An alien baby is abandoned on Sarah Jane's doorstep, prompting her to investigate the child's mysterious background. She soon discovers the presence of two aliens in London: a humanoid Fleshkind and a boron-based Metalkind, representatives of species which have been at war for generations. The baby, whom Sarah Jane names “Sky,” is a genetically-engineered weapon which was developed by the Fleshkind. However, their plan to wipe out the Metalkind will endanger not only Sky, but the entire Earth -- and when Sky abruptly ages to teenagerhood, Sarah Jane realises that time is running out.

Long before it went into production, various plans had been considered for the fifth season of The Sarah Jane Adventures. At one stage, the production team intended to write Rani Chandra out of the programme at the end of season four. A concern had developed that the show's younger castmembers were outgrowing the target audience, and so Rani would be recruited for a journalism internship in Manchester. Anjli Mohindra would subsequently make only occasional appearances, in much the same way as Tommy Knight, who played Luke Smith. This development would have to led Haresh and Gita Chandra fostering a younger boy named Alfie at the start of season six; Alfie would become the latest addition to Sarah Jane's circle. However, it soon became clear that the CBBC brass were happy for The Sarah Jane Adventures to feature the established characters even as they aged towards adulthood, and the plans involving Rani and Alfie were dropped.

Nonetheless, the reduction of Knight's involvement from the start of the fourth season still meant that The Sarah Jane Adventures was effectively down a main character. Moreover, it diminished Sarah Jane's maternal role in the series. As such, the production team began to develop a story arc which would unfold throughout the fifth season. The previous run of episodes would conclude with Sarah Jane finding an alien baby left on her doorstep. It was then hoped that a special episode would air at Christmas 2010 in which the “starchild”, called Astra, would rapidly age into her teens, and it would be revealed that she had been left in Sarah Jane's care by none other than the Doctor.

Originally, the “starchild” was brought into being via the machinations of the Trickster

However, the 2011 episodes would then be structured to reveal that the Doctor -- and, by extension, Sarah Jane -- had been duped. Astra had been brought into being via the machinations of the Trickster, the reality-bending entity who had unsuccessfully opposed the Bannerman Road gang in several stories starting with 2007's Whatever Happened To Sarah Jane?. The Trickster would attempt to use Astra to fully manifest in our reality, and the fifth season would culminate in a battle between Sarah Jane and the Trickster for the girl's soul. In the end, Sarah Jane would win and Astra would evolve into a being with the power to banish the Trickster forever. Thus the character would exit at the end of the 2011 season; it was also intended that this would be the final encounter between Sarah Jane and the Trickster.

These plans changed when the CBBC simultaneously commissioned both the fourth and fifth seasons of The Sarah Jane Adventures, with the intention of cutting costs by filming all of the 2010 episodes alongside the first six 2011 editions. With the option for a Christmas special no longer available, it was decided to defer the introduction of Astra to the start of the fifth season. Lead writer Phil Ford agreed to give up one of his planned 2010 serials -- Death Of The Doctor, which would instead be developed by executive producer Russell T Davies -- and contribute two of the first three stories for the 2011 run, including the season premiere. As he worked on these scripts, he refined the character of the starchild, who was now called Sky. In particular, it was felt that Sky could offer a new take on the fish-out-of-water naivete previously embodied by Luke. Both were newborn innocents in the bodies of teenagers but, whereas Luke had been instilled with a genius-level intelligence, Sky had extraordinary electrical abilities.

Ford's script, appropriately entitled Sky, was paired with the season's second adventure, The Curse Of Clyde Langer, to form the year's fourth production block. The director would be Ashley Way, who had recently completed work on Death Of The Doctor and The Empty Planet. As recording began, it was still hoped that Matt Smith would be available to film a scene in which the Eleventh Doctor revealed that he had left Sky on Sarah Jane's doorstep. The Time Lord would then offer to take Sky to a school in outer space, before capitulating to her request to stay on Bannerman Road. Cast as Sky was eleven-year-old Sinead Michael, who had already amassed a number of credits for her work on stage, television and film, including the movie blockbuster Clash Of The Titans.

The first several days of production on Sky were spent at the programme's regular studio facilities in Upper Boat. First, the footage of Luke in his dorm room was recorded on June 17th, 2010. Next, the 21st saw Michael join the cast dor filming on the standing attic set. June 22nd was dedicated to scenes in the Chandra home, after which it was back to the attic on the 23rd and 24th; material in Luke's room was also completed on the latter day. Way's attention then turned to The Curse Of Clyde Langer, although shots from the Metalkind's perspective were taken on St Asaph Close in Cardiff on July 2nd.

The major location for Sky was Aberthaw Power Station in West Aberthaw, which represented most areas of the Summerwell Nuclear Power Station from July 4th to 6th. On the 7th, Bannerman Road scenes were recorded on Clinton Road in Penarth. Work on July 8th concentrated on the scrapyard, which was actually the premises of Longlife Spares on the Graddfa Industrial Estate in Caerphilly. The only part of the power station not taped at West Aberthaw was the power exchange room; these sequences were filmed at the Millennium Centre in Cardiff on the 13th and 14th.

By now, it had been determined that Smith would be too busy filming the 2010 Doctor Who Christmas special, A Christmas Carol, to appear in Sky. As a result, on July 15th, the scripts were amended to replace the Doctor with the Shopkeeper, the enigmatic character played by Cyril Nri in Lost In Time, which had been made as part of the third block. The same day, more Bannerman Road sequences were taped at Clinton Road; this work continued to the 16th. Nri returned to The Sarah Jane Adventures to record his appearance on the attic set at Upper Boat on July 21st. The same day, a revised version of Sarah Jane and Rani's entry into Summerwell was filmed at a power substation on Hadfield Road in Cardiff. This remount accommodated new dialogue in which Sarah Jane explicitly ruled out the possibility of the Doctor's involvement in Sky's appearance. Inserts for Sky were completed on July 28th at both Upper Boat -- where the scene in Rani's bedroom was also recorded -- and Clinton Road.

Elisabeth Sladen's family agreed that the three remaining serials should be broadcast as a tribute to her career

In post-production, a new version of Clyde's introductory monologue was prepared for the 2011 episodes. It was essentially the same as in the two preceding years, but was interspersed with footage from the three 2011 serials which had already been recorded. Before viewers could see this revised greeting, however, tragedy struck The Sarah Jane Adventures when Elisabeth Sladen died on April 19th, 2011 following a two-month battle with pancreatic cancer. After consulting with her family, the production team agreed that the three remaining serials should be broadcast as a tribute to Sladen's career. However, this truncated version of the fifth season would draw The Sarah Jane Adventures to a close.

For the CBBC digital channel, the same scheduling pattern established in 2010 was retained, with episodes debuting on Mondays and Tuesdays. However, the BBC One and BBC One HD repeats were delayed, taking place on Thursdays and Fridays five weeks after their CBBC premiere. As such, the season got under way on CBBC with Episode One of Sky on October 3rd, but BBC One viewers had to wait until November 10th.

• Doctor Who Magazine Special Edition #32, Autumn 2012, “Episodes 5.1/5.2: Sky” by Andrew Pixley, Panini UK Ltd.