In March 1947, Canada's Department K and remnants of the United States Operation: Rebirth pooled their resources to become Project: Sentry, an attempt to not only recreate the Super Soldier Serum, but in post-war hubris, magnify its effects a thousand times over. Unfortunately, within a thirty years it had been divided nearly a thousandfold into isolated sub-projects, with lab work farmed out to private contractors. The overall administration of the Project was virtually nil. Precisely where all of its resources ended up over the decades, what they might have begun, and where they ultimately led, is currently unknown.
Over ten years later, Robert Reynolds, a meth addict, broke into the laboratory of an unnamed professor. It was there that he discovered the Golden Sentry Serum. Consuming the compound, Robert Reynolds gained the power of a million exploding suns. This act killed his partner and the lab's guards. He then asked the Professor to make more for himself.
Project: Sentry, appalled at what their irresponsibility had wrought, persuaded Reynolds to let them test his powers at their facility, but they could not contain him for long, nor curb his desire to use his powers for the greater good of all, and the Sentry debuted before a world that had almost forgotten superhuman Samaritans. After trouncing the school bully who had tormented him, Reynolds sewed together a costume and made his debut as the heroic Sentry.
The Sentry was an optimistic and socially accepted hero who stood in marked contrast to the mostly freakish nature of the other heroes. He had connections to Mister Fantastic, Iron Man, the Hulk, Spider-Man and Professor X. One of very few super heroes active during the years just prior to the Fantastic Four's emergence, Sentry gained new importance when the new wave of heroes rose to prominence. Almost instantly deducing Spider-Man's secret identity, he became a role model for the young hero, as well as an ally to the X-Men, an equal to Reed Richards, and a friend to the outcast Hulk. The Sentry even battled Doctor Doom alongside the Fantastic Four and defeated his greatest enemy, the General, with the X-Men's aid.
During this period, the Sentry married the love of his life, Lindy, and took the young Scout as his sidekick. Lindy got swept up in it all, excited at the prospect of meeting other heroes. She eventually learned of Robert's addiction but did nothing about it. She even feared that taking the serum must have awakened something of biblical proportions. She later stated that she should have killed him on their wedding night.
The arrival of the Void, a shadowy monster that exploited its enemy's greatest fears, ended the Sentry's charmed life. The Void nearly killed Scout, drove Hulk into a rampage, and murdered over one million people in Manhattan. Discovering that the Void had been the dark aspect of his own powers, the Sentry teamed with Reed Richards and Dr. Strange to create a system which made Earth's entire population -- including Richards, Strange, and the Sentry -- forget his existence. With the Sentry inactive and forgotten, the Void vanished.
In reality the Void was Reynolds' own repressed persona, powered by the Sentry's own powers. The General had captured Reynolds and brought him to mutant illusionist Mastermind, who implanted a "mental virus" in the Sentry's mind. This virus created delusions that if the Sentry used his powers, a devil would appear, intent on destroying the world.
As a subconscious cry for help, Reynolds implanted his memories into the mind of comic book writer Paul Jenkins, who wrote the Sentry's repressed memories into a series of comic books.
Years later, Robert's memories returned, and along with them the Void, who rampaged across Europe, murdering many members of the Super Heroes of Europe (SHE). The Sentry contacted his former colleagues, but the only one who recalled their shared history was the Hulk. As the Void returned to Manhattan, the pieces began to fall in place and the Sentry's former friends assembled to defend him and the city. Remembering the dual nature of Robert's powers, Sentry and Reed Richards reactivated the Watchtower, dispelling the Void and making the world forget the Sentry once again.
Later, however, Robert's memories of the Sentry again returned, and he demanded to be placed on the Raft, S.H.I.E.L.D.'s high-security prison for super-villains, claiming that he had killed his wife. Lawyers Matt Murdock and Foggy Nelson, along with bodyguard Luke Cage and S.H.I.E.L.D. agent Spider-Woman, traveled to the Raft to visit the Sentry in order to find out what had happened to him and his wife. Soon after their arrival the supervillain Electro staged a massive jailbreak, freeing almost ninety supervillains who then attacked Murdock, Nelson, Cage and Drew, along with the newly arrived Captain America and Spider-Man. During the ensuing melee the group was confronted by the alien symbiote, Carnage. Foggy Nelson, thrown into the cell containing Robert Reynolds, attempted to convince the Sentry to help them. Without saying a word, the Sentry blasted through nine levels of the complex carrying Carnage into space, where he ripped the alien symbiote in half.
