Avengers: Dissembled , House of M, Civil War, Secret Invasion, and Dark Reign have been building up to the Siege.
Following "The List", Norman Osborn saw Asgard as the last stopping point for his reign. Norman's intentions are clear, to remove Asgard so he can consolidate his power.
This brings the reunion of the Avenger's main three: Captain America, Iron Man and Thor. Showing the beginning of a new "Heroic Age" that was first hinted by Eternity to Hank Pym.
Doctor Doom opens his doors to Asgardian Gods in Latveria, which angers Thor.
Loki joins Osborn's evil cabal with one intent , to become the ruler of Asgard. Norman solicits the President for an invasion of Asgard , and is infuriated by the President's refusal to invade. Loki goes out of his way to arrange some of the Hood's flunkies to create a disastrous situation. Volstagg, while roaming the Earth, ends up in Chicago, which is ironically, the home town of the President of the United States. Members of the Hood's gang attack Volstagg, resulting in an explosion in Soldier Field. finally has the opportunity he has been waiting for. Without the President's approval, he invades Asgard with his Avengers, members from the Hood's gang, the Thunderbolts, and a horde of evil. Once the invasion starts, Loki goes to Balder to "warn" him of the incoming attack. Loki mentions he tried to stop Osborn, but he is too mad and corrupt to listen. Balder does not believe Loki, thinking that he is playing another one of his tricks. Just when Balder says he could not think of a human that could threaten the Asgardian Gods, the Sentry flies through the walls, causing the castle begin to cave in. The next scene is Norman Osborn, with his Dark Avengers and an immense amount of backup heading for Asgard. The people down in Oklahoma begin to flee, knowing that this day would eventually come. Dr. Donald Blake, and Maria Hill are looking after Tony Stark in his hospital bed. Hill looks out the window and sees Osborn's minions blanketing the sky. Blake realizes that he is not there for Stark, but to attack Asgard. Hill orders the transport of Stark immediately, then runs outside, and then transforms into Thor. Thor gets close to striking, but the Sentry throws him out the opposite end of the castle. Osborn and his evil minions begin a vicious assault on Thor, beating him unconscious. All this is caught on live on t.v., and is seen by Steve Rogers, who is immediately readies for battle as Captain America.
At Asgard, while Thor is being beaten and bloodied, Norman Osborn begins to put Thor under arrest. Suddenly a missile strikes Osborn, hurling him miles away from Thor and his Avengers. The missile was fired by Maria Hill. She begins firing at Moonstone, and the Hood's gang. Moonstone gets closer to Hill until Thor smacks Moonstone away with his hammer. Hill and Jason, a truck driver assisting her, help Thor to safety. Meanwhile at the Avengers hideout, Captain America holds a meeting with the Avengers including Nick Fury, Stature, Speed, Stonewall, Slingshot, and many other heroes.
On Asgard, Ares is battleing Balder, and tells him he is there because Loki had taken control of Asgard. Heimdall informs Ares him that is not the case. Learning the truth, Ares, realizes he must kill Norman Osborn. Osborn's lie forced Ares into battle against his fellow Asgardians.. Osborn sends the Sentry after Ares. The battle ensues, but the Sentry is too much for Ares, and he is gruesomely torn in half.
In Brooklyn, as the Avengers prepare to go to battle, Bucky returns Steve Rodgers' shield to him, saying the world needs Captain America. Nick Fury tells Phobos he needs to stay out of the battle because of the severity of the situation.
In the Bronx, Daken is tracking Maria Hill's scent, and then begins to switch to look for Osborn. He finds him, and Thor fries him with lightning. Osborn is then struck by a fast incoming Captain America.
In Washington, the President is shown footage of Osborn's attack on Asgard above Oklahoma. He and the rest of the leaders decide to arrest Osborn for treason. The military is sent to Broxton to take down Osborn's army. Captain America is leading tthe Avengers to assist the Asgardian Gods. The Hood's Gang arrives, with The Hood being given the Norn Stones allowing him to increase his troops' power. The Sentry attacks just as Thor was beginning to get back on his feet. Thor swears he will defend Asgard with his life, while the Sentry allows the Void to take over. Osborn ordered Sentry to defeat Thor at any cost, so the Void could achieve a greater demonic form, and completely destroys Asgard. As Osborn is fighting Captain America, his armor system begins to be corrupted by Tony Stark who is wearing his classic armor from the 70's. All of the sudden, Osborn's armor breaks into pieces, shattering everywhere, revealing a Green Goblin mask. Osborn continues to say he was the only one who knew of the Sentry's ultimate power, and how to defeat him. Osborn says the heroes were doomed because of the where the Sentry's power levels were at. At that moment, the Void overtakes the skies above him, and all the surrounding heroes.
As the Void starts attacking the heroes, Loki realizes that what he did was wrong. He only wanted the throne of Asgard, and never expected the mortals to have such power. Loki then takes the Stones of Norn from the Hood and uses them to empower the heroes. The Void notices this, and attacks Loki. Just before he dies, Loki apologizes to Thor. Inspired by his brother, Thor starts mercilessly attacking the Void. Iron Man then asks Thor to get the Void out to the fields, and Thor does so, allowing Iron Man to bring the HAMMER Helicarrier down on the Void. This causes the Void to revert to Robert Reynolds, who begs the heroes to kill him. Thor at first refuses, wanting Bob to pay for his crimes, but Bob then starts turning back into the Void. Having no other choice, Thor then brings a massive lightning bolt down on Bob, killing him once and for all. Upon Bob dying, the Watchtower at Avengers Tower disappears. Thor then wraps Bob's body up in his cape, and flies into space. He then throws Bob's body into the sun.
In the aftermath of Siege, Norman's troops are captured and placed under arrest. Steve Rogers gives his shield to Bucky, saying that he should be Captain America. Afterwards Rogers is taken to the President's office, where he has a list of demands. One of them is for the Super-Human Registration Act to be abolished; the President complies.
The heroes then all celebrate at Avengers Tower, when a group of Asgardians, including Thor, arrive. Heimdall connects his watchtower to the top of Avengers Tower, signifying a new alliance between Earth and Asgard.