In the realm of Asgard, a world of gods connected to Earth by a rainbow bridge called the Bifrost, the mighty Odin prepares to pass the throne to his firstborn son, Thor. But Thor is arrogant, brash, and eager for battle. When Frost Giants from Jotunheim infiltrate Asgard's weapons vault during the coronation, Thor defies his father's orders and leads a small group—including his brother Loki and friends Sif and the Warriors Three—to Jotunheim to demand answers.
The confrontation quickly turns violent. Thor's reckless attack reignites an ancient war between Asgard and Jotunheim that Odin had ended centuries ago at great cost. Odin arrives to save them but is furious at Thor's arrogance. He strips Thor of his powers and his enchanted hammer Mjolnir, banishing him to Earth. Odin casts a spell on Mjolnir: "Whosoever holds this hammer, if he be worthy, shall possess the power of Thor."
Thor lands in New Mexico, where he's hit by a van driven by astrophysicist Jane Foster, her mentor Erik Selvig, and assistant Darcy Lewis. Disoriented and powerless for the first time in his life, Thor struggles to understand the mortal world. Meanwhile, Mjolnir has also fallen to Earth, creating a crater that draws the attention of S.H.I.E.L.D., who quarantine the area.
In Asgard, Loki makes a shocking discovery: he is not Odin's biological son but a Frost Giant, adopted as an infant after the war. The revelation shatters his sense of identity. When Odin falls into a deep restorative sleep called the Odinsleep, Loki takes the throne, refusing to end Thor's banishment.
Thor attempts to retrieve Mjolnir from the S.H.I.E.L.D. facility, fighting through agents to reach it—only to find he cannot lift it. He is not worthy. Captured and interrogated by Agent Coulson, Thor is eventually freed when Selvig vouches for him. Defeated and humbled, Thor begins to change, falling in love with Jane and learning what it truly means to protect others rather than seek glory.
Loki visits Thor, lying that Odin is dead and that Thor's banishment can never be lifted. Believing he has nothing left, Thor accepts his fate on Earth. But Loki's schemes grow darker: he makes a secret deal with the Frost Giants to let them kill Odin, planning to betray them at the last moment and appear as Asgard's savior.
When Loki sends the Destroyer, an indestructible automaton, to Earth to kill Thor, Thor's friends defy Loki's orders and travel to warn him. Thor faces the Destroyer without his powers, offering his life to save the town of Puente Antiguo and the people he's come to care about. The Destroyer strikes him down—but this selfless sacrifice proves Thor worthy. Mjolnir flies to his hand, restoring his powers.
Thor defeats the Destroyer and returns to Asgard via the Bifrost, arriving just as Loki has killed the Frost Giant king and turned the Bifrost into a weapon to destroy Jotunheim entirely. Thor is forced to destroy the Bifrost to stop the genocide, stranding himself in Asgard, separated from Jane. In the battle's aftermath, Loki chooses to fall into the void rather than face justice, apparently dying.
Thor reconciles with his father but mourns his brother. On Earth, Jane searches for a way to reopen the bridge between worlds. In a post-credits scene, Selvig is brought to a S.H.I.E.L.D. facility where Nick Fury shows him a glowing blue cube of immense power: the Tesseract. In a reflection, Loki is visible, influencing Selvig's mind and whispering, "Well, I guess that's worth a look."