It is a dark time for Asgard. The All-Father is trapped in a bewitched Odinsleep, inspiring an all-out assault from the Frost Giants. They evade the gods' defenses with uncommon ease, as if guided by augury. Heimdall, a quick-witted young warrior still finding his place amongst Asgard's defenders, believes it no coincidence that Odin lies enchanted and that the Giants are so well-informed. Sneaking into Odin's inner chambers, he discovers that the severed head of Mimir, a great source of wisdom, is missing. Accompanied by his sister, Lady Sif, Heimdall must quest across the Nine Realms to retrieve it, lest mighty Asgard fall.