In the 2000 limited series The Sentry, and its related one-shots, Bob Reynolds remembers that he is the Sentry, a superhero whose "power of one million exploding suns" derives from a special serum. Realizing that his arch-nemesis the Void is returning, Reynolds seeks out several prominent Marvel characters to warn them and to discover why no one remembers the Sentry.
The characters' memories of the Sentry and the Void resurface when Reynolds talks with them. The Sentry had taught Angel how to conquer his fear of falling. Peter Parker's photograph of the Sentry earned him a Pulitzer Prize and fame. The Hulk had never forgotten the Sentry, whom he called "Golden Man". Under the Sentry's influence, the Hulk had been a force for good which had redeemed his violent actions and won the adoration of the public. Reed Richards remembers the Sentry was his best friend and that the Fantastic Four had teamed up with him on many adventures. Meanwhile the general public themselves gradually come to remember the Sentry as does Reynolds' old sidekick, Billy Turner, formerly known as the Scout.
During the course of his investigation, Reynolds and Reed Richards discover what had happened: as the Void had threatened the Earth, the heroes learned that the Sentry and the Void were two halves of the same person, and in order to save the world, Robert Reynolds erased his memory from the mind of nearly every person on Earth. As the heroes stand along the United States' East Coast, united against the coming Void, Reynolds realizes that he must make the sacrifice again, and with the help of his mechanical servant CLOC, Richards, and Dr. Strange, Reynolds erases the Sentry from the world's memories once more.
In 2004's New Avengers #1, Reynolds re-appears inside the supervillain prison the Raft, voluntarily imprisoned for murdering his wife Lindy Lee. During a massive jail break in which several Marvel superheroes are caught, the Sentry defends several other characters from Carnage, whom he flies to space and rips in half.
Eventually, the Avengers learn that Mastermind, under the direction of Sentry's enemy the crazed General, implanted a virus in Reynolds' mind that created delusions and the existence of the Void, which is actually Reynolds' repressed persona. The mental virus impairs Reynolds' ability to remember life, and, as a cry for help, he subconsciously implants his memories into the mind of comic book writer Paul Jenkins, who then transferred those memories to comic books. Emma Frost releases Reynolds from the virus, and the Sentry joins the Avengers (though the world no longer remembers him or the Void) and discovers that he had never killed his wife. Additionally, the Sentry's Watchtower appears atop the Stark Tower, where it had been all along.
In the 2005 Sentry limited series by Paul Jenkins and John Romita, Jr., the Sentry has captured the hearts of the public, newspapers refer to him as "the Golden Guardian of Good", and he saves hundreds of lives on a daily basis; Reynolds' psychological problems, however, have worsened. Unable to reconcile that Robert Reynolds, the Sentry and the Void are the same being, the Sentry contains the Void in a vault in the Watchtower. At C.L.O.C.'s urging, Reynolds' psychiatrist Dr. Cornelius Worth enters the vault and finds only a chair and a mirror. When Cornelius confronts Reynolds with this, Reynolds becomes confused and runs to the fairgrounds where he first gained his powers.
Cornelius follows Reynolds to the fairgrounds where the Sentry and the Void have been fighting. The Void claims that Reynolds transforms into him rather than the Sentry: the Sentry is merely a guilt-borne byproduct. With only half of Robert Reynolds' memories (the Void possessing the other half), the Sentry isn't sure the Void is wrong. The Sentry chases the Void away and then explains to Cornelius that Reynolds hired him because he 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 then tells Cornelius how Reynolds really gained his powers as a teenager: by stealing the Professor's serum and ingesting it to get high. He further reveals he knew all along that the Void was never really in the Watchtower's vault, but if Reynolds believed the Void to be there then the Void would be dormant.
In a final battle at Antarctica, the Void claims 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, enough for the entire world. Several failed attempts were made to kill him. Enraged by this revelation, the Sentry throws the Void into the Sun, telling his enemy that he no longer needs him to balance his own actions of good. The Void promises to return.
