It's 965 AD. The king of Asgard, Odin (Anthony Hopkins), wages war against the Frost Giants of Jotunheim, led by Laufey (Colm Feore), who intend to open the "Casket of Ancient Winters" and plunge the world into a new ice age. Odin wins.
A while later, it's six months after Tony Stark became Iron Man, the first Super Hero of the Marvel Cinematic Universe. On Asgard, a realm of Sufficiently Advanced Aliens, Odin's son Thor (Chris Hemsworth) is about to ascend to the throne, but is interrupted when Frost Giants invade. They have secretly been let in by Loki (Tom Hiddleston), Thor's brother, and are after the Casket. In defiance of Odin's orders, Thor leads a Cast Herd into Jotunheim to confront Laufey. The battle only ends when Odin intervenes. To punish his Hot-Blooded son, Odin has Thor Brought Down to Normal and exiled to the worst place in the galaxy: New Mexico, America. Thor is accompanied only by his warhammer, Mjolnir, which has now been ensorceled so that only the worthy may wield it.
Thor is found by several humans: astrophysicists Dr. Jane Foster (Natalie Portman), Darcy Lewis (Kat Dennings) and Dr. Erik Selvig (Stellan Skarsgård). Mjolnir, and the data about the wormhole which brought Thor to Earth, are commandeered by Agent Phil Coulson (Clark Gregg); Thor tries to retrieve it but, of course, cannot yet lift it. Selvig secures Thor's release from S.H.I.E.L.D. custody, and Thor settles in on Earth, beginning a romance with Jane.
Loki learns that he is adopted and that Laufey is his real father. Loki confronts Odin, who falls into a very long "Odinsleep" to recharge himself. Now king in his "father'''s stead, Loki negotiates with Laufey, who wants to kill Odin and take the Casket. Thor's Cast Herd, displeased with Loki's rule, bribe Heimdall (Idris Elba), who controls the "Bifröst" teleportation bridge leading in and out of Asgard, to send them to Earth where they can contact Thor. Loki learns of this and sends a robot which is Made of Indestructium to attack them; Thor performs a Heroic Sacrifice to save them, and the action makes him worthy to wield Mjolnir, allowing him to fly, summon Shock and Awe, and boomerang the hammer back to his hand after throwing it.
Back in Asgard, Loki has now stabbed Laufey In the Back and is planning to destroy Johunheim entirely. Thor stops him by destroying the Bifröst bridge, leaving both him and Loki on a Literal Cliffhanger. Odin awakens and comes to save them both, but Loki decides on a Disney Villain Death in the face of his "father"'s disapproval. Thor, having had Character Development, admits to his father that he is not yet worthy to be king, but Odin promises he will grow into it.
Loki lands on Earth and begins to manipulate his way into positions of power. Jane Foster continues her work on opening a portal to Asgard. And, in The Stinger, Selvig is taken to a S.H.I.E.L.D. facility, where Nick Fury (Samuel L. Jackson) asks him to examine a glowing blue cube, which he calls The Tesseract...
