In Bellvue Hospital, Dr. Donald Blake works to save the life of a young boy, Randy Mossberg, who had been hit by a car. After a successful operation, a female colleague compliments Blake on his impressive work and invites him out for the weekend. Blake, however, responds that he already has plans to go skiing, which confuses her because it’s summer time. When he’s alone, Blake changes to Thor and opens a portal to Asgard.
Next, we see Thor and his companions racing other Asgardians in their iceboats across the frozen Sea of Marmora. As Thor’s boat takes the lead, the race is interrupted by a group of sea serpents who suddenly break through the ice. Thor leaves Sif and the Warriors Three to defend themselves, as he flies over to another ship to defend the less capable. During the struggle, Thor sees an illusion of Sif in peril. When he rushes to save her, the image disappears and Thor is knocked unconscious and falls through a hole in the ice. As soon as he is out, the sea serpents retreat. Sif dives into the water and rescues Thor, but he is seriously injured.
Back in Asgard, Thor is recovering from his wounds. A number of Asgardians were killed, and Thor laments that he couldn’t have saved them all. The Warriors Three help Thor out of bed and walk him to the Odin’s palace. On the way, Thor is alarmed to see a streak of red “Witchfire” above the palace, but when he tells the Warriors Three, they see nothing. Entering the palace, Thor sees the guards as demons, and begins to attack them. The Warriors Three and Sif break up the fight quickly, but after they take Thor back to his palace Bilskirnir (here written as “Silskirnir”) to rest, the guards reveal that when Asgard learns that what Thor saw was real, and everyone else sees is an illusion, it will be too late!
When Thor finally wakes up, Sif has fallen asleep at his bedside. He places her in his bed to sleep, and steps outside only to discover that the sky is now full of Witchfires, but nobody notices. Thor flies to Odin’s palace, fights through the demons who guard his throneroom, and bursts in. Inside, he finds Odin sitting face to face with Dormammu over a chessboard. Above them hover the faces of two cosmic entities: Lord Chaos and Master Order. At this point in the game, Dormammu is winning, so the image of Lord Chaos is massive, compared to the shrinking face of Master Order. Thor cannot communicate with Odin, who is within a mystical force field, so he goes to the Grand Vizier for answers. The Grand Vizier states that the throne room is empty and that Odin is resting, so Thor then goes to the home of Loki (who is bound at the wrist to his wife Sigyn), where Loki claims that he has had nothing to do with Dormammu’s presence in Asgard. Finally, Thor goes to Mimir for answers. Thor asks if Odin can beat Dormammu in the chess game, to which Mimir responds that he cannot, as Dormammu can only be defeated in the Dark Dimension. Thor immediately opens a portal with Mjolnir and disappears. Meanwhile, Odin’s ravens show him what has transpired, and Odin thinks to himself that Thor asked Mimir the wrong question: not can Odin beat Dormammu, but should he. Odin then sends his ravens to awaken Sif and tell her to follow Thor into the Dark Dimension, which she does.
When Sif arrives in the Dark Dimension, she is immediately taken prisoner by Umar, Dormammu’s sister. Elsewhere, Thor defeats one of the phantasmagoric monsters of the Dark Dimension and follows the directions that Mimir implanted into his mind. As Thor wanders through the mystical realm, Umar holds Sif prisoner with the G’Uranthic Guardian. Umar reveals that she was responsible for the illusion that led to Thor’s injury, and eventually led him here, so that he could not interfere with the Great Game. Umar leaves Sif to the mental torture of the guardian, but Sif breaks free, only to face the bizarre hordes of the realm. Nearby, Thor hears Sif’s scream and rushes to her aid, fighting off the demons as she lies unconscious. When the demons are gone, Thor finds not Sif—but Jane Foster! Sif had earlier joined her lifeforce with Jane in an effort to save her, but Jane disappeared into Sif’s psyche.
In that moment, Umar attacks Thor and Jane with numerous spells, which Thor must fight off to protect Jane. After a mystical battle, Umar yields to Thor. She reveals that the easiest way to defeat Dormammu is to force him to leave the Great Game early, thereby forfeiting. Jane, however, begins to regain her memories, and also vaguely recalls that there was some important mission that brought Sif here. Umar tells Thor that releasing the Mindless One from its prison would be more than enough to bring Dormammu back to this realm, so Thor begins to attack the mystical barrier that he is kept in. Meanwhile, Jane is attacked by a nearby demon, but defends herself with Sif’s sword. When she defeats the demon, Jane throws Sif’s sword to block Mjolnir from breaking the Mindless One’s barrier. Jane tells Thor that this is all a trick by Umar to aid in her goal, at which Umar admits her plan: by forcing Dormammu to leave the game, Thor would have made it look like Odin cheated, thereby causing him to forfeit and lose the match. Her plan foiled, Umar once again attacks Thor and Jane.
As Umar casts a spell to destroy them, Thor quickly opens a portal to bring Thor and Jane to safety. They appear on the roof of the World Trade Center, where Jane struggles to understand her true identity: is she Jane, or Sif, or both? Suddenly, she transforms back into Sif, as the effect of the Dark Dimension wears off. Then, Thor and Sif return to Asgard to see the outcome of the Great Game. In the throne room, Odin reveals that the game ended as it must—in a draw. Chaos and Order must always be equal to maintain balance in the cosmos. Finally, Odin gives each Thor and Sif a token to remember the part each played in the Great Game. Sif, who served Odin’s side, receives a white pawn, while Thor, who unwittingly served Dormammu’s side, receives a black pawn.