Monitor ( Sarath ) Monitor was an artificial intelligence on the planet Sarath . History . Monitor was an AI program created to help city officials coordinate and control the city of Arkhaven , one of the largest cities on Sarath . Although he might have been content to exist in this manner for centuries , Monitor ' s purpose changed when an asteroid struck Sarath ' s moon , leaving it in a gradually descending spiral . Eventually it would strike Sarath , eliminating all life on the planet . Professor Tovel Jarrasen , one of Arkhaven ' s leading scientists , worked on the Ship , a massive rocket capable of taking the surviving population of Arkhaven to Mirath , the next planet in the system . The city ' s officials found themselves facing refugees attempting to gain access to the Ship . This crisis came to a head when the city was attacked by the Taklarian s , a brief but devastating war that resulted in Arkhaven being reduced to around eight hundred thousand people from more than five million , although the Taklarian fleet was destroyed . At the same time , Jarrasen realised that his calculations had been flawed due to lack of funding , and he had thus incorrectly calculated the power of the engine he could create . With the Professor only able to build an engine with half the power required , the Functionary class — those responsible for the Ship ' s construction — discreetly chose a group . These would travel to Mirath in the lunar lander , the ship that would have taken passengers from the Ship to Mirath . To subdue riots , a camp was established for the NC2s : Non - Citizen , Non - Conformist people from outside Arkhaven or those who caused trouble in the city . The threat of the camp limited the risk of rebellion , since the NC2s would be left behind when the Ship left . Monitor began to take independent action . He could have survived if the Ship had been in working order , but with only five hundred people carried in the Lander , the population would regress to a low - technology level and Monitor would " die " as his systems decayed without repair . Inspired by Mayor Brantus Draad ' s efforts to make the city seem more populous than it was , Monitor implemented his own version of this plan . Arkhavian medical technology put some people in stasis when they faced imminent death , and Monitor discreetly removed people from stasis and replaced them with convincing android duplicates , not violating his programming as they were technically dead while in stasis . This created a secret group of " Trojan Horses " that Monitor could use to infiltrate and control the final evacuation to secure his own survival . Monitor ' s plans were jeopardised a month before the day that the moon would hit Sarath , when the TARDIS arrived . The TARDIS crew were separated when the building that they had materialised on top of was hit by a meteor . Barbara was lost in the sewers under the building . Susan was taken to hospital when she was injured in the building ' s collapse . The First Doctor and Ian were sent to the NC2 camp . Susan was replaced by one of Monitor ' s androids , but since she was in a healing coma rather than actually dying she regained consciousness before her body was cremated and managed to escape . The Doctor and Ian told their story to one of the wardens of the NC2 camp . The discovery of the TARDIS made him think that their story was true ( Although The Doctor was unable to show them the interior as Barbara had the only available key ) . Draad offered The Doctor access to the city ' s equipment to create a new TARDIS key in exchange for his advice in the final stages of the ship ' s construction . The Doctor quickly deduce d the truth about the Ship when allowed a closer look at it , but was forced to continue his work . The situation became even more complicated when a group of Church functionaries abducted the Susan android to question her about her religious beliefs , with their actions confirming her android status . The real Susan and Barbara were reunited , only to be captured by the group of Functionaries responsible for developing the Lander . The moon ' s descent was accelerated when it was unexpectedly pulled apart by tidal stresses , resulting in Zero Day being over a month early . The Doctor and Ian delayed their return to the Ship to continue their search for Susan and Barbara . They missed an attack on the ship mounted by the surviving Taklarians . Draad destroyed the Ship after the Taklarians had boarded it . The Doctor led the NC2s to the Lander , but an attempt to appeal to Draad was interrupted when Monitor finally revealed his true intentions , taking direct control of his androids . Despite Monitor ' s refusal to be controlled once again and the shock factor of the androids ' true natures , the addition of Susan ' s android to the equation proved to be Monitor ' s undoing . Monitor ' s androids included duplicates of the originals ' brainwaves . Susan had an undefined connection to her android . Her duplicate resisted Monitor ' s influence and disrupted his connection to the other androids . Monitor refused to be controlled again , but one of his androids regained enough memory of his human identity to destroy Monitor once and for all . Monitor tried to gain some vengeance by damaging the Lander ' s control computer in his last moments . The android Susan was able to connect herself up to the system and take his place . The Doctor used an " escape pod " from the TARDIS to provide the Lander with additional space so that all of the survivors present could depart in the ship . ( PROSE : " City at World ' s End " )