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 Taklarians, 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 deduced 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)