The Poison Sky (TV story)

Synopsis
The Valiant clears the ATMOS gas as the Sontarans activate their master plan to turn Earth into a 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
Sylvia manages to free Wilf 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, Clone Martha has the hypnotized UNIT soldiers Harris and Gray transport the TARDIS to the Sontaran flagship. 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 "clone feed" 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. 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
 * Trinity Wells - Lachele Carl

Production crew

 * Uncredited: Alice Troughton (director, Billie Piper sequence, used again in Midnight)

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. She also wrote Ghost Machine & To the Last Man of Torchwood
 * 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. She receives screen credit, and is fourth-billed, for a performance that lasts less than one second and which was actually shot for another 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.

Studio

 * Upper Boat Studios, Trefforest
 * BBC Broadcasting House, Llandaff

Location

 * Margam Country Park, Port Talbot
 * Usk Valley Business Park, Pontypool
 * Nant Fawr Road, Cardiff
 * Orion Electric, Port Talbort
 * Roath Basin, Cardiff Docks

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.
 * After the Sontarans fill the area with gas the Doctor wears a gas mask. While being shown a gun by Colonel Mace, he is asked what he thinks. His reply is "Are you my mummy?", a reference to the episodes The Empty Child and The Doctor Dances, in which the child asks this question numerous times. It is worth noting that these episodes did not feature David Tennant as the Doctor, but instead Christopher Eccleston as the Ninth Doctor.
 * 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 tries to sacrifice 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, the Butler Institute first appeared in NA: Cat's Cradle: Warhead.
 * Gas was released over London previously in PDA: Amorality Tale.
 * 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 DW: The Age of Steel, when he gave clues to Mickey about how to stop the Cybus Cybermen.
 * In DW: 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 the Sontaran's home planet.
 * The destruction of the Sontaran ship will be shown again in SJA: The Last Sontaran.

Discontinuity, Plot Holes, Errors

 * When Martha's clone enters the mobile HQ, it can be seen that she entered through a normal building's door (with daylight coming in through the window) which isn't present on the back of the vehicle or in the previous episode.
 * During the end we see the Sontaran's weapons starting to target the Earth, they get ready and open up, yet, when it's destroyed, they're still closed.
 * 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. A beep is heard when the teleport disks are attached to the TARDIS. This sound is heard three times, and three disks can be seen on the TARDIS on board the Sontaran Warship (just about).
 * 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. The Sonataran gas was burnt up instantly on earth, so the fire could not continue burning as it had no fuel source (as in "real-world physics"). The Doctor had to recalibrate the device for Sontaran air, which may have been more flammable than the ATMOS gas, and the entire spaceship, not just the surface, was filled with this gas.
 * 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.
 * On that same note, wouldn't burning all the gas in the atmosphere also end up burning all the oxygen too, thus suffocating the whole planet? Air consists mainly of Nitrogen, Carbon Dioxide, Water, Methane, and Oxygen. Maybe the ATMOS gas was more reactive than air. Also, the doctor quite obviously realised it was very risky as he crossed his fingers, maybe it could have. The gas wasn't being burnt, exactly, it was being run through a chemical process by the atmospheric converter designed to create an Earthlike atmosphere on another planet. "Burning the sky" is a bit of whimsical language.
 * As confirmed in DW: 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 arm them so easily? Given the ATMOS crisis and the declaration of Defcon One, it's easy to believe an official agreement was speedily reached by the nations of the world offscreen; it was already mentioned by Mace that they were planning a nuclear strike.
 * Russia is not part of the nuclear grid, despite being a major nuclear power both in real life and in-universe. (Israel goes unmentioned as well, but Israel has never officially admitted to having nuclear weapons)
 * 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. The previous solution would be limited by units upload speed, but the software could be hosted on a server (cloud computing).
 * 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. Also, just because we don't see her express concern doesn't mean it doesn't happen.
 * If the TARDIS fixes itself to the spot it lands, how do the Sontarans teleport it, and how do they move it around the Sontaran Ship? It has been quite firmly established that the TARDIS can be moved by external forces (numerous examples exist of it being carried and teleported).'
 * During the warehouse battle, around when the Doctor tells Donna, "Hold on, I'm coming", the film crew can be seen reflected in a door at the right of screen.
 * When Colonel Mace confronts Commander Skorr, he raises his gun twice; Once in one shot, and immediately after in the other.

DVD and other releases

 * 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.