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 a wunderkind name Luke Rattigan, but the Doctor discovers that he is working with the Sontarans. Together, they have used ATMOS to turn four hundred million cars into deadly weapons. And, in the bowels of the ATMOS factory, Martha finds herself replaced with a Sontaran clone.

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 would mark the return of the Sontarans. Introduced by Robert Holmes in 1973's The Time Warrior, the Sontarans had last appeared in their creator's 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 Season Thirty's first two-part story, Davies assigned the scripts to Helen Raynor; she had written another monster-centric epic, Daleks In Manhattan / Evolution Of The Daleks, the year before.

In addition to the Sontarans, Davies indicated that Raynor should include several other elements in her storyline. Foremost amongst them was the return of Martha Jones, who had been the Doctor's companion throughout Season Twenty-Nine. Martha had departed in that year's finale, Last Of The Time Lords, which drew an end to the season-long storyline of her unrequited love for the Doctor. Davies was now keen to show Martha as a more mature character, to which end she had featured in the second season of the spin-off series Torchwood. Reset, Dead Man Walking and A Day In The Death had established Martha -- now a fully-fledged doctor in her own right -- as being on attachment to the United Nations Intelligence Taskforce. Better known as UNIT, the paramilitary organisation had been introduced in Doctor Who during the late Sixties, and had featured prominently throughout Jon Pertwee's time as the Third Doctor.

Russell T Davies viewed the Sontaran story as an opportunity to put UNIT centre-stage

The UNIT connection was another focus which Davies encouraged. Since Doctor Who's revival, the Taskforce had played a minor role in adventures such as 2005's Aliens Of London / World War Three and The Christmas Invasion, and 2007's The Sound Of Drums / Last Of The Time Lords. However, Davies viewed the Sontaran story as an opportunity to finally put UNIT centre-stage. In particular, he was keen to draw a contrast between their approach and the Doctor's more pacifist brand of heroism, because he felt that the Doctor's association with the Taskforce had seemed almost hypocritical in the past.

Finally, Davies wanted a key hook for the adventure to be something topical yet seemingly mundane. Originally, this was going to be a new kind of chimney, which would be attached to every home as part of a purported effort to cleanse the atmosphere of pollution. As Raynor developed her narrative, however, it was decided to combine the notion of green technology 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. It was agreed that modern viewers -- especially children -- would be better able to relate to a pollution-fighting satnav than a chimney. To help establish the ATMOS device within the fictional context, Davies included a reference to it in his script for the season premiere, Partners In Crime, where it was seen on a taxi driver's windshield. Likewise, the return of the Sontarans was foreshadowed with a mention in Eye Of The Gorgon, a story from the first season of Doctor Who's other spin-off, The Sarah Jane Adventures.

Raynor's episodes came to be called The Sontaran Stratagem and The Poison Sky. Initially, cloning was going to be a much more prominent aspect of the story, with the ATMOS factory workers all being clones. Davies felt that this element 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 reminiscent of the captured scientists seen in The Time Warrior. Luke's surname was originally Marlow, in reference to a young Doctor Who fan of Raynor's acquaintance; it was changed to Rattigan, which had been an early name for Miss Foster in Partners In Crime, and had previously been used by Davies on the soap opera Revelations. The reporter's surname was initially Rosser instead of Nakashima. Ross Jenkins, the UNIT driver, was an homage to script assistant Ross Sutherland and New Media assistant producer Richard Jenkins.

A long-simmering issue which Davies decided to confront in The Sontaran Stratagem / The Poison Sky was UNIT's name. Following its mention in Aliens Of London, the BBC was made aware that the real United Nations was now concerned about being associated with a fictional entity, and was especially wary of any confusion which might arise from people encountering online references to UNIT. Davies' initial reaction was simply to avoid any explanation of the UNIT acronym, but he no longer felt that this approach was viable. Having explored a range of options, it was finally agreed that the organisation would now be identified as the Unified Intelligence Taskforce.

