Sontaran invasion of Earth The Sontaran invasion of Earth was the attempted takeover of Earth by the Sontaran s for the purposes of breeding more clone warriors . Background . During the early 21st century , the Sontaran Empire was losing its war with the Rutan Host . They were running out of soldier s and needed to produce more . They decided that a large planet such as Earth would be a good target and set about their plans to eradicate human ity and convert Earth into a cloning factory . ( TV : " The Poison Sky " ) Beginnings . Part of their plan called for Luke Rattigan to market ATMOS devices across Earth . These would be activated at the appropriate time . They did so shortly after the Unified Intelligence Taskforce ' s storming of the ATMOS factory . After UNIT had taken over the factory , Martha Jones was captured and cloned to become an agent for the sontarans . When the ATMOS devices were activated , they pumped a thick gas into the atmosphere which had two purposes — to poison humans and to aid the growth of Sontaran clones . ( TV : " The Sontaran Stratagem " ) At this point , many humans died — the first deaths happened in Tokyo , Japan . As other urban areas filled with smog , refugees escaped to more rural areas or even into the oceans . ( TV : " The Poison Sky " ) Battle . UNIT prepared many plans for the Sontarans , including firing nuclear weapon s at the Sontaran flagship . This attack was thwarted by a clone of Martha Jones , who was given a device to wipe the nuclear launch codes and the order to do so repeatedly . This was not because it would damage the Sontaran ships — it wouldn ' t — but because it would ignite the clonefeed in the atmosphere and end the invasion . Around this time , a Sontaran attack squad retook the ATMOS factory from UNIT and killed all the humans in the area . Shortly after , UNIT prepared its soldiers from one of its nearby bases and used the " Valiant " to clear the smog from the area and shoot the factory , killing several Sontarans . Entering the factory once again , the human soldiers swiftly decimated the Sontaran attack squad . ( TV : " The Poison Sky " ) Sontaran defeat . The Tenth Doctor ignited the smog with Luke Rattigan ' s atmospheric converter and flames spread across the Earth . When they died down , the planet ' s atmosphere was restored to its previous state . The Sontarans prepared to attack the Earth with its fleet . However , Rattigan sacrificed himself by using the atmospheric converter on the flagship . He exploded the flagship ; its fragments destroyed the rest of the fleet . ( TV : " The Poison Sky " ) One Sontaran pod , piloted by Commander Kaagh , survived the fleet ' s destruction . When the flagship was destroyed , his pod was knocked into space by the explosion . It crash - landed on Earth , where Kaagh staggered out with a scarring gash across the right side of his face and part of his right ear mangled . ( TV : " The Last Sontaran " ) Aftermath . Many humans were afraid to travel by car ( as it had been cars that the ATMOS devices were connected to ) . Other than that , life on Earth apparently returned to normal . ( TV : " The Poison Sky " ) UNIT took advantage of the teleport pod s left behind in the ATMOS factory and the Rattigan Academy . This led to Project Indigo , which would prove useful during the War in the Medusa Cascade . ( TV : " The Stolen Earth " ) Commander Kaagh , the lone survivor of the Tenth Sontaran Battle Fleet , crash - landed near Goblin ' s Copse . Though severely injured , he repaired his ship in secret in the woods near the Tycho Project radio telescope . His drones looked like lights in the sky , attracting human attention . Kaagh used the drones to lure Professor Nicholas Skinner and his daughter out of the radio telescope . He seized them and fitted them with neural implants to control their actions . He put Professor Skinner to work to make all of Earth ' s satellites crash onto nuclear stations . This would cause a chain reaction and destroy the Earth , avenging the loss of his comrades - in - arms . His plans were foiled by Sarah Jane Smith , her son and his friends . Kaagh was sent home to Sontar in shame . ( TV : " The Last Sontaran " )