The Poison Sky

192b – "The Poison Sky"
Doctor Who episode
The Valiant firing at the ATMOS Factory.
Cast
Doctor David Tennant (Tenth Doctor)
Companions Catherine Tate (Donna Noble) Freema Agyeman (Martha Jones)
Others 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
Production
Directed by	Douglas Mackinnon
Written by	Helen Raynor
Script editor	Nikki Smith
Produced by	Susie Liggat
Executive producer(s)	Russell T Davies Julie Gardner Phil Collinson
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
Chronology
← Preceded by Followed by → "The Sontaran Stratagem" "The Doctor's Daughter"

"The Poison Sky" is the fifth episode of the fourth series (since revival) of British science fiction television series Doctor Who. It was broadcast on BBC One on 3 May 2008. The episode features both former companion Martha Jones and the alien Sontarans. It is the second of a two-part story, following "The Sontaran Stratagem".

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 windshield 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.