In 965 AD, Odin, the king of Asgard, battles the Frost Giants of Jotunheim, led by Laufey, to prevent their conquest of the Nine Realms, including Earth. After defeating the Frost Giants, Odin seizes their powerful artifact, the Casket of Ancient Winters. In the present day, Odin’s son Thor is about to ascend to the throne but is disrupted when Frost Giants, secretly allowed in by Thor’s brother Loki, attempt to reclaim the Casket. Defying Odin’s orders, Thor travels to Jotunheim with Loki, Sif, and the Warriors Three to confront Laufey, leading to a battle that destroys the truce between Asgard and the Frost Giants.
As punishment for his arrogance, Odin strips Thor of his godly powers and exiles him to Earth, where his enchanted hammer Mjölnir is placed under a spell that only allows the worthy to wield it. On Earth, Thor encounters astrophysicist Dr. Jane Foster, her assistant Darcy Lewis, and mentor Dr. Erik Selvig. The hammer is discovered by S.H.I.E.L.D. and taken into custody, and Thor, unable to lift it, is captured. With Selvig’s help, Thor escapes and begins to adapt to life on Earth, developing a romance with Foster.
Loki learns that he is actually Laufey’s biological son, adopted by Odin, and becomes the ruler of Asgard while Odin falls into a deep sleep to recover. Loki allies with Laufey, offering him the chance to kill Odin and retrieve the Casket. Meanwhile, Sif and the Warriors Three, displeased with Loki’s rule, persuade Heimdall, the gatekeeper of Bifröst, to allow them to return Thor to Earth. Loki sends the Destroyer to eliminate Thor, who, after being struck by the Destroyer, proves himself worthy of Mjölnir, regaining his powers and defeating the automaton.
Thor returns to Asgard to confront Loki, who has betrayed and killed Laufey and plans to use the Bifröst Bridge to destroy Jotunheim. Thor fights Loki and destroys the Bifröst Bridge to thwart his plan. Odin prevents the brothers from falling into the abyss caused by the bridge’s destruction, but Loki allows himself to fall when Odin rejects him. Thor reconciles with Odin, admitting he is not ready to be king. On Earth, Foster and her team continue searching for a way to open a portal to Asgard.
In a post-credits scene, Selvig is taken to a S.H.I.E.L.D. facility, where Nick Fury presents him with a mysterious cube-shaped object that Fury believes holds immense power. An invisible Loki, who survived his fall to Earth, subtly influences Selvig to agree to study the object, hinting at further conflicts to come.