Part of Helen Raynor's remit was to include a strong role for Donna's parents, Geoff and Sylvia Noble

Meanwhile, around the middle of October 2007, the scripts for The Sontaran Stratagem / The Poison Sky underwent a significant change. Part of Raynor's remit had been to include a strong role for Donna's parents, Geoff and Sylvia Noble, to help establish them for their prominent involvement in the season's last three episodes. She had drawn upon the relationship between her own parents, portraying Geoff as eternally henpecked by Sylvia. However, after filming his scenes for Partners In Crime, it had become clear that Attfield was extremely ill with cancer. After consulting with the actor's wife, Davies decided to replace Attfield with Bernard Cribbins, who would play Donna's grandfather, Wilfred Mott; 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 be produced by Susie Liggat, who had filled the same role on 2007's Human Nature / The Family Of Blood, and more recently on Planet Of The Ood and The Unicorn And The Wasp, the first two episodes of Season Thirty to go before the cameras. As usual, Collinson would be credited as an executive producer on The Sontaran Stratagem / The Poison Sky. The two episodes were originally designated Block Four of the recording schedule, under director Douglas Mackinnon. However, when Block Three -- Partners In Crime and The Fires Of Pompeii -- was split into two, the Sontaran story became Block Five instead.

The task of redesigning the Sontarans fell to Neill Gorton and his team at Millennium FX. Gorton felt that John Friedlander's original facemask design, seen in The Time Warrior and 1974's The Sontaran Experiment, required little modernisation. In greater need of updating was the Sontaran armour, with the quilted texture used in the Seventies now seeming very dated -- although both Gorton and Davies were determined to preserve the dome-like helmets which perfectly matched the shape of the Sontaran head. Recalling that Linx, the first Sontaran seen on-screen in The Time Warrior, had been mistaken for a knight, Gorton strove to give the Sontaran armour a metallic feel. To play the various Sontarans, a number of relatively short-statured artistes were required, only two of whom would be seen without their helmets. Christopher Ryan was cast as General Staal; he had been Kiv, an alien Mentor, in 1986's The Trial Of A Time Lord. The role of Commander Skorr went to Dan Starkey, a fresh graduate from drama school who was a longtime Doctor Who fan.

Filming for The Sontaran Stratagem / The Poison Sky began at Margam Country Park in Margam, which stood in for the building and grounds of the Rattigan Academy from October 23rd to 26th. Except for a day off on Sunday the 28th, the period from October 27th to 30th was spent at the Usk Valley Business Park in Pontypool, which served as the ATMOS factory. From October 31st to November 2nd, scenes in and around Donna's home were recorded at the same residence on Nant-Fawr Road in Cardiff which had been used for Partners In Crime.

After the weekend, production shifted back to the Usk Valley Business Park from November 5th to 8th for more scenes at the ATMOS factory. Additional ATMOS material was shot at the premises of Orion Electric in Margam on the 9th and 10th; the sequence in the supermarket car park was also recorded there on the second day. Mackinnon's team enjoyed a break on Sunday the 11th, before spending the following week at Doctor Who's usual studio facilities in Upper Boat. From November 12th to 15th, the main focus was the set for UNIT's mobile headquarters, although the concluding TARDIS scene was also recorded on the 15th. Cast and crew remained at Upper Boat on the 16th, with cameras now rolling on the Sontaran war room.

The following week started back out on location; on November 19th, the Cardiff Docks offered areas suitable for the Doctor and Ross escaping from their UNIT vehicle and Jo Nakashima's car going into the water. The remaining sequences aboard the Sontaran ship were filmed at Upper Boat from November 20th to 22nd, alongside further TARDIS material on the latter day. Part of November 22nd was also spent at BBC Broadcasting House in Cardiff, where the footage of the American newsreader was taped. Scenes involving Donna alone in the TARDIS were recorded at Upper Boat on December 18th. This left only a handful of inserts -- which were completed at Upper Boat on January 24th and February 29th, 2008 -- plus the cameo appearance by Newsnight journalist Kirsty Wark, who was a friend of Mackinnon's.

Although The Sontaran Stratagem / The Poison Sky was ready for broadcast by the time Doctor Who's 2008 season began transmission, a further change was made on April 8th. Three days earlier, Partners In Crime had surprised audiences with the return of Billie Piper as Rose Tyler, and the public reaction was enthusiastic. However, Rose's next scheduled appearance in Doctor Who 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 two-month gap, and so similar footage was added to one of Donna's TARDIS scenes in The Poison Sky. It had been recorded at Upper Boat on December 5th.

• 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 Complete History #58, 2018, “Story 192: The Sontaran Stratagem / The Poison Sky”, edited by Mark Wright, Hachette Partworks Ltd.
• Doctor Who: The Writer's Tale by Russell T Davies and Benjamin Cook (2008), BBC Books.