In 965 A.D., Odin (Anthony Hopkins), king of Asgard, wages war against the Frost Giants of Jotunheim and their leader Laufey (Colm Feore), to prevent them from conquering the Nine Realms, starting with Earth. The Asgardian warriors defeat the Frost Giants and seize the source of their power, the Casket of Ancient Winters.
In the present, Odin's son Thor (Chris Hemsworth) prepares to ascend to the throne of Asgard, but is interrupted when Frost Giants attempt to retrieve the Casket. Against Odin's order, Thor travels to Jotunheim to confront Laufey, accompanied by his brother Loki (Tom Hiddleston), childhood friend Sif (Jaimie Alexander) and the Warriors Three; Volstagg (Ray Stevenson), Fandral (Joshua Dallas) and Hogun (Tadanobu Asano). A battle ensues until Odin intervenes to save the Asgardians, destroying the fragile truce between the two races. For Thor's arrogance, Odin strips his son of his godly power and exiles him to Earth as a mortal, accompanied by his hammer Mjolnir — the source of his power, now protected by an enchantment to allow only the worthy to wield it.
