Ever since Secret Invasion, a Dark Reign had fallen over the Marvel Universe. Norman Osborn was simultaneously the head of HAMMER, leader of the Dark Avengers and the secretive Cabal, and had the sinister Dark X-Men under his control. With this much power, Osborn was now more dangerous than ever before. There was only one final thing standing in his way of complete domination of the United States: Asgard, the realm of Thor and Loki that was currently floating over Broxton Oklahoma, and the last place that could openly defy him. Osborn knew he needed to conquer the goldly realm, and with Iron Man in hiding, Steve Rogers dead, Thor banished from his home realm, and the Avengers a shell of their former selves, it seemed that nothing was going to be able to stop him.
To begin his plans, Osborn calls together what remains of the Cabal to discuss the issue. Loki convinces Osborn to create a tragedy involving Asgard so that he can gain approval from the President to invade the realm as a matter of national security. To this end, Loki and Osborn manipulate the Asgardian Volstagg into battling the villains U-Foes, resulting in an accidental explosion at Soldier’s Field in Chicago, killing the thousands of civilians inside. This action gives Osborn all the justification he needs to launch a military assault on Asgard, aided by his Dark Avengers, including the Sentry who has been manipulated into serving Osborn in return for treatment of his insanity. Loki meanwhile works from the shadows, eliminating Heimdall, Asgard’s watchman, to make way for the invasion. Osborn, as Iron Patriot, strike quickly, stunning Asgard’s defenders. Thor tries to intervene, but being greatly outnumbered, falls to the Cabal’s forces. This attracts the attention of the fugitive Avengers, organized by a man the world had thought dead... the original Captain America, STEVE ROGERS! Rogers, who had been lost in time, not dead, formally grants Bucky Barnes the name and shield of Captain America, allowing him to lead the fight against the Dark Avengers.
In the background, Osborn was starting to come under suspicion by his own people, his insanity showing through more and more. Ares, one of the Dark Avengers realizes that Osborn had tricked him into joining the Dark Avengers, and tries to kill him, but Osborn has the Sentry kill him instead. This and other actions leads to public opinion turning against Osborn. Finally, the President realizes how insane Osborn truly is, and orders the US Army to Asgard to arrest Osborn for treason.
The Avengers arrive at Asgard and immediately go on the offensive, despite infighting between Iron Man and Captain America, the two bitterly remembering the Civil War. Once the Avengers and Dark Avengers meet, Captain America goes for Iron Patriot, taking him down before being attacked the Sentry. Thor comes to Cap’s aid, but he is still wounded, and is soon brought to the Sentry’s mercy. Sentry, driven completely mad by the battle, into a being known as the Void, and leaves Thor to destroy the infrastructure of Asgard, bringing it crashing down into the Earth.
In the chaos, Cap finds Osborn and places him under arrest. Osborn tries to resist, arrogantly thinking that the arrest will mean nothing, and he will be a free man soon enough. Iron Man arrives then, and with the entire United States watching on live television, Stark explodes the Iron Patriot armour, revealing Osborn’s face painted to look like the Green Goblin. His true nature revealed, the Void turns on him, and Osborn is left a raving lunatic.
His master vast aside, the Void turns his attention to the Avengers, hammering them into submission. Loki, seeing the horror he had wrought, goes to his father Odin, and repents for his work. Odin gives him the mystical Norn Stones to boost the Avenger’s powers to better fight against the Void. This move turns the tide, and in vengeance, the Void kills Loki. The sight of his brother dead sends Thor into a rage, and with the help of the other Avengers, he beats the Void nearly to death, forcing Void to revert to the Sentry. Sentry, realizing what he has done and the danger he is, begs the heroes to kill him less he become the Void again. Thor reluctantly does so, ending the battle.
In the aftermath, Thor takes the Sentry’s body to the sun while the New Avengers place the surviving members of the Dark Avengers and the Cabal under arrest while Steve Rogers is pardoned of his crimes during the Superhero Civil War, and is appointed the new head of SHIELD. Additionally, Rogers and Tony Stark reconcile, and reform the Avengers with Tony, Bucky, and Thor to spearhead the team. Finally, the Superhero Registration Act is repealed to public approval, and bringing Marvel into a new age.
Siege was the end of long road that had started with Secret War, coming to fruition in the best way possible, heralding the arrival Heroic Age of Marvel Comics. The Avengers were back on top, the heroes of Marvel no longer fighting each other, and the bad guys were cast down. From here, Marvel launched a wave of new titles, building and expanding on the last half-decade of stories. Sure, there would be challenges on the horizon, but for now, the Avengers were back in business!