UNIT is investigating ATMOS, a new device which somehow cleanses the exhaust from automobiles. The Taskforce believes that ATMOS is alien technology, and so their newest recruit -- Martha Jones, now a fully-credentialled physician -- calls the Doctor back to Earth for assistance. ATMOS is purported to be the invention of child genius Luke Rattigan, but the Doctor soon discovers that Rattigan is working with the Sontarans, who have used ATMOS to turn four hundred million cars into deadly weapons.

Having successfully resurrected the Daleks, the Cybermen and the Master during Doctor Who's first three years back on television, executive producer Russell T Davies decided that 2008 was the year that the Sontarans should make their return. Introduced in 1973 in Robert Holmes' The Time Warrior, the Sontarans had last appeared in Holmes' 1985 serial The Two Doctors. Davies was eager to hew closely to Holmes' vision of the aliens as a short, stocky clone race obsessed with war. Deciding that the Sontaran adventure should be the first two-part story of Doctor Who's 2008 season, Davies assigned the scripts to Helen Raynor, who had written another monster-centric two-parter, Daleks In Manhattan / Evolution Of The Daleks, the year before. In the interim, Raynor had contributed To The Last Man to the second season of Torchwood.

In addition to the Sontarans, Davies asked Raynor to include several other elements in her storyline. Foremost amongst these was the return of Martha Jones, who had been the Doctor's companion throughout the 2007 season. Martha had departed in the previous year's finale, Last Of The Time Lords, drawing to an end the season-long storyline of her unrequited love for the Doctor. Davies was now keen to show her maturation as a character, and to this end Martha had featured in three episodes -- Reset, Dead Man Walking and A Day In The Death -- of Torchwood's second season. These had established Martha (now a fully-fledged doctor in her own right) as being on attachment to UNIT, the paramilitary organisation which had been introduced in Doctor Who in the late Sixties and which had featured prominently throughout Jon Pertwee's time as the Doctor.

Indeed, UNIT was another traditional aspect of Doctor Who that Davies wanted to place more firmly in the spotlight. UNIT had been seen in recent adventures such as Aliens Of London / World War Three, The Christmas Invasion and The Sound Of Drums / Last Of The Time Lords, but Davies viewed the Sontaran story as an opportunity to finally put UNIT centre-stage. In particular, he was keen to draw contrasts between the Doctor's more pacifistic brand of heroism and that of UNIT, because he felt that the Doctor's association with the organisation had seemed almost hypocritical in the past.

Finally, Davies wanted to base the adventure around something seemingly mundane and topical. Originally, this was going to be a new kind of chimney attached to every home which purported to cleanse the atmosphere. As Raynor's scripts developed, however, it was decided to combine the environmentally-friendly notion with the newfound popularity of satellite navigation in automobiles. There had been several recent reports of SatNav devices giving drivers incorrect information, leading to accidents and even near-fatalities, and it was agreed that modern viewers -- especially children -- would be better able to relate to this notion. As such, Davies and Raynor conceived the ATMOS device, and Davies included it in his script for the season premiere, Partners In Crime (where it was seen on a taxi driver's windshield) as a way of foreshadowing the Sontaran adventure.

Raynor's episodes came to be called The Sontaran Stratagem and The Poison Sky. Initially, cloning was going to be a much more persistent element of the story, with the ATMOS factory workers all being clones. Davies felt that the clone idea would be more effective in small doses, however, which led to the introduction of the cloned Martha. The ATMOS workers would now be hypnotised, in a manner of reminiscent of the captured scientists seen in The Time Warrior. Luke's surname was originally Marlow, a reference to a young Doctor Who fan of Raynor's acquaintance; this was changed to Rattigan, which had been an early name for Miss Foster in Partners In Crime. Raynor also tacitly acknowledged script assistant Ross Sutherland and New Media assistant producer Richard Jenkins, by naming the UNIT driver “Ross Jenkins”.

