It is the year 866 and Osbert is the 10-year-old second son of English Ealdorman Uhtred, Lord of Bebbanburg in the Kingdom of Northumbria. Whilst exploring with his father outside the Bebbanburg gates, Osbert spots Viking long ships off the coast. At this point in time, the Danes have already raided English cities and monasteries, so Lord Uhtred quickly retreats to the Bebbanburgh fortress to raise the alarm. He dispatches his first son, also named Uhtred, to scout the long ships and specifically instructs him to not engage in any combat. Uhtred disobeys and taunts the Vikings from the shore. The leader of these oncoming Danes, Earl Ragnar the Fearless, is then seen riding Uhtred's horse to the gates of Bebbanburg, only to toss the bloodied head of the younger Uhtred in front of his father. As a result, Osbert becomes the new Uhtred and is baptized for the second time in front of the common folk so that they may recognize him as the new heir of Bebbanburg.
Earl Ragnar is accompanied by his son, Ragnar the Younger; a fellow Danish warlord, Ubba; as well as his ship master, Kjartan. They sail to Eoferwic (York) to meet up with Guthrum, another Danish warlord. Whilst feasting, Ubba's pagan priest, Storri, upon whom Ubba places all confidence, informs him that he foresees a battle to come. As a result, Ubba enthusiastically declares that they will fight the Northumbrians the following day.
Lord Uhtred and his fyrd (local army men) have already marched south from Bebbanburg towards Eoferwic. The young Uhtred, who even at the age of 10 possesses the heart and spirit of a warrior, has secretly followed, and luckily as well, for his uncle, Ælfric (brother of Lord Uhtred), has already given instructions to kill the younger Uhtred to seize lordship of Bebbanburg for himself.
Danish and Northumbrian forces meet on the battlefield. Lord Uhtred leads the offensive attack as the Danes wait patiently. The English forces rush downhill but their blows are rendered useless by the ever-effective Danish shield wall. The second half of the Danish army comes out of hiding to close in on the English from behind, trapping them between two shield walls. The English are slaughtered and Lord Uhtred is killed. Little Uhtred then rushes the battlefield in a futile attempt to kill a Dane, and is instead captured as a slave along with a young Saxon girl named Brida.
Whilst in the Danish longhall that night during celebrations, Uhtred is treated with kindness by Ravn, blind father of Ragnar the Fearless. When Ælfric, Uhtred's uncle, learns that he is still alive, a meeting is arranged to ransom the boy back to Bebbanburg. Uhtred's childhood priest and teacher, Father Beocca, warns him of his uncle's treachery and Uhtred is purchased by Ragnar to live with his family as a slave. But Ragnar treats Uhtred and Brida with kindness, as if they were his own children.
One day while playing in the woods with Brida and Thyra (Ragnar's daughter), they encounter Sven Kjartansson, the son of Ragnar's ship master, Kjartan. Sven rips open Thyra's dress, leaving her half naked and afraid as Uhtred rushes to her rescue, knocking Sven to the ground. Uhtred is almost punished by Ragnar for starting a fight until he learns the truth of what happened. Ragnar then makes a visit to Kjartan's house to punish Sven by blinding him in one eye and banishing Kjartan from his lands. Later that night, Ragnar gives Uhtred a necklace of Thor's hammer, letting him know that he is proud of him.
Years later, Uhtred is grown up. He has a close relationship with his Danish family and is encouraged by Ragnar to marry Brida. That night he goes to Brida in the woods to make love to her. However, it is that same night, the eve of Thyra's wedding, that Kjartan and his men, now allied with Ælfric of Bebbanburg, have come: Kjartan to exact his revenge on Ragnar, and Ælfric to kill Uhtred and consolidate his hold on the Bebbanburg seat. Ragnar's hall is lit on fire, trapping Ravn and Ragnar's wife within. Those who were able to escape are slaughtered anyway while Thyra is kidnapped by Sven. Uhtred and Brida arrive from their spot in the woods just as Ragnar valiantly attempts to kill as many as he can before burning to death (Ragnar the Younger is in Ireland trying to claim his own land at this time).
The next morning, Uhtred and Brida look on helplessly in the smouldering aftermath of the murder scene as Kjartan and his men search for Ragnar's wealth (silver). Bloodthirsty, Uhtred sees Ælfric's right hand man, Weland, and sneaks in to murder him. Thyra sees Uhtred but is too scared to call out for help. She is carted away with Kjartan and Sven. Uhtred rides to Bebbanburgh with Weland's head and tosses it at the gates just as Ragnar the Fearless did years before. He declares that Bebbanburg belongs to him and will take it back.