The Poison Sky
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. 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.
The Tenth Doctor continues using the sonic on 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 hypnotized 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 theValiant’s turbofan engines to clear an area around the factory. A Sunglider weapon — smaller than that used against the Sycorax — is their main offence and with steel-encased bullets in lieu of copper, they begin a successful retaliatory strike.
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. 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 tha 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 detonates the device.
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…