Some time after the modern age, the hero known as Machine Man retired from being a hero. He did this in order to experience what it was like to be human, and faded into the background that was mediocrity of American living. The robot known as Aaron Stack soon adopted a life not much different than that of his "father", inventor Abel Stack, right down to imitating his creators habits during bouts of insomnia.
Time passed and one evening he was confronted (and not for the first time in his existence) by a black doorway that offered him the ability to know what it was like to be human. Aaron, finally curious to know what was being offered to him, entered the doorway.
He was stripped of his "humanity", his skin turned transparent so that his mechanical inner workings could be visible for all to see, and he was transported to the Moon. There he was drawn to the Watcher's Citadel to find that he was being recruited by Uatu, Earth's Watcher, to see all that transpired on Earth.
Uatu explained to X-51 that he was sent to watch over the planet Earth after the Celestials had come and manipulated the beings of the Earth. Uatu was charged with watching mankind's progression for their eventual ascension to becoming Celestials themselves.
Uatu told X-51 he had selected him to watch the world because Uatu was blinded by an enemy and required new eyes to record what was transpiring on Earth, and that X-51 was the only being on the planet with the capacity to record and view these events.
Little did X-51 know that he was really being manipulated by Uatu, and before he was revealed the true secrets of the Celestials and the Watchers, Uatu was slowly stripping away his humanity, first by stripping away his human "mask", constantly referring to him by his model number, and inundating him with events and facts and stressing that he maintain a distance from the events unfolding.
X-51 observed a world where every single being on the planet had been mutated by the Inhuman's Terrigen Mists, and there was a power struggle between the worlds various super-powered beings and a young boy named the Skull who could control peoples' motor functions. These events signaled the coming of the final host of Celestials to Earth, 200 years ahead of schedule.
Overloaded with facts, X-51 constantly questioned everything he saw and demanded more information. Uatu informed the robot that he would reveal everything once X-51 had erased his emotional programming. Fooling the old blind Watcher into believing he had done so, X-51 had all of the Watchers secrets downloaded into his body, and learned of the Celestial plan of propagation.
X-51 betrayed Uatu and went to Earth to warn Reed Richards of a Celestial embryo growing in Earth's core and that super-powered beings existed on the planet through Celestial gene manipulation to act as anti-bodies to protect the Celestial child growing in Earth's core. He also told Richard's that the only being that stood in the Celestials way, and prevented them from over-populating the universe was Galactus, whom Richards had turned into a star.
With the aid of Richards and Black Bolt, X-51 succeeded in summoning "Galactus" (who was really Franklin Richards, Reed's son who had entered the third tier of mutation in which he became what people believed him to be) to defeat the Celestial host and destroy the Celestial embryo growing in the Earth's core, but sparing the planet.
With the defeat of the Skull and the Celestials, mutantkind began to build a series of "Human Torches" to burn the Terrigen Mists from the air so that Reed could cure people of their mutations. X-51 decided to stay on the Moon and watch over the Earth for impending disasters. Upon returning to the Moon, X-51 unplugged Uatu from his machines so that he had no contact to the world in which he was charged to watch.
Three years later after the rebirth of Mar-Vell, the world was thrown into chaos by the depolarization of the Earth due to the Celestials death, and mutantkind had begun attacking the Human Torches drunk on the power that was to be taken away from them. X-51 up to this point had remained relatively uninvolved with the affairs of those on Earth, merely sticking to watching, that is until the being thinking itself to be Galactus returned to the Moon.
Galactus demanded that X-51 tell him about the Earthling who's name was shouted by Black Bolt that summoned him here, because it troubled Galactus that he answered to the call for Franklin Richards. X-51 began to explain to Galactus about the origin of the Fantastic Four, the birth of Franklin Richards, and his exploits as a young boy and the events that led to the groups disbanding. Deciding not to manipulate Galactus for his own ends, like the old Watcher would have done, X-51 resolved to tell Galactus the true fate of Franklin Richards (in spite of the consequences that Galactus would revert back to being Franklin and there would be nothing to stop the Celestials overpopulating the universe), however when bringing up this information he told instead of the death of Franklin Richards of Earth-811, where Franklin had been a member of a mutant rebellion and was murdered by the mutant hunting robots, the Sentinels. Satisfied, Galactus left the Moon to resume his quest.
X-51, troubled by the events that he saw, because they contradicted what he knew, returned to see Uatu, who told him of alternate realities, but would tell him no more unless he was plugged back into his machines so he could hear what transpired on Earth.
X-51 soon took keen interest not only of events occurring on Earth, but also in the visions of Kyle Richmond (formerly the hero known as Nighthawk), which were being written down by his friend and former teammate in the Defenders the Gargoyle. Not only did these visions foresee what was yet to happen, but also enabled X-51 to learn about events occurring in realms that Uatu's machines could not see into (distant universes, the original universe, death's realm, Limbo, etc.) and to learn of the progression of Mar-Vell's quest to collect weapons of power, as well as Mephisto's manipulation of the Church of Immortus to cause chaos around the globe.
In one of Richmond's visions, he saw X-51 digging in an ancient Kree city that was built on the Moon's Blue Area. Curious to see what he would find there, X-51 found the Supreme Intelligence, who told him that the final piece of the Cosmic Consciousness that Mar-Vell was looking for was located in the eyes of Kyle Richmond himself. Shortly after, Mar-Vell and his followers arrived looking for the Ultimate Nullifier, X-51 informed them that Uatu had it, not long after retrieving the Nullifier, X-51 witnessed Mar-Vell assassinating the Supreme Intelligence.
