The Poison Sky (TV story)

The Poison Sky was the fifth episode of the fourth series of Doctor Who. It was the only episode of the season to not reference the Missing Planets arc. It saw the Doctor's life saved again by and at the cost of the life of another person; it was also another moment where his desire to spare his enemies overrides his logic, as he knew Sontarans wouldn't back off if threatened. There was also a second, fleetingly brief cameo made by Billie Piper as Rose Tyler, foreshadowing a major story arc development ahead.

This episode left a loose end about the Sontarans' defeat that was used in The Sarah Jane Adventures television story The Last Sontaran.

Synopsis
As the poisonous gas from the Tenth Sontaran Battle Fleet's ATMOS continues to thicken throughout the world, a clone of Martha Jones works in the shadows to support them. Will the Tenth Doctor learn about Martha's replacement? And can he figure out what the gas is made of, and destroy it before the entire planet chokes to death? More importantly, can the Doctor stop UNIT from starting an interplanetary war with the deadly Sontarans? It's all down to Donna Noble and child genius Luke Rattigan to fix things before it's too late.

Plot
The Tenth Doctor continues sonicking the car in his attempt to free Wilf from it before he chokes on the gas. However, it proves useless as Sylvia frees Wilf from the car by smashing the window with an axe she keeps in case of burglars. Ross arrives in a taxi, the only thing he could find not fitted with ATMOS. Despite Sylvia's protests, Donna goes with the Doctor back to the ATMOS factory with her grandfather's support.

Arriving at the factory, the Doctor gives Donna a key to the TARDIS so she can wait without choking on the air. Donna manages to enter the TARDIS just as she began coughing hard from the gas. In the meantime, Clone Martha overhears a radio transmission of the Doctor's arrival and tells Colonel Mace the previous message the Doctor gave her: Code Red Sontaran. Rushing into the mobile base, the Doctor tells Mace not to engage the Sontarans in battle. When questioned about what he plans to do, the Doctor says he'll use the TARDIS to get on their ship to talk with them.

Martha's clone subtly gives orders to the hypnotised UNIT soldiers Harris and Gray to put teleport relays on the TARDIS for the Sontarans to beam it up. Once done, Donna feels a shake from the teleport. The Sontarans gloat over having the Doctor's infamous vessel. The Doctor and Clone Martha arrive to find the TARDIS gone. Knowing he is trapped, the Doctor tries to goad the Sontarans into revealing their plan. The Sontarans do not to fall prey to this ploy, but they move 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. The Sontarans mobilise 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 flagship to gather his students. He plans to take them to another planet and begin the human race anew. The students laugh him off, even when he brandishes a gun. When he returns to report his failure, the Sontarans too ridicule his efforts, having never planned to take him. Rattigan teleports back to his mansion before they can kill him, and the Sontarans lock the teleport pods against him.

Meanwhile, the Doctor instructs Donna to reopen the teleport pods. UNIT uses the Valiant's turbofan engines to clear an area around the factory. A Sunglider weapon — smaller than that used against the Sycorax — is their main offence, killing many Sontarans in the blast, and with steel-encased bullets in lieu of copper, they begin a successful retaliatory strike, during which Colonel Mace personally confronts and executes Commander Skorr with a pistol.

The Doctor makes his way to the cloning room where Martha is being held. Having figured out long before 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. 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 reactivates the teleport pods, rescues Donna, steals back the TARDIS, and teleports into Rattigan's mansion.

With Rattigan's equipment, the Doctor builds an atmospheric converter, igniting the atmosphere to clear out the poison gas, much to the delight of UNIT and the rest of humanity; Wilf and Sylvia come out of their house and start cheering along with their neighbours, while Captain Marion Price kisses Colonel Mace as the UNIT soldiers cheer their victory, though she quickly composes herself. However, he knows the Sontarans won't accept defeat so easily — they are beginning their standard invasion stratagem. He tells Donna and Martha to lead good lives, tells Rattigan to go on and do something clever, and teleports to their ship with the converter to give them the choice between retreat and death. The Sontarans choose the latter, as a countdown begins. The Doctor threatens that he will do it, but Staal defiantly tells him the Sontarans do not fear death. The begin chanting "Sontar-ha!" However, with a few seconds left, Rattigan simultaneously teleports himself to the Sontaran ship and the Doctor back to Earth. The Sontarans stop their chant with the new development. Luke says "Sontar? Ha!" and triggers the device. The explosion blows out the windows of the bridge, killing Staal and Rattigan, before consuming the entire ship; the resultant shockwave destroys most of the Sontaran space pods launching from the main vessel.

