The Sontaran Stratagem
The episode takes place on present-day Earth, where Martha and UNIT summon the Doctor (David Tennant) for assistance concerning ATMOS, a revolutionary piece of green technology installed in 400 million cars worldwide.
Showrunner Russell T Davies had considered bringing the Sontarans back since the revival of the series and wanted to show changes in Martha's personality after her departure.
Minutes after the Doctor's craft, the TARDIS, materialises in contemporary Britain, Martha authorises the raid of an ATMOS factory.
UNIT are also concerned about fifty-two simultaneous deaths occurring spontaneously several days earlier, all inside vehicles equipped with ATMOS.
The Doctor travels to Rattigan's private school to investigate the system and discovers that the recent events have been plotted by an alien warrior race known as the Sontarans.
Meanwhile, Donna returns home to explain to her mother Sylvia and her grandfather Wilfred that she has been travelling with the Doctor, after being advised to do so by Martha.
The Doctor Who spin-off show: The Sarah Jane Adventures features the two part episodes The Last Sontaran in which the only survivor of The Sontaran Stratagem / The Poison Sky: Commander Kaagh (also called Kaagh the Slayer) attempts to take revenge on Earth and The Doctor by crashing Earth's satellites into the nuclear weapon storage facilities to cause a chain reaction resulting in a cataclysmic nuclear explosion killing the entire world's population.
The brief that executive producer Russell T Davies gave to writer Helen Raynor included the terms "Sontarans", "military", and "Martha's back".
[5] Raynor initially envisioned the poisonous gas would be emitted by factories, but changed it in later drafts to cars for several reasons: the episode would provide social commentary and the idea of an "evil satnav system" was "much more engageable" and "irresistible".
Unlike the Tyler or Jones families, both Sylvia Noble and Wilfred Mott had met the Doctor before (in "The Runaway Bride" and "Voyage of the Damned", respectively), providing Raynor with an additional subplot.
The Doctor, working with UNIT again, alongside two great companions, and taking on a faithfully realised monster from the classic series.
Mark Wright of The Stage commented overall that the episode was "about as deliciously old-fashioned as new Who gets," stating that the script was "a deftly simple premise that makes you wonder why you didn't think of it yourself", and that Tate's character moved away from the caricature in "The Runaway Bride".
Wales noted that Tennant and Tate "relished" the perpetual swings from "square-jawed serious" to "faintly sadistic subversion", to mess with audience expectations.
Ryan's performance as Staal was again praised, but Wales felt Agyeman still displayed "the charisma and range of a dead fish, despite Martha's transformation from lovesick sap to Ripley-esque super soldier."
Wales summed up that the episode "did pretty much everything a two-part opener should – swiftly shifting the pieces into place for next week's inevitably bombastic showdown.
"[14] Alan Stanley Bear of Airlock Alpha called "The Sontaran Strategem" an "exciting and nostalgic adventure", and said that the episode lives up to the show's reputation for transforming a normal object, in this case the car, to a "global killer".
However, Bear wrote that it took too long to "get to the point", and criticized the "bland Sontaran dialogue and the clichéd simplicity of mind control", which he felt "reduced what could have been a fantastically intricate story piece into Saturday morning cartoon material.
