Three scientists—Jane Foster, Erik Selvig (Stellan Skarsgard), and Darcy Lewis (Kat Dennings)—are traveling in a 4WD van through New Mexico when they spot a lightning storm brewing. Jane believes she can predict the appearance of the Einstein-Rosen Bridge, which is essentially a wormhole. Their journey takes a dramatic turn when they accidentally hit a well-built, semi-conscious man caught in the storm.
Flashback to 965 AD, the Frost Giants invade Norway, aiming to use the Tesseract's immense power to turn Earth into a frozen wasteland. The Asgardians, led by Odin, rally their armies and join forces with the humans to push back against the Frost Giants. As they wage war, the Asgardians successfully force the Frost Giants to retreat and reclaim the Casket of Ancient Winters, containing the Tesseract. Odin speaks to his two sons—one blonde and the other dark-haired—asserting that although both are worthy, only one can ascend to the throne. He wisely concludes, "A wise King never seeks war, but he is always ready for it."
Fast forward to the present day: Asgard is in celebration for Thor's (Chris Hemsworth) crowning as crown prince, much to the jealousy of his brother Loki. But the festivities are interrupted when intruders breach the secure room housing the Frost Giant's power casket. A massive Destroyer robot annihilates the intruders, leading to a tense moment where Odin remains composed, while Thor, brimming with rage, wishes to retaliate against the Frost Giants. Odin intervenes, ordering Thor to stand down, asserting he’s not yet worthy of the title of King.
Undeterred, Thor gathers Loki and friends—Volstagg (Ray Stevenson), Fandral (Josh Dallas), Hogun (Tadanobu Asano), and Sif (Jaimie Alexander)—and travels to the icy world of Jotunheim. With Heimdall’s (Idris Elba) reluctant approval, they confront King Laufey (Colm Feore) and engage in a fierce battle against the Frost Giants. While Loki’s unique heritage allows him to blend in, the Asgardians find themselves in peril, prompting Odin to teleport them back to safety, though not without angering Thor.
As punishment for his reckless actions, Odin banishes Thor to Earth, stripping him of his powers and sending his enchanted hammer, Mjolnir, to a location only the worthy can access. Stranded in the desert, Thor’s aggressive behavior leads to an encounter with Darcy, who tasers him and takes him to the hospital. There, Jane finds herself intrigued by the handsome stranger. Thor escapes but is soon caught again, drawn by a lead on his missing hammer, which remains under governmental protection.
Meanwhile, back in Asgard, Loki grapples with his newfound lineage after discovering he is not a true Asgardian. As Odin enters a deep slumber known as Odin's Sleep, Loki seizes the opportunity to become acting King. He manipulates events to allow the Frost Giants back into Asgard with the intent to eliminate Odin. Loki then sends the Destroyer to Earth with orders to eliminate Thor.
As chaos unfolds in New Mexico, Thor confronts the Destroyer and pleads for Loki to spare humankind, only to be brutally struck down, appearing lifeless as Jane rushes to his side in despair. However, Thor’s selfless act of protecting his friends proves his worthiness, causing Mjolnir to return to him, rejuvenating him for battle. With newfound strength, he defeats the Destroyer and vows to return to Jane.
Back in Asgard, the Frost Giants invade while Loki pretends to be the hero by killing Laufey. Meanwhile, Heimdall, capturing the chaos, escapes his frozen state and summons Thor back home. In a fierce showdown, Thor attempts to stop Loki from destroying the ice realm. A battle ensues where Thor ultimately subdues Loki. As the Bifrost Bridge collapses, Loki, accepting defeat, plunges into the abyss below.
As the dust settles, Odin mourns the price of their struggles, and Thor grows pensive, longing for Jane. Back on Earth, Jane embarks on a new scientific endeavor, unaware of the cosmic events unfolding.
In a post-credits scene, Selvig finds himself in a S.H.I.E.L.D. facility, where Nick Fury reveals a mysterious cube-shaped object of potential power, prompting Selvig, influenced by an unseen Loki, to concur with Fury’s quest.