In New Avengers Annual #1, Yelena Belova attacks the Avengers and absorbs the Sentry's powers. After Belova defeats each of the Avengers, she is defeated by the manifestation of the Void, which envelops and incapacitates her. The Sentry tells Belova that absorbing his powers has exposed her to the Void, but if she answers his questions, he can send the Void away.
The Sentry continues to be troubled by his psychological issues, including his fear of the Void. In New Avengers #17, Captain America has to give the Sentry a pep talk before he can work up the nerve to join a fight.
In Iron Man (vol.4) #9-10, the U.S. government sends Sentry to apprehend Iron Man, who has been mind-controlled to assassinate a number of high-profile former terrorists. Unable to find any physical weaknesses or outrun the Sentry, Iron Man attacks the Sentry's mind; he remotely hacks C.L.O.C. and has Sentry barraged with unfiltered warnings about multiple devastating disasters occurring simultaneously throughout the world. Unable to prioritize which alarm to deal with first, Sentry collapses to the ground in tears, utterly incapacitated.
Trying to escape from the battle, believing that every path he can choose will ultimately lead to the death of people he knows (one of his thoughts at this point consists of himself and Hulk triumphantly returning to Earth and 'ending' the war via killing all the heroes) Sentry is confronted by the Inhumans. Believed a threat, he is ordered to follow them to Black Bolt's presence. Then, after a discussion of the Civil War events with the (still unaware) Inhumans, he rekindles his friendship with them and almost resumes his past relationship with Crystal. He is then confronted by Iron Man himself, who finally convinces a still reluctant Sentry to join him.
It is stated that the Sentry publicly announces his support of the Registration Act three days after the climactic battle of the Civil War limited series.
Later in a confrontation between the Hulk and the Fantastic Four, Sue Storm tries to call the real Sentry for help, but he does not answer the call and is sitting in his apartment watching television. The President of the United States tries to convince the Sentry to fight against his longtime friend the Hulk. It is still unclear in this issue whether or not he will take the call.
After the Hulk turns Madison Square Garden into a gladiatorial arena and forces Mister Fantastic, Doctor Strange, Black Bolt, and Iron Man to fight one another, the military turns to the Sentry for help once again. The Sentry admits to his fear of his tremendous power mixed with his agoraphobia, stating,
The Sentry engages the Hulk and unleashes his power. During the prolonged fight they both expend massive amounts of energy. In the last few seconds they both revert to human form. After Bruce delivers the final punch, the broken-faced Robert Reynolds thanks Bruce before immediately collapsing before his feet.
The Sentry is recruited by Tony Stark to be part of the Mighty Avengers, the newest incarnation of the Avengers team. While at first there is some dispute between the Sentry and his wife, Robert joins the team while Tony Stark and Ms. Marvel offer him assistance to battle his mental issues. He is described to be the most powerful member of the team, but lacks proper training on how to use his abilities, usually apologizing for his mistakes (apologizing for damage to a building and being thrown into a blimp in #1).
In the battle against Ultron the two prove to be evenly matched. Neither is able to win until Ultron uses a virus to down Stark's helicarrier. Ultron then initiates "Plan B" and kills Lindy, the Sentry's wife. '' An enraged Sentry attacks Ultron once more. In an exchange of blows Sentry is knocked away as Ares and Ant-Man proceed to infect Ultron with a virus intended to destroy it. Soon after, Sentry once again attacks Ultron, almost compromising the Avengers plan, nearly destroying Ultron by tearing its head off. Before he can finish, he is knocked away by Ms. Marvel. After Ultron's defeat he returns to the Watchtower to find his wife, Lindy, alive and well — having apparently revived her himself. Stark is later shocked when a terrified Lindy secretly requests that he find a way to either de-power or kill her husband.
