The game opens in medias res as Asgard is attacked by frost giants from Jotunheim. Thor Myrom Kingery trains with Sif and Loki when the assault hits, their routine breaking apart under the sudden crisis. Loki, Daniel O'Reilly, races to inform Odin Vernon Wells about the threat, while Thor fights with a fearless resolve. Sif is overwhelmed and frozen by one of the invaders, and Thor pushes through the danger to free her, then hurries back to Odin's castle to defend Asgard. The invasion is halted, but Sif is left gravely wounded. Thor is denied retribution for the attack by Odin, a decision that fuels a dangerous mix of pride and duty inside him, and he disobeys orders to seek answers on his own.
Driven by a fierce need to understand what has happened, Thor heads for Niflheim with the aid of one of Loki's projected clones, only to discover that this clone is part of a larger scheme masterminded behind the invasion. Meanwhile, Odin manages to rescue Sif from death and later falls into the Odinsleep, setting the stage for a power vacuum and unsteady leadership back home.
In Niflheim, Loki urges Thor to find Ymir, Lord of the Jötun and ruler of this shadow realm. A long, brutal battle follows and Thor defeats Ymir, uncovering the source of a power that, according to Loki, could annihilate the entire realm. Ignoring Ymir's warnings, Thor activates the source, which morphs into Mangog—an immense, golden, minotaur-like entity that vaporizes Ymir in an instant. Loki then sends an unknowing, guilt-ridden Thor to Vanaheim rather than Asgard, hoping to buy time and seize Mangog’s threat. Behind the scenes, Loki had struck a pact with Ulik, the Vanir-trolls’ ruler of Vanaheim, to delay Thor's return in exchange for fixing Vanaheim’s Frostgrinder and letting Ulik slip away.
Thor makes his way through Vanaheim as Mangog wreaks havoc on Asgard. Loki attempts to halt Mangog using the Casket of Eternal Winter from Niflheim, but the plan backfires; Mangog shatters the ice’s grasp with its fiery power. Mangog summons an army of Jötun and Infernir, forcing Loki to pivot and urge Ulik to let Thor use the Frostgrinder. Yet Ulik betrays Loki, waiting for decades to depart, and Thor pushes forward to the Frostgrinder doors. There, a hidden history of Vanaheim reveals that Odin created Mangog after the Vanir declared war on Asgard. The Vanir could not control or defeat Mangog, but they discovered Mangog’s only weakness lies in Scabrite, a god-made mineral Odin forged into chains to restrain the beast.
Thor defeats Ulik and, as Loki fills him in on the state of Asgard, reveals Mangog’s potential vulnerability to Loki. Loki explains that Scabrite can only be found in one realm: Muspelheim, home of the Infernir.
Thor then travels to Muspelheim, where he faces Surtur, Lord of the Infernir and ruler of the fiery realm. He defeats Surtur and claims the Twilight Sword. When he destroys the Twilight, its energy—rich with Scabrite—surges into Mjolnir, empowering Thor even further.
Returning to Asgard, Thor learns from Loki that his next move must be strategic: help Heimdall and a fully recovered Sif retake the Bifrost’s observatory before facing Mangog. With Heimdall and Sif secured, Thor advances to Mangog’s stronghold, aided by Loki and a weakened Odin. After a grueling duel, Thor defeats Mangog, and an uneasy reconciliation follows: he apologizes to Odin for releasing Mangog in his anger, while Odin admits responsibility for creating the threat in the first place. In the game’s final act, Odin releases the souls of the Vanir trapped within Mangog, granting them peace as the realm begins to heal and the story ends.
