Doctor: David Tennant (Tenth Doctor)
Companions: Catherine Tate (Donna Noble)
Freema Agyeman (Martha Jones)
Billie Piper – Rose Tyler
Christopher Ryan – General Staal
Rupert Holliday-Evans – Colonel Mace
Bridget Hodson – Captain Price
Dan Starkey – Commander Skorr
Bernard Cribbins – Wilfred Mott
Jacqueline King – Sylvia Noble
Ryan Sampson – Luke Rattigan
Christian Cooke – Ross Jenkins
Clive Standen – Private Harris
Wesley Theobald – Private Gray
Meryl Fernandes – Female Student
Leeshon Alexander – Male Student
Kirsty Wark – Herself
Lachele Carl – US Newsreader
Jack Steed – Lieutenant Skree
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 4.5
Series Series 4
Length 2nd of 2-part story, 45 minutes
Originally broadcast 3 May 2008
Immediately after the events of "The Sontaran Stratagem", the Doctor attempts to deactivate the ATMOS devices while Sylvia frees Wilfred from her car by smashing the windscreen with an axe. The Doctor tells Donna's family to get inside and seal the windows and doors as best they can. He and Donna return to the ATMOS factory, where the Doctor warns UNIT not to engage the Sontarans. The Doctor tells Donna to stay in the TARDIS for her own safety, but the Sontarans locate and teleport the TARDIS aboard their ship. Meanwhile, the clone of Martha has given the Sontarans information and control over UNIT's computer systems.
The Doctor speaks to General Staal and learns that the Sontaran war with the Rutan Host is not going well, so the Sontarans need to convert Earth to a breeding planet where they can clone more troops. When the Sontarans prevent UNIT from launching a nuclear missile at their ship with help from the cloned Martha, the Doctor works out that since their ship was quite safe, the Sontarans were really preventing the disruption of their atmospheric conversion. UNIT attempt an attack on the ATMOS factory, but the Sontarans easily overwhelm their troops. UNIT manages a counterattack and calls in the aircraft carrier Valiant, which puts the Sontarans on the defensive. Finding the TARDIS missing, the Doctor sends Donna a coded message while speaking to General Staal and tells her to find and re-engage the teleport pods. Following UNIT's offensive, the Doctor ventures into the factory and discovers the real Martha in one of the Sontaran's cloning devices. Having long suspected the truth, the Doctor awakens the real Martha, killing the clone in the process. He tells Martha to keep UNIT from launching any missiles and rushes off.
The Doctor tells Donna how to use the teleport pods to return to Earth, and then uses the teleport to return the TARDIS to Earth. They teleport with Martha to Rattigan's institution, finding him distraught over the potential disaster to Earth that he caused. The Doctor collects the necessary equipment to construct his own atmospheric converter, which would harmlessly ignite the poison gas across the globe and end the threat. The poison gas burns off in the atmosphere and the Earth is saved. The Doctor is aware the Sontarans will not concede defeat but feels that he needs to give them a chance to withdraw. He teleports to the Sontaran ship and offers Staal the chance to retreat, but Staal calls the Doctor's bluff and encourages him to destroy them. Humbled, Rattigan reactivates the teleport and switches places with the Doctor, sacrificing himself to activate the device and destroy the Sontarans.
Afterward, Martha says goodbye to Donna and the Doctor inside the TARDIS and prepares to head home. Before she can leave however, the TARDIS doors suddenly snap shut and it dematerialises. Trapped on board, Martha is transported with the Doctor and Donna to an unknown destination, as the Doctor's severed hand begins to bubble in its jar under the console.