The Sentry then aids the team when they attack Latveria but ends up stuck in the past with Dr. Doom and Tony. The Sentry finds them and Stark sends Rob into the Baxter Building so that they can use Mister Fantastic's time machine; since all memory of his past actions were erased by his 'spell', he can do anything in the past and be sure that it won't impact on the present. The Sentry gets to it, first having to deal with the Thing. After he returns to present time along with Iron Man, he finds the rest of the Avengers engaged in battle with Doctor Doom. The Sentry quickly subdues Doom, who is then taken into S.H.I.E.L.D. custody.
When a Skrull spacecraft is discovered approaching Earth's atmosphere, the Mighty Avengers and New Avengers simultaneously head to its predicted crash area in the Savage Land. Inside are numerous superheroes dressed in past regalia. When fighting one who looks like the Vision, it transforms into the Void, blaming the entire situation on the Sentry's hidden desires. Panicking, the Sentry flees. At the same time, a full-scale Skrull invasion begins, with one Skrull attacking the Watchtower where Lindy is. Before he can attack however, the Void appears and defends Lindy and tells her that the Sentry can't handle the situation and that "Whatever he can't do, I can."
The Sentry's powers derive from a serum that moves his molecules an instant ahead of the current timeline. Even though most of his powers and their limits are unknown, he displays several abilities including vast superhuman strength, speed, stamina, durability and senses, the power of flight, and energy projection. The Sentry has been shown to be able to easily lift tremendous weights, but generally greatly restrains his full power. He has been able to effortlessly defeat, and break the axe of Terrax, a herald of Galactus shown as powerful enough to slice planets in two,, subdue, and easily break through the shields of, Dr. Doom, and his unrestrained power even overloaded the Absorbing Man. His speed is great enough to catch bullets, and through flight he can travel to the sun and back in a matter of minutes. Nick Fury has stated that S.H.I.E.L.D. has not yet found a way to kill the Sentry, and Iron Man's scanners have found no physical weaknesses in his body. His durability rivals that of Thor, and Hercules. By unleashing his full potential he managed to fight an extremely enraged Hulk for a prolonged time until both fighters reverted to their human forms. The Sentry also has superhuman senses as he once tells an opponent that he can see his nerve centers and claimed to be able to "hear a butterfly sneeze in Central Africa" from New York City. He can project energy fields and control light, and he uses his mental powers in his daily life, although they are primarily used for holding his physical form together. He was once able to implant his memories inside another person's mind. He appears to be able to bring the dead back to life under extreme emotional distress. After Ultron murdered his wife, Sentry was able to resuscitate her by simply touching her. He can also emit a radiation that pacifies the Hulk, severely limiting the latter's rage-fueled power. His main weakness may be that he's been shown as susceptible to mental manipulation.
Sentry possesses tremendous energy projection, capable of harming even the Hulk, who has withstood the equivalent of solar flares unharmed. The serum he drank was stated as being a hundred thousand times stronger than the original used on Captain America, and was modified by Weapon X.
His Void 'counterpart'/nemesis possesses the ability to shape-shift, and through its control over the weather and darkness it can create destructive storms and deadly "infini-tendrils" attacking the mind. Victims impaled on the tendrils experience traumatic visions of the past, present, and future. Its regular appearance varies between a shadowy, trench coat-wearing villain to a massive hurricane of darkness. It can also assume powers dependent on shape, like a flame form that breathes fire, alternately an armored monster with super strength and toughness. It is at its strongest during the night and in the Negative Zone. Coincidentally, Sentry is at his weakest in the negative zone, although he can still withstand blasts of N-Zone energy.
In Marvel Zombies vs. The Army of Darkness, it is revealed that The zombie Sentry was undead in the afterlife, eating everyone that came into the "light" and knocking Ash into the not yet infected world of superheroes. The Sentry finds its way through the same portal and infects the Avengers.
Sentry is also featured in the 12th wave of Marvel Minimates.
Danish indie band Waking Life, who in 2007 won the Emergenza Festival DK, wrote two songs called "Sentry" and "Void" about the hero and his struggles with his dark alter ego. The songs can be found on the band's 2005 EP The Awful Courage.