah Jane would act as a mentor even as she found herself struggling to cope with the attendant responsibilities. Davies also hoped that K·9 would serve as a major attraction for the new programme. Davies had previously considered a regular role for the robot dog when he was planning the Doctor Who relaunch, and the character had become closely identified with Sarah Jane ever since the ill-fated K·9 And Company, playing a notable role in School Reunion. However, the rights to the character remained with his creators, one of whom -- Bob Baker -- had long been developing an animated K·9 project outside the auspices of the BBC. Indeed, he was still pursuing this project with Park Entertainment and Jetix Europe. Although Baker agreed that K·9 could make a cameo appearance in Sarah Jane Investigates in order to explain his whereabouts, he did not want the character to feature prominently in the narrative. In place of K·9, Davies created a talking computer called Mr Smith, which would be housed in Sarah Jane's attic; Mr Smith was inspired by the biological computer TIM from the Seventies science-fantasy series The Tomorrow People.

Gareth Roberts developed Kelsey Hooper, and consideration was given to retaining her beyond the special

The scripting duties for the introductory episode of Sarah Jane Investigates were shared between Davies and Gareth Roberts. Roberts had written the interactive Doctor Who game Attack Of The Graske and the prequel TARDISodes for the 2006 season; he was also developing The Shakespeare Code for transmission the following year. Davies roughed out the storyline and devised the main characters. The young leads would include Maria Jackson and Luke, whom the alien Bane referred to as the Seedling. Maria's father was initially called Kevin rather than Alan, and the Jacksons and Sarah Jane lived on Hillview Road in South Croydon -- Sarah Jane's address as established in dialogue from The Hand Of Fear -- instead of Bannerman Road. Roberts began working on the script during the summer of 2006, before Davies took over for the later drafts. It was Roberts who developed Maria's new friend, Kelsey Hooper, and consideration was given to retaining her beyond the special.

At an early stage, the Bane did not attack Sarah Jane's home directly, but instead tried to smuggle an alien creature called the Gloor into her house by hiding it in a delivery parcel. For Bubbleshock's name, Roberts drew on a beverage called “bubbleshake” which he had invented for his 1992 Doctor Who: The New Adventures novel The Highest Science; Bubbleshock was originally a high-end product which satirised the organic foods craze. As noted by Sarah Jane in dialogue, the villainous Mrs Wormwood was named for a star or comet described in the Bible's Book Of Revelation. It was prophesied that it would fall to Earth during the End Times and poison the world's freshwater rivers and springs; the name Wormwood traditionally connoted “bitterness”.

Over the summer, the new programme's title was amended to The Sarah Jane Adventures. On August 1st, the first official word of the project's existence appeared in the pages of the BBC's in-house magazine Ariel. The production team started to come together, with Davies joined by Gardner and Doctor Who producer Phil Collinson as executive producers. Appointed to the producer's chair for the introductory special was Susie Liggat. She had been an assistant director on several Doctor Who episodes, and had also been chosen to temporarily replace Collinson as Doctor Who's producer for one recording block during the 2007 season.

Tommy Knight had made his West End debut in 2001

A key challenge was finding the right mix of young actors to play Maria, Luke and Kelsey. Cast as Maria was Yasmin Paige, who had been a regular on CBBC's The Mysti Show. Tommy Knight won the role of Luke; he had made his West End debut in 2001, and had subsequently taken several parts on television. His character would now be known to the Bane as the Archetype, rather than the Seedling. Coming to the series with a more unusual background was Porsha Lawrence-Mavour. A finalist in the inaugural 2002 children's edition of the reality competition Stars In Their Eyes, Lawrence-Mavour was chosen to play Kelsey.

Apart from Sladen, several other adult actors would also feature prominently in The Sarah Jane Adventures. Joseph Millson, playing Alan, had been a regular in Peak Practice. The role of Maria's mother, Chrissie, went Juliet Cowan; her credits included recurring roles in This Life, Family Affairs and The Bill. Heard but not seen as the voice of Mr Smith would be Alexander Armstrong, who was best known for his successful comic partnership with Ben Miller.

