Anlaf, the cunning Viking king from Ireland, leads his formidable army to Northumbria, drawn by the news of King Edward's grave illness. Tragically, Edward passes away, leaving behind three sons who are left without a clear aetheling. In the midst of turmoil, Lady Eadgifu and the youngest son, Edmund, make a desperate escape to Bebbanburg, seeking refuge with Uhtred, Edward’s former ally. Despite his retirement from the battlefield, Uhtred is thrust back into conflict upon learning that Edward's eldest son, Aethelstan, plans to confront his sibling, Aelfweard, for the throne. In a foreboding moment, the seer Ingrith forewarns that "seven kings must die" for England to find unity.
As Uhtred makes his way to Aegelesburg, tensions rise as Aelfweard prepares his defense against Aethelstan's advancing forces. Aethelstan, accompanied by his advisor Ingilmundr, cunningly dismantles Aelfweard's defenses and, after convincing him to surrender, unleashes a brutal slaughter of Aelfweard and his men, subsequently claiming the title of king of the Saxons. Meanwhile, Ingilmundr, secretly in love with Aethelstan, encourages him to realize his grandfather's vision of a united England under one Christian banner.
In Eoferwic, Aethelstan's rule grows as he demands tribute from foreign kings or threatens conquest and forced conversion. Uhtred, witnessing Aethelstan destroy Pagan landmarks, learns of King Hywel Dda's unfortunate submission triggered by his son’s captivity. Uhtred, suspecting deception, becomes entwined in a dangerous game as Ingilmundr plots to eliminate him, leading to a desperate escape back to Bebbanburg after receiving a warning from Aldhelm.
Conflict erupts further when Ingilmundr attempts to seize Bebbanburg but falls victim to a trap of his own making, leading to his capture. In a turn of betrayal, he persuades a servant to lead Uhtred into a precarious situation, resulting in Uhtred’s imprisonment. With Bebbanburg under threat, Osbert is forced to surrender, and both Osbert and Edmund are taken away to Thelwael while Uhtred is banished from England by Aethelstan, who struggles with his own moral conflicts.
As the Isle of Man becomes the meeting ground for Anlaf and other regional kings opposing Aethelstan's threats, an alliance forms, albeit hesitantly as they grapple with forming a coalition with a Pagan ruler. Aethelstan’s relentless campaign into Scotland catalyzes King Constantin and others to unite against him. Meanwhile, a weakened Uhtred finds refuge with the Danes and is introduced to Anlaf’s proposition of assassinating Aethelstan to prevent widespread war.
Uhtred’s attempts to reason with Aethelstan in Wintanceaster prove fruitless, and returning to Bebbanburg brings heartbreaking news of Anlaf’s assault, which has left Ingrith and commoners to face dire fates. In a grim turn, Ingilmundr commits a massacre at Thelwael, leading Uhtred to discover that Osbert and Edmund have survived against all odds. Although Aethelstan makes peace with Uhtred, he adamantly refuses assistance in facing Anlaf's imminent invasion.
As the Battle of Brunanburh unfolds, Uhtred finds himself leading a vastly outnumbered force into desperate tactics, culminating in chaos when the heirs of other kings are tragically killed. Amidst the raging battle, Uhtred suffers grave wounds while attempting to protect Aethelstan from Ingilmundr’s assault. As the dust settles, the remaining allies abandon Anlaf, leading to Ingilmundr’s execution, a testament to the shifting allegiances in this turbulent era.
In a poignant moment of closure, Aethelstan brings Uhtred back to Bebbanburg, contemplating the weight of their loss as they ponder the prophecy of “seven kings must die.” Uhtred’s oath of loyalty to Aethelstan and the promise of Edmund as his sole heir symbolically unites Northumbria, heralding Aethelstan as the first king of a unified England, fulfilling the long-held aspirations of King Alfred. As Uhtred teeters on the brink of death, he is greeted in a visionary experience of Valhalla, surrounded by familiar faces from his past, awaiting their next journey together.