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, the Doctor's hand jar bubbling...

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/Lieutenant Skree - Dan Starkey
 * Colonel Mace - Rupert Holliday Evans
 * Luke Rattigan - Ryan Sampson
 * Ross Jenkins - Christian Cooke
 * Captain Price - Bridget Hodgson
 * Private Harris - Clive Standen
 * Private Gray - Wesley Theobald
 * Herself - Kirsty Wark
 * Trinity Wells - Lachele Carl

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 story also occupies episode four and five.
 * Raynor was originally reluctant to write this story, having read online comments about her previous Dalek two-parter and come away "physically shaking" at the level of vitriol. Russell T Davies convinced her not to take the comments too seriously and to write this story. (REF: The Writer's Tale)
 * This is the second part of a story which began with The Sontaran Stratagem.
 * This episode and The Sontaran Stratagem are Douglas Mackinnon's directorial debut.
 * When the Doctor cuts off Staal's speech on the video screen in mid flow, a clip from cartoon  is featured. The original plan to use a clip from  fell through.
 * Billie Piper appears very briefly as Rose Tyler on the TARDIS' 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 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 Murray Gold's musical themes for the Master. The Master helped design the Valiant.
 * This is the first episode to feature Billie Piper, Freema Agyeman, and Catherine Tate all together as Rose, Martha, and Donna, respectively. It is not, however, the first story in which all three actresses appear, as they all appear in Army of Ghosts/Doomsday. 
 * The Doctor says to the clone Martha, "Avanti", which means "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", the French equivalent.
 * The Brigadier is said to be stranded in Peru. He has been knighted as Colonel Mace refers to him as "Sir Alistair". This is the first reference to the character in Doctor Who since the 1989 story Battlefield, although several references occurred in the first season of The Sarah Jane Adventures, broadcast in 2007. The Brigadier was knighted in the book The Dying Days and called "Sir Alastair" in Big Finish Productions' audio dramas. The Doctor wishes he was there.

Ratings

 * 6.5 million viewers

Myths

 * 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.
 * The Tenth Doctor's remark of "Are you my mummy?" while wearing the gas mask was an adlib by David Tennant who had forgotten his line, and the production team subsequently opted to keep it in. This is a myth spread by a user on Tumblr and is not true.

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

Production 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.
 * During the warehouse battle, around the time 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 again in the immediate next shot.
 * Near the end when Luke Rattigan teleports onto the Sontaran ship, he catches the detonation device, then catches it again in a close up.
 * When the corridor is blown up by the Valiant, one of the visual effects department's right shoulder can be seen.

Continuity

 * The Sontaran High Command are mentioned. (TV: The Two Doctors)
 * Rose Tyler appears on the TARDIS scanner screen for a split second. She appears to be shouting, "Doctor!" (TV: Partners in Crime)
 * This is not the first time Rose has attempted to communicate with the Doctor though a television screen. (TV: 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?" (TV: The Empty Child, The Doctor Dances)
 * The Valiant's laser appears similar to one that Torchwood used. (TV: The Christmas Invasion)
 * The Doctor thanks Donna and Martha just before he tries to sacrifice his life. (TV: The Sontaran Stratagem)
 * In the New York scene a building with the name "Butler Institute" on it can be seen. The Butler Institute first appeared in PROSE: Cat's Cradle: Warhead.
 * Gas was released over London previously in PROSE: 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 when he gave clues to Mickey about how to stop the Cybermen created by Cybus Industries. (TV: The Age of Steel)
 * In a parallel world, 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, who were killed in the process. Jack Harkness was taken to the Sontaran home planet. (TV: Turn Left)
 * The destruction of the Sontaran ship would be referred to again by a surviving Sontaran. His scout ship can be seen being knocked away from the blast force of the exploding flagship. He would crash land on Earth, heavily injured. (TV: The Last Sontaran)

Home video releases

 * This story was released in the Series 4 DVD box set in November 2008 along with the rest of the series.
 * It was released as part of Series 4 Volume 2 in a vanilla edition with The Sontaran Stratagem, The Doctor's Daughter and The Unicorn and the Wasp.