Throughout the original Doctor Who series, UNIT's full name had been the United Nations Intelligence Taskforce. When UNIT featured briefly in 2005's Aliens Of London, however, the BBC was made aware that the real United Nations was now concerned about the use of its name for a fictional entity, and was especially wary of any confusion which might arise from people encountering references to UNIT online. Davies' initial response was simply to avoid any references to what UNIT stood for, but he now felt that this approach was no longer viable. The production team consequently discussed various options for what else UNIT might stand for, before finally settling on rebranding it the Unified Intelligence Taskforce.

Meanwhile, in mid-October 2007, Raynor's scripts had to undergo a significant alteration when it became clear that Howard Attfield would not be able to play Donna's father, Geoff Noble, as planned. Attfield, who was suffering from cancer, had filmed scenes as Geoff for Partners In Crime but had been visibly unwell, and soon thereafter had been forced to pull out of the rest of the season. Davies decided to replace Geoff with Donna's grandfather, Wilf, who would be played by Bernard Cribbins; The Sontaran Stratagem / The Poison Sky was therefore rewritten to account for the change.

To afford regular Doctor Who producer Phil Collinson some time off, Raynor's adventure would instead be produced by Susie Liggat, who had filled the same role on Human Nature / The Family Of Blood the year before. Collinson would be credited as an executive producer on these two episodes. Directing The Sontaran Stratagem / The Poison Sky would be Douglas Mackinnon, whose extensive body of work included episodes of London's Burning, The Bill, The Last Detective and Jekyll.

Filming for the two-part story began with four days at Margam Country Park in Margam -- standing in for Rattigan Academy -- beginning on October 23rd. Work at the park included Freema Agyeman's return to Doctor Who as Martha, with the 24th being her first day back on the series. October 27th, 29th and 30th saw material at the ATMOS factory recorded at the Usk Valley Business Park in Pontypool. These days concentrated on scenes in the clone room, with Mariam Nundy doubling for Freema Agyeman in shots which involved both Martha and her clone. Mackinnon's team then travelled to Nant Fawr Road in Cardiff, where scenes in and around Donna's home were recorded at the same location used for Partners In Crime. Work there spanned October 30th to November 2nd.

Production shifted back to the Usk Valley Business Park from the 5th to the 8th for more scenes at the ATMOS factory. Additional ATMOS material was shot at Orion Electric in Margam on the 9th and 10th. The supermarket car park sequence was recorded there, as well, on the second day. The next week was spent at Upper Boat Studios, with work on the UNIT HQ set spanning November 12th to 15th. The concluding TARDIS scene was also shot there on the 15th, followed by some material in the Sontaran war room on the 16th.

November 19th took cast and crew to the Roath Basin at the Cardiff Docks, when the scenes of the Doctor and Ross escaping their UNIT vehicle and Jo Nakashima's car going into the water were both performed. Then it was back to Upper Boat from the 20th to the 22nd, completing the sequences aboard the Sontaran ship. The last day also dealt with additional TARDIS material, while the footage of the American newsreader was taped in Studio C2 of BBC Broadcasting House in Llandaff. Finally, most of the scenes involving Donna alone in the TARDIS were recorded at Upper Boat on December 18th. This left only a handful of inserts, which were captured on January 24th and February 29th, 2008, plus the cameo appearance by Newsnight journalist Kirsty Wark.

Although The Sontaran Stratagem / The Poison Sky was ready for broadcast by the time Doctor Who's 2008 season began transmission, one further change was made on April 8th, three days after Partners In Crime kicked off the new set of episodes. The surprise appearance of Rose Tyler in this story had proved hugely popular, but her next scheduled appearance was not until Midnight (originally planned to be the season's eighth episode, but subsequently rescheduled as the tenth), when she would be glimpsed on a monitor screen trying to attract the Doctor's attention. It was felt that an extra Rose sighting should bridge the gap between these two episodes, and so similar footage -- recorded at Upper Boat on December 5th -- was added to one of Donna's TARDIS scenes in The Poison Sky.

• Outpost Gallifrey: New Doctor Who Series News Page, edited by Shaun Lyon.
• Doctor Who Magazine Special Edition #20, 19th November 2008, “Episodes 4 & 5: The Sontaran Stratagem / The Poison Sky” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Writer's Tale by Russell T Davies and Benjamin Cook (2008), BBC Books, ISBN 978 1 846 07571 1.