- Sam Worthington as Thor/Donald Blake
- Benedict Cumberbatch as Loki
- John DiMaggio, James Earl Jones, Steve Blum, and Grey DeLisle voice Mangog
- Thomas Jane as Skurge the Executioner
- John Hurt as Odin
- Amy Adams as Jane Foster
- Stephen Fry as Volstagg
- John Barrowman as Hogun
- Stephen Amell as Fandral
- Anna Paquin as SHIELD Agent Pia Marina
- Matt Dillon as Nick Fury
Thor is the Prince of Asgard and its most celebrated warrior. One day, his father Odin approaches him and tells him that since he is getting old, he will soon appoint Thor as the new king. Loki, Thor's brother, is jealous, and hatches a revenge scheme. A few weeks later, the day before his ceremony, Loki tells Thor that a evil troll named Ulik has escaped the Asgardian prison and is attacking random citizens. However, it is only an illusion conjured by Loki. Not knowing this, Thor goes and fights the monster, causing innocent citizens to die in the battle. Thor then discovers that Odin has been killed, and goes to the Throne room to see the evidence. Moments after he arrives, Loki comes in and accuses Thor of killing Odin, and so Loki gains Odin's power as king. Loki tells Thor that he created an illusion of Ulik, and that Odin isn't dead, but rather captured. Thor is cast to Earth, transforming his body into a that of a 150-pound weakling, and stripping him of his power. However, Odin is revealed to be contained in Asgard's dungeon, held captive by Loki.
Loki then finds Asgard's second best warrior, the Executioner and asks him to help in his plan, to which Executioner agrees. Meanwhile, on Earth, Thor discovers that he has been found by a nurse named Jane Foster. He tries to escape and find Mjolnir, but remembers recent events. He stays at Foster's house and discovers that his idetity is Donald Blake. He sees a criminal rob a bank, and tries to stop him, but is defeated in his feeble form. Loki is shown to be watching. He tells Executioner to go to Earth and kill Thor in case he somehow manages to get back to Asgard.
SHIELD Aent Pia Marina shows up and questions Foster about the mysterious energy portal that brought Thor to Earth because Foster was spotted near it. Marina sees Blake and realizes that he looks similar to the man who fell out of the portal. She starts to question him, but he runs off so she detains him and interrogates him. In jail, he sees Executioner attacking downtown. He tries to escape and pleads to Marina to let him go because he knows how to stop Executioner. She lets him go.
Meanwhile, on Asgard, Loki brings back the souls of the civilians killed in Thor's fight with Loki and combines them into a monster called Mangog. He will use Mangog to help in his conquest of Asgard. He unleashes the creature and the Warriors Threee (Hogun, Volstagg, and Fandral) try to stop it but are failing. We then see Foster taking Blake to where Executioner is. He attempts to fight Skurge but is easily knocked over. Executioner gloats but Thor sees a metal pipe a few feet away and grabs it. He tells the people, including Jane, to evacuate, and he hts Executioner on the head with the pipe. It transforms into Mjolnir and Skurge is knocked hundreds of feet away. Loki sees this but doesn't understand. It is revealed that Odin had sent Mjolnir in disguise to Eath just before his capture , so Thor would have backup.
Thor and Executioner fight until they get to the water supply plant and Thor supercharges the water with lightning, putting Executioner in a coma. Marina then arrests Skurge. Thor tells Jane that he will return, and goes to Asgard, where he finds Mangog. They fight and Thor sends the Warriors to free Odin which they do. However, Loki still has the power of the king. The battle spills over to Earth and the Bifrost is destroyed in the process. Thor and Mangog continue to fight, and Loki (who also came to Earth) uses his power to kidnap Jane, Marina, and 8 others as hostages. He also destroys buildings at random and proclaims himself king of Earth and Asgard. He tells Thor to choose between saving the hostages or stopping Mangog. Thor saves the hostages, flies them to a SHIELD base, and returns. He tells Loki that since Bifrost is destroyed, it is now a portal that could send them anywhere. Thor also tells Mangog that Loki engineered the beast that killed them. They then gra Loki and Thor creates a wind tunnel that sends them through the portal. Mangog seperates and the individual spirit find peace in Valhalla, while Loki is trapped in the core of Jotunheim, the ice realm. Because Loki is now trapped, his power goes back to the dying Odin, bringing him back to full health. The Asgardians close the portal from their end, and set about to rebuild Bifrost. Thor is trapped on Earth, but he can change between Blake and Thor at will, plus he now has Jane, and the move in together and start dating.
In a post-credit scene, Marina is shown telling the story to a shadowy figure, who is revealed to be Nick Fury.