Inspired by this adventure, the other heroes soon formed the new Avengers, but Sentry vanished before they could invite him to join them. The Sentry stayed in hiding, but remained a person of great interest to S.H.I.E.L.D. and the Avengers, both of whom were fascinated by his powers and worried that they may lead to another Scarlet Witch situation.
Eventually, S.H.I.E.L.D. and the New Avengersconfronted the Sentry with several disturbing facts: the wife he claimed to have killed was alive, and the only reference in the world that could be found of the Sentry was in comic books, stories invented and written down by the imagination of a comics writer. Unable to face these shocking events, the Sentry fled. Resurfacing as Robert Reynolds at his home, the Sentry was again confronted by the Avengers and S.H.I.E.L.D., along with the Inhumans and the X-Men. He cried, saying that he had warned them of the Void's coming and that it was now too late. The group was suddenly attacked by the Void. Robert, along with the help of Emma Frost, discovered that the reason no one remembered him was not because of his own attempts to erase the Void, but because of Mastermind's manipulation. With this new-found knowledge, the Void disappeared, and Emma Frost resurrected the Sentry. The New Avengers offered him membership, partly to keep an eye on him and partly because they would need his power, which he accepted. The Sentry's headquarters the Watchtowerreappeared on top of Stark Tower, where it supposedly had been originally built.
Later, Yelena Belova attacked the Avengers and absorbed the Sentry's powers. After Belova defeated each of the Avengers, she was defeated by the manifestation of the Void, which enveloped and incapacitated her. The Sentry told Belova that absorbing his powers had exposed her to the Void, but if she answered his questions, he could send the Void away. A.I.M. liquefied Belova before she should divulge any info.
Later still, the Sentry was sent by S.H.I.E.L.D. to subdue a rogue Iron Man. During the course of the battle, Iron Man hacked into CLOC - the Sentry's robot prioritizing and warning system - and fed alerts of multiple false emergencies into the Sentry's head, leaving him incapacitated for a few hours.
Looking for the Truth
Having returned to the world, the Sentry captured the hearts of the public, with newspapers referring to him as "the Golden Guardian", and he saved hundreds of lives on a daily basis; Reynolds' psychological problems, however, had worsened. Unable to reconcile that Robert Reynolds, the Sentry and the Void were the same being, the Sentry contained the Void in a vault in the Watchtower. At C.L.O.C.'s urging, Reynolds' psychiatrist Dr. Cornelius Worth entered the vault and found only a chair and a mirror. When Cornelius confronted Reynolds with this, Reynolds became confused and ran to the fairgrounds where he first gained his powers.
Cornelius followed Reynolds to the fairgrounds where the Sentry and the Void had been fighting. The Void claimed that Reynolds transformed into the Void rather than the Sentry, and that the Sentry was merely a guilt-borne byproduct of that transformation. With only half of Robert Reynolds' memories (the Void possessing the other half), the Sentry was not sure the Void was wrong. The Sentry chased the Void away and then explained to Cornelius why Reynolds had hired him in the first place. Reynolds subconsciously wanted someone to reveal the Void's existence to the world so he would no longer have to hide that aspect of himself. The Sentry told Cornelius how Reynolds really gained his powers as a teenager: by stealing the Professor's Golden Sentry Serum and ingesting it to get high. He also told Cornelius that he knew all along that the Void was never in the Watchtower's vault. He knew that if Reynolds believed that he believed the Void to be in the Watchtower, the Void would be there. This kept the world safe from the Void for as long as the Sentry could keep up the charade.
The Sentry then visited Dr. Strange to determine if he was real or not. Strange tricked the Sentry into traveling to his old Professor's research facility where the two men confined the Sentry in an illusionary realm where he was a mental patient and superheroes didn't exist in an attempt to prevent any further destruction. The Sentry broke free of the illusion and after confronting the Professor who made the original serum, he traveled to Antarctica to confront the Void and discover what memories of Reynolds the Void has in his mind.
The Void claimed that Reynolds had actually ingested a super-saturated, exponentially more potent, version of the Super-Soldier formula that created Captain America. This was considered dangerous by the government because the Sentry's blood could be used to create more of the serum, perhaps enough for the entire world. Several failed attempts were made to kill him. Enraged by this revelation, the Sentry threw the Void into the Sun, telling his enemy that he no longer needed him to balance his own actions of good. The Void promised to return.
