The Poison Sky (TV story)

The Poison Sky was the fifth episode of series 4 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, purging humanity one country after the next, 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 Nobles' car in his desperate attempt to free Wilf from it before he chokes on the gas. It proves useless until Sylvia frees Wilf by smashing the car window with an axe she keeps in case of burglars. Ross arrives in a taxi, the only vehicle 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 safely inside without choking on the poisoned air. Donna manages to enter the TARDIS just before she begins coughing hard from the gas. In the meantime, Martha's clone overhears a radio transmission of the Doctor's arrival and belatedly relates to 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. Donna feels a shake from the teleport as the ship is transported to the Sontaran war room, and General Staal gloats over his capture of the TARDIS. The Doctor and Martha's clone arrive to find the TARDIS gone. Realising he is trapped on Earth, the Doctor returns to UNIT headquarters and establishing visual contact with the Sontarans, trying to goad them into revealing their plan. Staal does not fall prey to this ploy, but he orders the TARDIS moved out of the main war room when the Doctor threatens to summon it via remote control on his sonic screwdriver, 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 NATO launch codes and stops every attempt made by Captain Marion Price to fire the weapons. Led by Commander Skorr, 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 with their cordolaine signal, the UNIT troops are quickly slaughtered and the factory is secured, with privates Jenkins, Harris and Gray among the dead.

Rattigan leaves the Sontaran flagship to gather his students at the Rattigan Academy, planning to take them to another planet, Castor 36, and begin the human race anew; everything they were building was the foundation of said new world. The incredulous students laugh him off, and Rattigan attempts to keep them from leaving by brandishing a gun. However, when he shows no initiative to use it, they desert him en masse. Rattigan dejectedly returns to the ship to report his failure, only to then learn from the Sontarans that he was only needed to install ATMOS and that they had no further plans for him and his students beyond using them for target practice. Rattigan teleports back to his mansion before they can kill him, and the Sontarans lock the teleport pods, isolating him on Earth.

Meanwhile, the Doctor calls Martha's mobile phone, which is on the TARDIS console. Donna answers, and the Doctor instructs Donna over the phone to reopen the teleport pods from the Sontaran warship. She is able to incapacitate the Sontaran standing guard outside the TARDIS by striking its probic vent with a mallet from the console. Outside the ATMOS factory, UNIT troops have regrouped wearing gas masks. Mace calls down the Valiant, whose engines are strong enough to clear the area around the factory; this is enough to earn the Doctor's approval. A Sunglider weapon — smaller than that used against the Sycorax — is UNIT's main offence, killing many Sontarans in the blast, and with steel-encased bullets in lieu of copper, Mace's soldiers begin a successful retaliatory strike, during which the colonel personally confronts and executes Commander Skorr with a pistol.

With Martha's clone in tow, the Doctor tracks a signal on his sonic screwdriver and makes his way down to the cloning room where the unconscious Martha is being held. Having figured out long before the clone wasn't the genuine article from her lack of concern for her family and bad non-human smell, he severs its connection to Martha, leaving it to die. Wrapped in the Doctor's coat, Martha convinces her 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. As her clone dies, Martha reclaims her engagement ring. With Donna's help, the Doctor reactivates the teleport pods, rescues her from the Sontaran ship, steals back the TARDIS, and teleports into Rattigan's mansion.

Using Rattigan's equipment, the Doctor builds an atmospheric converter, igniting the Earth's 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 to cheer along with their neighbours, while Captain Marion Price kisses Colonel Mace as the UNIT soldiers celebrate their victory, although she quickly composes herself. However, the Doctor knows the Sontarans won't accept defeat so easily — they are beginning their standard invasion stratagem to wipe out the human race in retaliation.

Recalibrating the atmospheric converter to Sontaran air, the Doctor tells Donna and Martha to lead good lives, asks Rattigan to go on and do something clever with his life, and teleports to the Sontaran ship with the intention of giving them the choice between retreat and death. The Sontarans choose the latter, as a countdown begins to deploy their weapons upon the Earth. Unafraid of death, Staal remains defiant in the face of the Doctor's threats and leads his warriors in another "Sontar-ha!" war chant. However, with a few seconds left, Rattigan acts on the Doctor's advice and rewires the teleport pod to transport himself to the Sontaran ship in the Doctor's stead, mocking the Sontarans' chant with a sarcastic "Sontar? Ha!" before he triggers the device. The ensuing explosion blows out the windows of the bridge, killing Staal and Rattigan, before consuming the entire Sontaran ship; the resulting shockwave destroys most of the Sontaran space pods launching from the main vessel.

