In 965 AD, Odin, king of Asgard, wages war against the Frost Giants of Jotunheim and their leader Laufey, 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 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, childhood friend Sif and the Warriors Three: Volstagg, Fandral, and Hogun. 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, now protected by an enchantment that allows only the worthy to wield it.