The Sontaran Stratagem

Martha Jones calls the Doctor to ask for assistance during an investigation by UNIT. Minutes after the TARDIS materialises in contemporary Britain, Martha authorises the raid of an ATMOS (Atmospheric Omission System) factory. The Doctor introduces his companion Donna Noble to Martha and UNIT; Donna instantly befriends Martha, but is concerned about UNIT's ethics and asks the Doctor why he is associated with them; the Doctor ambiguously replies he used to work for them in the late twentieth century.
ATMOS is marketing a satellite navigation system developed by child 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 potentially alien. UNIT are also concerned about fifty-two simultaneous deaths occurring spontaneously several days before the narrative. The Doctor travels to Rattigan's private school to investigate the system, and discovers that the episode's events are being influenced by the Sontarans.
The Sontarans depicted in the episode are part of a battlegroup led by General Staal, "the undefeated". Instead of an instant invasion, they are tactically approaching an invasion with a combination of human clones, mind control, and ATMOS; Martha is captured by two of the controlled humans and cloned to provide a tactical advantage against UNIT.
A subplot depicts Donna returning to her home to warn her mother Sylvia and grandfather Wilfred Mott about the Doctor, having been advised to do so by Martha. Concerned about the implications of telling the truth, Donna reneges from warning her mother. At the end of the episode, 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. The episode's cliffhanger depicts Donna shouting for help while the Doctor stares helplessly at a street full of cars emitting the gas, while on their ship orbiting the planet, the Sontarans prepare themselves for battle.