With the day saved, life gradually returns to normal as the ATMOS devices are removed from the world's cars. Donna returns to her home in Chiswick and shares a tearful farewell with Wilf, who promises not to let Sylvia know of her travels with the Doctor. Martha turns down an offer to travel with them and bids them goodbye, 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
 * Luke Rattigan - Ryan Sampson
 * Colonel Mace - Rupert Holliday Evans
 * General Staal - Christopher Ryan
 * Commander Skorr - Dan Starkey
 * Private Harris - Clive Standen
 * Private Gray - Wesley Theobald
 * Ross Jenkins - Christian Cooke
 * Female Student - Meryl Fernandes
 * Male Student - Leeshon Alexander
 * Captain Price - Bridget Hodgson
 * Kirsty Wark - Kirsty Wark
 * US Newsreader - Lachele Carl

Worldbuilding

 * The Doctor mentions the Rutans and the Rutan-Sontaran War.
 * The Doctor addresses the Sontarans "under Jurisdiction Two of the Intergalactic rules of engagement" which is very similar to the Shadow Proclamation.
 * The Doctor watches Tommy Zoom while waiting for the Sontarans to stop chanting.
 * Donna has to use the Vulcan salute on a hand recognition device in order to open a door in the Sontaran ship.
 * UNIT uses the Valiant to blow away ATMOS' gases.
 * The Doctor mentions Jack Harkness when he confronts the clone of Martha.
 * Luke Rattigan believed the Sontarans would take him and his followers to Castor 36, a planet orbiting Alpha Geminorum.
 * One of Rattigan's students tells him to shoot her.

Story notes

 * 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, this cameo 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.
 * During production, Douglas Mackinnon intended to have the episode's climactic scene in the TARDIS show the moveable column in the centre console move up and down much more rapidly than normal. However, when attempting to accomplish this, Mackinnon ended up breaking the prop, which took thirty minutes to repair.
 * Donna was originally meant to use a shoe to knock out the Sontarans, but because Catherine Tate only wears trainers they didn't want it to be a trainer, or they thought a trainer would be too soft and just bounce off the back of the neck. She got to use a mallet instead. Later, Chrissie Jackson would use a high heel to knock out Commander Kaagh in The Last Sontaran.
 * When preparing for the nuclear strike, all countries with known nuclear capabilities check in, except for Russia. The very last to check in is North Korea, of which it is disputed they have nuclear weapons, at which point the Colonel looks very surprised.
 * Susie Liggat produced this episode in order to give Phil Collinson time off.
 * When interviewed on Friday Night with Jonathan Ross, Catherine Tate stated that she had been filming alongside ten actors playing Sontarans for two weeks before she realised that there were actors inside the Sontaran costumes. She had assumed the Sontarans "ran on electricity". It was not until an actor removed his helmet to reveal his real face that she realised her mistake. She stated she was "freaked out" by this and said she "nearly died".
 * This two-parter was originally designated Block Four of the recording schedule. However, when Block Three - Partners in Crime and The Fires of Pompeii - was split into two, it became Block Five instead.
 * Kirsty Wark was a friend of Douglas Mackinnon.
 * The exploding ATMOS jeep was supposed to be a more impressive faire, but it would've been too expensive to blow up the Rover (and the Army wouldn't have approved), so it was rewritten to have the ATMOS controller give off a few sparks.
 * As in many previous episodes of the revived series, supposed BBC News 24 footage is used featuring reports of unfolding events. However, as with the more recent appearances of such footage in Doctor Who, the channel is simply captioned on screen as 'News 24' devoid of the BBC logo. Since this episode was produced, the BBC News 24 channel was rebranded in real life as BBC News.
 * Donna's mispronunciation of Sontaran stems from the original production of The Time Warrior. Kevin Lindsay pronounced the word as it has always been used, with emphasis on "Son-TAR-an", whereas Alan Bromly wanted it pronounced with no emphasis. Lindsay won the argument, claiming "I'm from the bloody planet, I think I know how to pronounce my own name!"

Ratings

 * 6.53 million viewers (UK final)

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 "Are you my mummy?" line was improvised by David Tennant because he forgot this line. There is no known behind-the-scenes source to back up this erroneous claim, and appears to originate with a Tumblr post that went mildly viral in the early 2010s. Georgia Moffett debunjed this on Instagram.

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 the 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 through a television screen. (TV: The Idiot's Lantern)
 * General Staal is under the impression that his fleet is the first in history to capture a TARDIS. In fact, other fleets had managed to do similar previously. (AUDIO: The Sontarans)
 * One such fleet not only managed to capture a TARDIS but also managed to enter it. They threatened to use it to annihilate the Rutans and to correct past defeats. (AUDIO: The First Sontarans)
 * 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)
 * The Sontarans show their tendacy to break promises and betray 'allies'. (TV: The Time Warrior, The Sontaran Experiment, The Two Doctors; HOMEVID: Shakedown: Return of the Sontarans)
 * The Doctor is able to tell when a companion is not themselves. He previously identified when Cassandra had taken over Rose's body. (TV: New Earth) The Eleventh Doctor would later recognise Amy Pond as a Ganger. (TV: The Almost People)
 * In a parallel world, the Sontaran invasion was stopped by Torchwood Three. 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 named Commander Kargh. 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)
 * Rattigan would be among those remembered when Davros asked the Doctor: "How many have died in your name? (TV: Journey's End)
 * The Tenth Doctor has had his TARDIS stolen before. (TV: Blink, Utopia)

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.