On September 14th, the BBC officially announced that The Sarah Jane Adventures was about to begin filming, with the special now set to air in early January 2007. The director would be Colin Teague, who had made the Torchwood episodes Ghost Machine and Greeks Bearing Gifts during the summer. As with Doctor Who and Torchwood, the production was based in Cardiff, Wales, with studio recording taking place at a facility in Upper Boat, near Pontypridd.

The first day of recording for The Sarah Jane Adventures was September 17th. It was dedicated to the footage for the Bubbleshock advertisement, with filming taking place at the Aberdare Skate Board Park in Cardiff, and then the Cogan Leisure Centre and Penarth Pier in Penarth. Indeed, Penarth would feature prominently in The Sarah Jane Adventures as the venue for its key location, Bannerman Road, which was actually Clinton Road. Various scenes set inside both the Smith and Jackson residences would also be taped there, with the significant exception of Sarah Jane's attic. The first work on Clinton Road took place on the 18th, when Sladen, Paige and Lawrence-Mavour all went before the cameras. The same day, Sarah Jane followed the bus carrying Maria and Kelsey on Stanwell Road and Station Approach. In many sequences of Sarah Jane driving her Nissan Figaro, a stunt double replaced Sladen; although the series star possessed a valid licence, she had driven so rarely in the years since obtaining it that she now felt uncomfortable behind the wheel.

Filming on Clinton Road continued through to September 22nd -- with Tommy Knight making his debut on the 20th -- and resumed after the weekend, from the 25th to the 27th. The latter day also saw Windsor Road and Glebe Street host scenes of the zombified Bubbleshock drinkers. Cast and crew returned to Cardiff on September 28th for the first of two days at St William House, where the premises of LTSB Finance Division masqueraded as Mrs Wormwood's offices. Also on the 28th, material involving K·9 and the black hole was recorded at Upper Boat, with Mat Irvine -- who had provided visual effects for Doctor Who during the Seventies -- supplying the original K·9 prop. The exterior of the Bubbleshock factory was actually the British Telecom Data Centre at Elinia House in Cardiff, with Teague's team present on the 30th.

As October dawned, production moved to Trident Park in Cardiff Bay, where the former home of Nippon Electric Glass UK offered spaces suitable for the Bubbleshock factory floor, data room, vat area, pipe room and various corridors. Work there took place from the 3rd to the 6th, and continued after the weekend, on the 9th and 10th. The rest of the special was then filmed at Upper Boat, starting with material in the Bubbleshock ladies' toilets and Maria's bedroom on October 11th. The 12th and 13th marked the debut of the set for Sarah Jane's attic, which was actually a modified version of the Carrionites' lair from The Shakespeare Code. It was dressed with various items alluding to Sarah Jane's past, including photographs of Brigadier Lethbridge-Stewart (from 1971's Terror Of The Autons and The Daemons) and Harry Sullivan (from 1974's The Sontaran Experiment). Another star of classic Doctor Who joined the team on October 12th, as John Leeson was present to provide K·9's dialogue from off camera. Leeson had been the robot dog's regular voice throughout the character's history, with the exception of Doctor Who's 1979-80 season.

Now given the title Invasion Of The Bane and fronted by a theme tune devised by Doctor Who composer Murray Gold, the inaugural edition of The Sarah Jane Adventures premiered on New Year's Day 2007. It garnered an impressive viewership just shy of three million people; this would be the largest audience the programme would ever attain, although it would never again be provided with such an auspicious timeslot. With the fledgling spin-off already proving to be a success, appetites were whetted for its first full season, which was now just a few months away...

• Doctor Who Magazine Special Edition #23, 24th December 2009, “Episode 1.X: Invasion Of The Bane” by Andrew Pixley, Panini Publishing Ltd.