"The Sontaran Stratagem" is the fourth episode of the fourth series of the British science fiction television series Doctor Who, which depicts the adventures of a time-travelling humanoid alien known as The Doctor. The episode was broadcast on BBC One on 26 April 2008. The episode and its sequel, "The Poison Sky", were written by Helen Raynor, who previously wrote the linked episodes "Daleks in Manhattan" and "Evolution of the Daleks" in the third series.
"The Sontaran Stratagem" features the first appearance of the alien Sontarans in the series since the 1985 story The Two Doctors, as well as former companion Martha Jones (Freema Agyeman), last seen in "Last of the Time Lords." 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. ATMOS is later revealed to be part of a Sontaran plot to poison the atmosphere.
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. The episode was viewed by 7.06 million viewers after its original broadcast, with an Appreciation Index of 87. Critics gave generally positive reviews, praising the return of the Sontarans, Christopher Ryan's portrayal as Staal, and Raynor's writing; some stated Raynor improved from her previous episodes.
Plot
Martha Jones, a former companion of the Tenth Doctor, calls him to ask for assistance during an investigation by UNIT. Minutes after the Doctor's craft, the TARDIS, materialises in contemporary Britain, Martha authorises the raid of an factory. The Doctor introduces his companion Donna to Martha and UNIT; Donna instantly befriends Martha, but is concerned about UNIT's ethics.
ATMOS is marketing a satellite navigation system developed by young prodigy Luke Rattigan. The system also reduces carbon dioxide emissions to zero; UNIT requested the Doctor's help because the technology is not contemporary and might be alien. UNIT are also concerned about fifty-two simultaneous deaths occurring spontaneously several days earlier, all inside vehicles equipped with ATMOS. Whilst everyone else is studying the technology at the plant, Donna goes through the personnel offices and discovers there are no recorded absences by anyone in the plant: no-one is getting sick.
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. The Sontarans are part of a battlegroup led by General Staal, "the undefeated", and Luke is his Earth Asset. Instead of an outright invasion, they are taking control with a combination of human clones, mind control, and ATMOS; Martha is captured by two of the controlled humans and cloned to provide a mole within UNIT.
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. Concerned about the implications of telling the truth, Donna decides against telling her mother. The Doctor investigates the ATMOS device attached to Donna's car and discovers a secondary function: the device can emit a poisonous gas. Wilfred attempts to take the car off the road, but is trapped when Staal activates all 400 million ATMOS devices installed in cars worldwide.
Continuity
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.
Production
Conception
The episode features the return of the Sontarans, who last appeared in the 1985 serial The Two Doctors; a central appearance by UNIT; and Martha Jones, who had last appeared in "Last of the Time Lords" and made special guest appearances in the Torchwood episodes "Reset", "Dead Man Walking", and "A Day in the Death." The brief that executive producer Russell T Davies gave to writer Helen Raynor included the terms "Sontarans", "military", and "Martha's back".