The Poison Sky (TV story)

"We will ravage this planet!"

- Staal

Synopsis
The Valiant clears the ATMOS gas as the Sontarans activate their master plan to turn Earth into their cloning planet. UNIT is left defenceless, with a traitor in their ranks. And as interplanetary war edges closer, Doctor has to fight to keep both Martha and Donna alive - but will he have to make the ultimate sacrifice?

Plot
Following from the previous episode, Sylvia Noble manages to free Wilfred Mott from the car by smashing the window. The Doctor sends Donna Noble back to the TARDIS while he sets off to figure out what the Sontarans are up to. After studying the gas, UNIT determines that it will need to reach 80% density to become poisonous. Elsewhere, Martha helps the Sontarans to seize the TARDIS. Realising that he is trapped, the Doctor attempts to goad the Sontarans into revealing their plan: the Sontarans are smart enough not to fall prey to this ploy, but the Doctor does trick them into manoeuvring the TARDIS out of the main war room, placing Donna in a position to help.

Against the Doctor's advice, UNIT decides to use nuclear weapons against the Sontarans; however, Martha's clone has covertly copied the launch codes, and stops every attempt they make to fire the weapons. This in itself shows a hidden agenda: nukes would not have harmed them in the first place, which combined with the unidentifiable elements in the gas, suggest that the Sontarans have an interest in keeping anything from disrupting the atmospheric conversion. At the same time, the Sontarans mobilize a contingent of troops to retrieve and protect the clone. With the Sontarans' ability to jam most conventional firearms by expanding the copper-lined bullets, the UNIT troops are quickly slaughtered and the factory is secured.

Rattigan leaves the Sontaran mothership to gather his students. He plans to take them to another planet and begin the human race anew. The students merely laugh him off, even when he brandishes a gun. When he returns to report his failure, the Sontarans likewise ridicule his efforts, having never planned to take him or his students anywhere. Rattigan teleports back to his mansion before they can kill him, and the Sontarans lock the teleport pods behind him.

Meanwhile, the Doctor instructs Donna on how to reopen the teleport pods. As she makes her way through the ship, UNIT begins a counterattack, loading their weapons with non-copper bullets and using the aircraft carrier Valiant to clear the gas. The counterattack is a success, and the UNIT troops are able to put the Sontarans on the defensive. The distraction allows the Doctor to make his way to the cloning room where Martha is being held. Having figured out long before that the clone wasn't the genuine article, he severs its connection to Martha, leaving it to die. Martha convinces the clone to betray the Sontarans in its last moments, and the clone reveals that the poison gas is actually "food" for Sontaran clones: they are converting the planet into a giant breeding world. With Donna's help, the Doctor is able to reactivate the teleport pods, allowing him to rescue Donna, steal back the TARDIS, and teleport into Rattigan's mansion.

With the equipment Rattigan's students built, the Doctor builds his own atmospheric converter, igniting the atmosphere to clear out the poison gas as shown in the picture. However, he knows the Sontarans won't accept defeat so easily, and teleports to their ship with the converter, planning to give them the choice between retreat or death. The Sontarans choose the latter, but at the last moment, Rattigan teleports himself to the Sontaran ship and brings the Doctor back to Earth, sacrificing himself to destroy the Sontarans.

With the day saved, the Doctor and Donna say their goodbyes to Martha, only to have the TARDIS spring to life and begin piloting itself to places unknown before she can leave.

Cast

 * The Doctor - David Tennant
 * Donna Noble - Catherine Tate
 * Martha Jones - Freema Agyeman
 * Rose Tyler - Billie Piper
 * Wilfred Mott - Bernard Cribbins
 * Sylvia Noble - Jacqueline King
 * General Staal - Christopher Ryan
 * Commander Skorr - Dan Starkey
 * Colonel Mace - Rupert Holliday Evans
 * Luke Rattigan - Ryan Sampson
 * Ross Jenkins - Christian Cooke
 * Captain Price - Biddy Hodson (as "Bridget Hodgson")
 * Private Harris - Clive Standen
 * Private Gray - Wesley Theobald
 * Newsreader - Kirsty Wark
 * Mal Loup - Lachele Carl

Production crew
To be added

Story notes

 * Helen Raynor wrote this episode. Her Series 3 episodes Daleks in Manhattan and Evolution of the Daleks both occupied fourth and fifth episode in the series. This two part episode also occupies episode four and five.
 * This the second part of a story which began with The Sontaran Strategem.
 * This story and The Sontaran Stratagem are Douglas Mackinnon's first stories as director.
 * When the Doctor cuts off Staal's speech on the video screen in mid flow, a clip from CBeebies cartoon Tommy Zoom is featured. The original plan to use a clip from Shaun the Sheep fell through.
 * Billie Piper appears (very briefly) as Rose Tyler on the TARDIS's screen in this episode.

Ratings
To be added

Myths
to be added

Filming locations
To be added

Continuity

 * The Valiant last appeared in DW: Last of the Time Lords.
 * At the end of the episode, when the TARDIS flies off, the Doctor says his "What, what, what" line from Donna's first appearance in DW: The Runaway Bride.
 * Rose Tyler appears on the TARDIS scanner screen for a split second, she appears to be shouting 'Doctor!' and was last seen in DW: Partners in Crime.
 * The Valiant's laser appears to be the same that Torchwood used in DW: The Christmas Invasion
 * The Doctor thanks Donna and Martha just before he sacrifices his life. He last said this to Donna in The Sontaran Stratagem' when she was supposedly leaving him.
 * In the New York scene a building with the name Butler Institute can be seen on one of the buildings Cat's Cradle: Warhead

Discontinuity, Plot Holes, Errors

 * A corridor explodes with Sontarans inside it. This is the same corridor that exploded in the Torchwood episode Sleeper. The corridor in Sleeper was in Cardiff but the one in this episode is in London. A similar mistake occured in Partners in Crime. (Perhaps these buildings are built to the same design.)
 * Commander Skorr is killed by Colonel Mace with a pistol, but minutes later he can be seen open skinned next to General Staal in the Sontaran ship. (The second Skorr, however, could simply be an identical-looking clone.)
 * The world wide nuclear grid includes minor nuclear powers such as India, Pakistan and North Korea, yet the Russian Federation, as well as Israel, the other nuclear state, are conspicuously absent.

DVD and other releases

 * It will be released on the Series 4 boxset DVD in November 2008.