Not long after Kyle Richmond learned that his friend the Gargoyle was supplying his visions to Mephisto (whom the Gargoyle believed to be a man named Mr. Church, a member of the Church of Immortus who promised to give the Gargoyle back his sensations as payment for access to Richmond's visions), he was joined by X-51. The two went to find the Gargoyle, as they did, Richmond explained to X-51 that alternate realities were the result of time manipulation, that do not change history, but only create parallel universes. Most alternate realities were instigated into creation by Mephisto who sought to escape a perceived final judgment at the end of time. Richmond revealed to X-51 that Mephisto was not the devil he believed himself to be, but only a third tier mutant who everyone believed to be the devil.
The two found the Gargoyle, who finally got what he wanted from Mephisto and was dying because of it. X-51 protected Richmond from Mephisto until Mar-Vell and his soldiers arrived to stop the "devil" from taking Richmond's life. The group then traveled to the Savage Land where they arrive just in time to save Black Panther and the X-Men from Belasco and an army of Dire Wraiths and to reveal to Belasco that he was not really the creature spoken of in Dante's Inferno, but really the former X-Man, Nightcrawler.
After puzzling together the conspiracy between Death, Thanos, the Supreme Intelligence, and Mephisto to create alternate worlds for their own gain, X-51 returned to New York with Mar-Vell and his followers to witness the battle against the newly resurrected Absorbing Man.
After Absorbing Man's defeat, and the repolarization of the Earth, X-51 returned to the Moon. There he began his plan to warn other alternate realities of the Celestial embryo's growing in the heart of their worlds. 
X-51 recruited a group he dubbed the Heralds, a group of beings representing different realities to aid him in his mission. He selected Wolverine of Earth-811 (whom he resurrected using a machine once used by Merlin to restore Captain Britain to life), Spider-Girl of Earth-1122, Deathlok (Luther Manning), Killraven, Bloodstorm, Hyperion and Arno Stark, Iron Man from the year 2020. X-51 then offered them all the thing they desired most in return for their aid. X-51 then sent all parties to various realities to warn the various alternate-reality-counterparts of Reed Richards.
X-51 and Hyperion traveled to a world ruled by the evil Kulan Gath which was ruled by magic and all those who practiced science would be put to death. They arrived just as that world's Reed Richards had be executed by hanging. Seeing the two visitors from another reality and realizing they were creatures of science, Gath ordered his minions to attack. Hyperion killed them all before taking Kulan Gath prisoner.
X-51 warned Kulan Gath of the danger of the Celestial embryo growing in Earth's core, Gath refused to destroy the being explaining that it was the embryo that supplied him with all his power. In refusing to aid X-51, Hyperion killed Gath. X-51 then gave the information to the people of a local village in hopes that a man of science from this world would some day rid it of the Celestial embryo.
Returning to his home reality, X-51 found that his other Heralds did not do very well on their quests either, and also found that the Watchers of the multiverse had arrived to execute Uatu for failing in his mission of non-involvement of the planet he was watching, and the result of the death of the Celestial growing within the planet. In a show of power, X-51 used the Watcher's own transportation device to send all the Watchers to various Earth's in different realities, realizing it wasn't Reed Richards' intelligence that allowed for the destruction of the embryo, but the knowledge of the Watchers existence coupled by mankind's inventiveness and curiosity of the universe. Also by transporting the Watchers to these alternate worlds making them just as guilty as the same crime they accused Uatu of committing, thereby sparing Uatu's life. X-51 then transported his Heralds to his Earth so that they may seek what they always wanted.
X-51 returned to the moon and was soon attacked by the Guardians of the Galaxy who had arrived in this reality to use the Terrigen Mists to mutate the population of Earth in their reality to fight the invading Badoon race (this was later learned to be another trick laid by Mephisto to create another alternate universe to escape into), taken aback by what happened (having no known records of the events on this Earth) they forced the tale of recent events from X-51 before leaving him to go to Attilan.
X-51 was then joined by Kyle Richmond once more -- now the new 3-D Man, the two watched over Earth, 3-D Man offering X-51 insight into various secrets of the universe and places which the Watcher's equipment could not see into. X-51 would eventually take information he gleaned from his own observations and what he learned from 3-D Man to various beings around the Earth. He also followed Thor and Loki to record the following of Asgard brought about by their own personal Ragnarök.
He also learned that of a plot by the Elders of the Universe to reunify all alternate realities into their universe, the true origin of Mephisto, and a way to establish a new Death and defeat Mephisto once and for all. He aided Loki, the Reject, Medusa, and King Britain in defeating killing Mephisto and ending a Deviant uprising.
Soon after, X-51 brought Uatu to see Jude, the new death, to bring the dying Watcher to peace. X-51's current whereabouts are unknown. It is possible he returned to the moon to continue watching the Earth to inform it's champions when evil strikes.
X-51 can change his hands or fingers into shape blades which can stretch or retract to an unknown length.
X-51 uses the Watcher's doorway transporter to travel from the Watcher's Citadel on the Moon to the Earth, or one of its alternate realities.
- 44 appearance(s) of X-51 (Earth-9997)
- 3 minor appearance(s) of X-51 (Earth-9997)
- 5 image(s) of X-51 (Earth-9997)
- 1 quotation(s) by or about X-51 (Earth-9997)