At the Battle of Brunanburh, Uhtred and Aethelstan's forces are heavily outnumbered and make a tactical withdrawal, allowing Saxon cavalry to flank from behind. In the chaos, Domnal and the heirs of the other kings are killed. -Anlaf, a Viking king from Ireland, arrives with his army in Northumbria after learning that Edward, King of the Anglo-Saxons, has fallen gravely ill. Edward soon dies, leaving behind three sons, none of whom were named as his aetheling. Lady Eadgifu and the youngest son Edmund flee to Bebbanburg where Edward's former ally Uhtred resides. Although retired from war, Uhtred is informed that Edward's eldest son Aethelstan will likely attack his younger brother Aelfweard and makes plans to stop him. Before Uhtred leaves, Ingrith prophesizes that "seven kings must die" before England is to be united.
Uhtred reaches Aegelesburg, where Aelfweard prepares to defend against Aethelstan's impending attack. Aethelstan and his advisor Ingilmundr soon arrive and thwart a trap set by Aelfweard. Uhtred convinces Aelfweard to surrender peacefully, but Aethelstan swiftly kills him and massacres his men. Aethelstan is crowned king of the Saxons and Ingilmundr, who is secretly Aethelstan's lover, urges him to fulfil his grandfather's dream of uniting England and all Christians.
In Eoferwic, Aethelstan has taken over following the death of the previous Northumbrian king Rǫgnvaldr and demands that all foreign kings pay him tribute or face conquest and forced baptism. Uhtred arrives and witnesses the destruction of pagan monuments before learning that King Hywel Dda has submitted to Aethelstan after his son was taken hostage. Uhtred then discovers Aethelstan's relationship with Ingilmundr and believes he's being deceived. Ingilmundr secretly plans to have Uhtred killed, but Aldhelm warns Uhtred who flees back to Bebbanburg. Aldhelm is subsequently labelled a traitor by Aethelstan and hanged for his actions. Ingilmundr arrives at Bebbanburg to take the castle, but unaware that Uhtred had escaped, is quickly captured. Ingilmundr later convinces a servant to lure Uhtred into a trap, where he is captured by Aethelstan's men, forcing Osbert to surrender Bebbanburg. Osbert and Edmund are taken captive and sent to Thelwael, while Aethelstan, unable to kill Uhtred, banishes him from England.
On the Isle of Man, Anlaf hosts a gathering of kings with Prince Domnal and the rulers of Strathclyde, Orkney, Shetland and Man, warning them of Aethelstan's impending conquest. Although they initially decline the offer of allying with a pagan, Aethelstan embarks upon an invasion of Scotland, forcing King Constantin and the other rulers to form an alliance. A weakened Uhtred is found by the Danes and taken to Shetland, where he learns that Ingilmundr is a Danish spy. He meets with Anlaf and the other kings, who request that Uhtred assassinate Aethelstan to avoid war.
Uhtred travels to Wintanceaster and unsuccessfully tries to convince Aethelstan to stop his conquest. Uhtred then returns to Bebbanburg, only to learn from Eadgifu that Anlaf had sacked the castle, and sealed Ingrith and the commonfolk in a cave to suffocate. Ingilmundr massacres the Saxons at Thelwael, and Uhtred later arrives to discover that Osbert and Edmund had managed to stay hidden and survive. A remorseful Aethelstan makes peace with Uhtred and plans to stop Anlaf's invasion, but refuses any help. Knowing that Aethelstan will likely die, Uhtred and his allies head to face Anlaf, who has amassed his forces at Brunanburh. At the Battle of Brunanburh, Uhtred and Aethelstan's forces are heavily outnumbered and make a tactical withdrawal, allowing Saxon cavalry to flank from behind. In the chaos, Domnal and the heirs of the other kings are killed. After witnessing Ingilmundr attack Aethelstan, Uhtred is gravely wounded whilst attempting to aid him.
In the aftermath of the battle, the other kings quickly abandon Anlaf and a captive Ingilmundr is executed. Aethelstan helps return the gravely wounded Uhtred to Bebbanburg and Finan concludes that the deaths of Edward and the five heirs—five kings who will never be crowned—only partially fulfil the prophecy that "seven kings must die", while Uhtred speculates that he will be the seventh. Uhtred agrees to swear Northumbria to Aethelstan in exchange for him agreeing to make Edmund his sole heir, making Aethelstan the first king of a united England, fulfilling the dream of his grandfather King Alfred. Seemingly on the verge of death, Uhtred experiences a vision of Valhalla, where he sees several of his deceased allies waiting for him.
