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 " . As shown in Donna ' s World in " Turn Left " , members of Torchwood Three ( Ianto Jones and Gwen Cooper ) would have given their lives in Luke Rattigan ' s place . However , even worse would be Captain Jack being transported to the Sontar , leaving the Earth without any more defenders against the aliens invasions . Synopsis . As the poison ous 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 Sontaran s ? 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 weapon s 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 firearm s by expanding the copper - lined bullet s , 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 , Castor 36 , and begin the human race anew ; everything they were building was the foundation of said new world . The students laugh him off ; he attempts to keep them from leaving by brandishing a gun . However , when Rattigan shows no initiative to use it , they leave . When he returns to report his failure , the Sontarans too ridicule his efforts , having never planned to take him ; they planned on using the students as target practice . Rattigan teleports back to his mansion before they can kill him , and the Sontarans lock the teleport pod s . Meanwhile , the Doctor instructs Donna to reopen the teleport pods . Everyone puts of gas masks , even the Doctor , who can ' t resisting remembering the Empty Child . Mace calls down the " Valiant ' s " , whose engines are strong enough to clear the area around the factory ; this is enough to impress the Doctor . 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 ( as she quite frankly smells badly to non - humans ) , 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 . The Doctor 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 . . .