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In 965 A.D., Odin, king of Asgard, defeats the Frost Giants of Jotunheim and their king, Laufey, and takes the Casket of Ancient Winters, the source of their power. More than a thousand years later, his son Thor is about to be crowned. The ceremony is cut short when Frost Giants break into Asgard's vault. It later turns out that Loki, Thor's brother, let them in.
Furious, Thor defies his father and takes Loki, the warrior Sif and the Warriors Three to Jotunheim to settle the score. The confrontation turns into a battle and Odin has to rescue them, breaking the truce between the realms. As punishment, Odin strips Thor of his powers and banishes him to Earth. He also throws down the hammer Mjolnir with an enchantment: only someone worthy can lift it.
In New Mexico, Thor gets hit by the van of astrophysicist Jane Foster, who is studying strange lights in the sky with her assistant Darcy Lewis and mentor Erik Selvig. He acts like a prince in a world he doesn't understand, smashing a mug in a diner and asking for another. The hammer lands in the desert and becomes a local attraction until S.H.I.E.L.D., led by Agent Phil Coulson, seals off the site and seizes Jane's research. Thor breaks into the compound, reaches the hammer and can't lift it. Crushed, he accepts his exile and grows closer to Jane.
In Asgard, Loki learns he is Laufey's son, a Frost Giant Odin adopted as a baby. Odin falls into the Odinsleep and Loki takes the throne. He lies to Thor that their father is dead and that he can never come home. Then he offers Laufey the chance to kill Odin. Sif and the Warriors Three travel to Earth to bring Thor back, and Loki sends the Destroyer, a living suit of armor, to finish them. Thor offers himself up to save the townspeople. That act makes him worthy: the hammer returns, his powers return, and he takes down the Destroyer.
In Asgard, Loki lets Laufey get close to Odin and then kills him, posing as the hero. His real plan is to wipe out Jotunheim with the Bifrost bridge. Thor returns, fights his brother and breaks the bridge with his hammer to save the enemy realm, knowing it cuts off his way back to Jane. Odin wakes up and catches both sons at the edge of the abyss. Loki sees he will never win his father's approval and lets go. Thor admits he isn't ready to be king. On Earth, Jane keeps looking for a way to reach him. After the credits, Nick Fury shows Selvig a mysterious cube, and Loki is there, unseen, controlling the scientist.