In AD 895, King Aurvandill returns triumphantly to the island of Hrafnsey after a series of conquests, joyfully reuniting with his beloved wife, Queen Gudrún, and their heir, Prince Amleth. To prepare Amleth for the responsibilities he will someday assume, both participate in a spiritually significant ceremony led by Aurvandill’s clever jester, Heimir the Fool. However, their joyous reunion is short-lived; the very next day, betrayal strikes as Aurvandill’s illegitimate brother, Fjölnir the Brotherless, brutally murders the king, plunders his stronghold, and abducts Gudrún. Devastated and enraged, Amleth escapes by boat, vowing to exact revenge.
Fast forward years later, Amleth is now a fierce berserker living among a volatile group of Vikings. After a violent clash in Gardariki, Amleth encounters a mysterious Seeress in the sacred temple of Svetovit, who prophesizes that revenge on Fjölnir is imminent and that Amleth's fate is linked to a Maiden-King. Discovering that Fjölnir has been ousted by Harald of Norway and is now living in exile in Iceland, Amleth takes the risk of masquerading as a slave to infiltrate Fjölnir's realm. Here, he meets the enigmatic Olga of the Birch Forest, a woman who claims to possess magical abilities, and learns that his mother has re-married Fjölnir and given birth to a son, Gunnar.
One fateful night, Amleth encounters a He-witch who facilitates a spiritual reunification with the ghost of Heimir, a suggestion leading Amleth to seek out the legendary Draugr, a powerful sword located at the Gates of Hel. After battling the undead Mound Dweller to claim the enchanted blade, Amleth finds himself drawn into the local customs; he gets selected to join a competitive game of knattleikr against another farm. Though the game escalates into violence and nearly costs Gunnar his life, Amleth rises to the occasion by saving him. In gratitude, Fjölnir's son, Thórir the Proud, grants Amleth additional privileges, allowing him to select a woman of his choosing.
During a night of revelry, Amleth and Olga share a passionate encounter, pledging to support each other against the treachery of Fjölnir. But chaos ensues when Amleth begins to eliminate Fjölnir’s men while Olga introduces a hallucinogenic element into their meals, further inciting turmoil. In the confusion, Amleth confronts his mother, who chillingly reveals the truth of her past with Aurvandill and her role in Amleth's conception—one born from violence. She had harbored a desire for both Aurvandill and Amleth to perish, leading to a turbulent climax where Amleth brutally takes Thorir's life.
As tensions escalate, Gudrún uncovers Amleth’s identity to Fjölnir and urges for his demise. In a desperate move, Fjölnir threatens to kill Olga, prompting Amleth to negotiate her life in exchange for Thorir’s heart. After enduring a savage beating, Amleth is rescued by a gathering of ravens, allowing him to escape with Olga by sea. A prophetic vision emerges, revealing that Olga is carrying twins, one being the destined Maiden-King foretold by the Seeress. Frightened for their future, Amleth determines to confront his uncle, plunging into action despite Olga's heartfelt pleas.
Back at Fjölnir's farm, Amleth liberates the captives and embarks on a quest to eliminate Fjölnir's men. However, an unexpected attack from his mother compels him to drive the Draugr into her heart. Amidst this mayhem, Gunnar fiercely assaults Amleth, but the tables turn, culminating in Amleth's victory. Consumed by grief and rage upon discovering his wife and son slain, Fjölnir challenges Amleth to a final resolution at the volcano Hekla—where fate awaits in the form of an epic duel. The climactic encounter concludes with Amleth decapitating Fjölnir even as he himself suffers a mortal wound. In his last moments, he envisions Olga cradling their twin children, before a valiant Valkyrie arrives to guide him into Valhalla's embrace.
