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, the 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

 * Uncredited: Alice Troughton (director, Billie Piper sequence)

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.
 * According to Russell T Davies in an interview in DWM Issue 396, the cameo by Rose was not in the original edit of the episode, but was added just before broadcast when Davies learned how successful her unbilled cameo in Partners in Crime was. Although it was reported that the clip came from "an untransmitted scene" from another episode, in fact, according to the DVD commentary for Midnight, the scene was shot especially for Midnight during production of Turn Left and Davies dropped the scene into The Poison Sky, too. Davies said that, like Partners in Crime, advance review copies of this episode did not include the cameo. This scene was directed by Alice Troughton.
 * The music that plays when the Valiant appears is strikingly similar to "The Master Tape", one of the Murray Gold themes for The Master. Of course, since The Master helped build the Valiant, this isn't too surprising...
 * When Martha Jones wears the Doctor's jacket at the end of the episode, she says she feels like "a kid in my Dad's clothes." The following episode features an actual daughter of the Doctor.
 * This is the first episode to feature Billie Piper, Freema Agyeman, and Catherine Tate all together as Rose, Martha, and Donna, respectively.
 * The Doctor says to the clone Martha: "Avanti" which is "Let's go" in Italian. This is probably to confirm his suspicion of her clone nature. The real Martha would know that the Doctor usually says "Allons-y" which is "Let's go" in French.

Ratings

 * 6.5 million viewers

Myths and Rumours

 * Billie Piper's brief cameo was taken from an earlier episode, most likely The Idiot's Lantern in which she was also shown shouting silently from a TV screen. In fact, the scene was filmed especially for Midnight, and inserted into this episode at the last minute.

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.And also from The Voyage of the Damned when the Titanic smashed through the TARDIS.
 * 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. This is not the first time Rose has attempted to communicate with the Doctor though a television screen, as she also attempted this in DW: The Idiot's Lantern.
 * The Valiant's laser appears to be the same that Torchwood used in DW: The Christmas Invasion (Similar, not the same - this one's much smaller)
 * 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
 * Gas was released over London previously in PDA: Amorality Tale.
 * In The Sontaran Stratagem Luke Rattigan was annoyed when The Doctor said ATMOS System and mentioned that is is a tautology because he is saying 'Atmospheric Emission System System'. Luke Rattigan is also annoyed when General Staal says ATMOS System.
 * The Doctor gave clues to Donna about what action to take via a visual link, but the Sontarans didn't know it was happening. The Doctor did a similar thing in The Age of Steel, when he gave clues to Mickey about how to stop the Cybermen.
 * In the episode Turn Left, the Sontaran invasion was stopped by Torchwood 3. The atmosphere was reverted to its previous state and the Sontaran flagship exploded by Gwen Cooper and Ianto Jones. Jack Harkness was stranded on Sontar.

Discontinuity, Plot Holes, Errors

 * 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 Russia and Israel, and undeclared nuclear states, are conspicuously absent. They may not have agreed to become part of the grid.
 * After The Doctor told Colonel Mace about the Sontaran's cordelane signal, how did Mace manage to produce new firing stock so quickly? U.N.I.T. probably had various types of ammunition at their disposal.
 * If Sontarans are a race bred for war and wear armour, how come they die so easily? This is why the Rutans are winning the war. Or maybe the armour is designed to withstand lasers not bullets also the doctor mentioned sontarans face bullets with dignity
 * Before sleeping, Martha saw her clone but when she woke up she was surprised to see it as though she never even saw it. Not alot of people can remember the moment before falling asleep. Maybe she was shocked because the clone was dying and it was like watching herself die
 * We can see the Sontaran teleport discs are on the TARDIS doors but when we see it in the background of Skorrs message one is on the front and one on the side. You can quite clearly see when the TARDIS is teleported to the Sontaran ship, there are discs on the side.
 * The thermal energy being released by igniting the entire planet's atmosphere should've had some negative effect as opposed to dissipating instantly; As in-story proof (not even counting real-world physics), when the effect was repeated on the Sontaran ship, it exploded violently.
 * When General Staal watches from the Sontaran spaceship, the ignition seems to start from the dark side of the Earth, although London is in clear daylight.
 * Let's also note that since the poison gas was even on ground level rather than just those high in the atmosphere, igniting the atmosphere could have also meant burning all the gas on ground level. Light a candle in a room where someone is smoking a cigarette and the smoke is attracted to the flame.
 * As confirmed in World War Three nuclear release codes are held by the United Nations and needs an official agreement to be able to release them so how is it that UNIT can get them so easily? They may by the Unified Intelligence Taskforce but they still need to go through the UN to get the codes. The previous use was sometime earlier; perhaps the "Homeworld Security" department now allowed for certain protocols to be bypassed in such an emergency. Also, despite the name change, UNIT may still be affiliated with the United Nations, perhaps giving them this authority.
 * How is it that the clone Martha can download enough control onto a handheld device so quickly and with no-one noticing? If the Sontarans can create a clone in a matter if minutes, they've devised a way for fast downloads. In any event, there's no reason at that point for anyone to doubt her identity or motives, especially as she is linked to working with the Doctor.
 * Martha's clone doesn't express any concern for "her" fiance, Thomas Milligan as events unfold, which is understandable; less understandable is the lack of any obvious concern for his well-being by the real Martha. Tom spent a year in an alternate timeline on the run from a renegade Time Lord; Martha knows he can look after himself

Behind the Scenes

 * Billie Piper receives screen credit, and is fourth-billed, for a performance that lasts less than one second and which was actually shot for another episode.

DVD and other releases
[[Image:Bbcdvd-s4-v2.jpg|thumb|250px|Series 4 Volume 2: The Sontaran Stratagem - The Poison Sky - The Doctor's Daughter - The Unicorn and the Wasp DVD Cover]
 * Released in the Series 4 DVD boxset in November 2008 along with the rest of the Series.
 * Released as Series 4 Volume 2 in a vanilla edition alongside The Sontaran Stratagem, The Doctor's Daughter and The Unicorn and the Wasp on 7th July 2008.