During the Superhuman Civil War, the Sentry sided with Iron Man's Pro-Registration program. While trying to escape from a battle between the New Avengers, believing that every path he could choose would ultimately lead to the death of people he knew, the Sentry flew to the moon. There he was confronted by the Inhumans; believed a threat, he was ordered to follow them to Black Bolt's presence in the so-called "Blue Area" on the Moon. Then, after a discussion of the Civil War events with the (still unaware) Inhumans, he rekindled his friendship with them and almost resumed his past relationship with Crystal. He was then confronted by Iron Man himself, who finally convinced a still reluctant Sentry to join him, stating that he'll help Captain America, who is opposed to registration, understand that the world is changing. He later accompanied a S.H.I.E.L.D. squad to battle Wolverine and told him that he didn't want to get involved but saw no choice - he claimed that he had to stop the ugly business even if that meant becoming part of it for a while. He then knocked Wolverine unconscious and handed him over to S.H.I.E.L.D. Three days after the climactic battle of the Civil War, the Sentry publicly announced his support of the Registration Act. Additionally, he was recruited by Tony Stark to be part of the Mighty Avengers, the newest incarnation of the Avengers team.
While at first there was some dispute between the Sentry and his wife, Robert joined the team while Tony Stark and Ms. Marvel offered him assistance to battle his mental issues. He was seen as the most powerful member of the team, but, lacking in proper training on how to use his abilities, he (consequently) frequently apologized for his mistakes (such as causing damage to the city building and then again when he was thrown into a blimp). When Iron Man morphed into the new female Ultron, she analyzed the Sentry, stated his high power level and pointed out he was very difficult to terminate. He battled Ultron, with neither one seeming to be able to win, until Sentry had to break off to stop the S.H.I.E.L.D. Helicarrier from crashing into Manhattan.
Ultron then initiated a "Plan B" which involved killing Lindy, the Sentry's wife. An enraged Sentry attacked Ultron once more, but in an exchange of blows was knocked away. His teammates Ares and Hank Pym in his Ant-Man costume proceeded to infest Ultron with a virus intent on destroying it and allowing Iron Man to reform himself. Sentry however, assaulted Ultron in retaliation for murdering his wife and almost compromised the Avengers plan by nearly tearing Ultron's head off. Before he could finish, he was knocked away by Ms. Marvel. After Ultron's defeat he returned to the Watchtower to find his wife, Lindy, alive and well - having apparently revived her himself after touching her. Robert was later shocked when he heard his wife, in a terrified state, secretly requesting that Iron Man find a way to either degrade her husband's powers or kill him before he would one day succumb to his insanity and possibly kill everybody else.
The next day, a symbiote attack infected everyone in New York, including a few of the Mighty and Secret Avengers, with the exception of Sentry, Wonder Man, Ares, Ms. Marvel, Luke Cage and Iron Man. Sentry eventually had no choice but to knock out Janet, who had grown to giant size and was infected. When Iron Man managed to create an antidote, they track the symbiotes from Latveria, so the Avengers and S.H.I.E.L.D. led an attack against Victor von Doom. Unfortunately, Sentry, Iron Man and Doom accidentally go back in time. To get back, they needed to use the Fantastic Four's time platform. That mostly required Sentry, because whatever he does any-when will be eventually forgotten.
When the Hulk and his Warbound attacked Earth, Sentry talked to Reed Richards and Tony Stark. Both men were able to convince Robert to fight off the Hulk upon his return to Earth, but not before the Sentry had a talk with Iron Man. Iron Man explained the Illuminati's actions in exiling the Hulk from Earth stating that no matter how strong a hero was, they would always be limited in preventing the criminals they fought from repeating their actions. The only solution they found was avoiding the law and deciding the criminal's fate themselves in order to save more lives. A confrontation with the Hulk and Iron Man caused Sentry's Watchtower to drop straight down through Stark Tower/Avengers headquarters, destroying it. Later in a confrontation between the Hulk and the Fantastic Four, Susan Storm tried to call the Sentry for help, but he did not answer the call. The President also attempted to convince the Sentry to fight the Hulk, but failed. The Sentry watched the battle between the Illuminati (minus Professor X and Namor) and a gigantic monster on the news as most of the Hulk's actions, as well as the recent events, were televised. The Illuminati were victorious, but the Hulk modified the obedience disks implanted in them, making them want to kill each other. After seeing this, the Sentry flew out of his home to face the Hulk.
Mister Fantastic was ordered to kill Iron Man, but was stopped by the Hulk declaring that his return was for justice. Sentry arrived and engaged the Hulk in combat. Robert was no longer holding back, having lost control of his powers while going into battle being in a frail mental state. After a while, he began to lose control of them. Sentry unleashed forces that nearly destroyed what was left of New York, but ultimately, the battle between the two titans was ended after they burned each other out at the end of the battle. With the Sentry reverting back to Robert Reynolds and the Hulk back into Bruce Banner.