Sword Song is the fourth book in The Saxon Stories.
The year is 885, and England is at peace, divided between the Danish Kingdom to the north and the Saxon kingdom of Wessex in the south. Warrior by instinct and Viking by nature, Uhtred, the dispossessed son of Northumbrian lord, has land, a wife and children—and a duty to King Alfred to hold the frontier on the Thames. But a dead man has risen, and new Vikings have invaded the decayed Roman city of London with dreams of conquering Wessex...with Uhtred's help. Suddenly forced to weigh his oath to the king against the dangerous turning side of shifting allegiances and deadly power struggles, Uhtred—Alfred's sharpest sword—must now make the choice that will determine England's future.
Uhtred is serving Alfred, King of Wessex, by building one of the fortified towns that will make up Alfred's system of defence against attacks by the Danes when he learns that two powerful Norse leaders have occupied nearby London, giving them the ability to interfere with traffic on the Thames to and from Wessex. He is contacted by his former friend, Danish chieftain Haesten, who invites him to a meeting across the Thames in Mercia. Haesten takes Uhtred to a graveyard, where a corpse appears to rise from the earth to tell Uhtred that the Fates have decreed he is to be King of Mercia.
Torn between his oath to Alfred, whom he dislikes, and the temptation to become a king in his own right, he follows Haesten to London, where he meets the Norse leaders Sigefrid and his brother Erik. Haesten and the Norse brothers have a proposition for Uhtred: if Uhtred convinces his foster-brother Lord Ragnar of Northumbria to bring Ragnar's men to join them in attacking East Anglia, Mercia and Wessex, then Uhtred will be given the throne of Mercia while the others rule East Anglia and Wessex.
Uhtred ponders this offer while Sigefrid invites him to watch the crucifixion of some Christian prisoners. Among the prisoners Uhtred recognises his old comrade at arms, the Welshman Father Pyrlig. Uhtred decides to save Pyrlig and lose his chance to join Sigefrid's plot. Knowing Pyrlig to be an experienced fighter, Uhtred tricks Sigefrid into promising the prisoners can go free if Pyrlig beats him in single combat - which he promptly does. Uhtred, Pyrlig and the prisoners leave London.
Returning to Wessex with Pyrlig, Uhtred swears to keep his oath to Alfred. In Wintanceaster, King Alfred's older daughter, Æthelflæd, is married to Mercia's Earldorman Æthelred. Uthred is summoned by Alfred and ordered to plan an attack on London to dislodge the Norse brothers and turn the city over to Alfred's son-in-law and ally Earl Æhelred of Mercia.
By stealth Uhtred's seaborne assault works and the defenders of London are caught out in the open as they sally forth to confront Æthelred's larger attacking army. Wedged between what was their safe London refuge, the Saxons in front of them and Uhtred's force behind, the Norse are defeated. A particularly cruel blow is struck by Osferth (King Alfred's illegitimate son) who leaps from the walls onto Sigefrid and injures him, leaving him crippled.
Sigefrid, Erik, and Haesten retire to East Anglia. Fortune smiles on them again when Æthelred mounts a seaborne raid on their hurt forces. After initial success against the Danes, Æthelred manages to lose his wife, Æthelflaed. Alfred is distraught at the threat to his daughter and is willing to ransom her from his foes. Uhtred is sent to negotiate the price and terms with Sigefrid. Whilst in their camp he learns that Erik and Æthelflaed have fallen in love, whereupon Erik and he plot to spirit her away from her captors; all without either of their leaders knowing what they plan.
Uhtred takes a ship with his household guard to help Erik and Æthelflaed escape. Upon arriving to the camp, Sigefrid's hall is ablaze and Haesten has kidnapped Æthelflaed. The battle in the mouth for the inlet where the Vikings have holed up is as desperate as they come, with it often being unclear who is fighting for whom. This climax to the narrative is fought over marshland, waterside, on ship and across ships. Erik is killed by Sigefrid, but Uhtred and his crew quickly gain victory over Sigefrid's own warriors, and Sigefrid himself is killed by Osferth. Æthelflaed is rescued and the story ends with Uhtred taking her back to her father.