David Tennant (Tenth Doctor)
Catherine Tate (Donna Noble)
Freema Agyeman (Martha Jones)
Christopher Ryan – General Staal
Rupert Holliday-Evans – Colonel Mace
Dan Starkey – Commander Skorr
Ryan Sampson – Luke Rattigan
Bernard Cribbins – Wilfred Mott
Jacqueline King – Sylvia Noble
Eleanor Matsuura – Jo Nakashima
Christian Cooke – Ross Jenkins
Clive Standen – Private Harris
Wesley Theobald – Private Gray
Ruari Mears – Clone
Directed by Douglas Mackinnon
Written by Helen Raynor
Script editor Nikki Smith
Produced by Susie Liggat
Executive producer(s) Russell T Davies
Incidental music composer Murray Gold
Production code 4.4
Series Series 4
Length 1st of 2-part story, 45 minutes
Originally broadcast 26 April 2008
Martha Jones, a former companion of the Doctor, calls him to ask for assistance during an investigation by UNIT (Unified Intelligence Taskforce). Minutes after the Doctor's craft, the TARDIS, materialises in contemporary Britain, Martha authorises the raid of an ATMOS factory. The Doctor introduces his companion Donna Noble (Catherine Tate) 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 "70s or 80s."
ATMOS is marketing a satellite navigation system developed by young prodigy Luke Rattigan (Ryan Sampson). 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. 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" (Christopher Ryan). 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 warn her mother, Sylvia (Jacqueline King), and grandfather Wilfred Mott (Bernard Cribbins) about the Doctor, having been advised to do so by Martha. Concerned about the implications of telling the truth, Donna decides against warning 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. 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.