Set in Scandinavia and across Europe around 980–1010 AD, the novel follows the life and voyages of a Viking called Red Orm, from his abduction as a teenager to his old age as a wealthy chieftain on the Swedish-Danish border.
The story opens in the Danish province of Skania, where Christian missionaries arrive with mixed success. In the border forests of Göinge, locals capture priests and sell them as slaves. King Harald Bluetooth, elderly and ailing, converts after Saxon bishops cure him and orders all his subjects baptized. The border people laugh at the decree. An old saying holds that no king will appear in their land until wild oxen return.
At a coastal homestead called the Mound, Thane Toste and his sharp-tongued wife Asa have lost three of five sons. Their youngest, Orm, is red-haired and quick-tempered. Asa forbids him to sail, haunted by prophetic dreams. When a chieftain named Krok raids the Mound for sheep, Orm rushes out, kills a raider, and is knocked unconscious. Krok takes him aboard to row in the dead man's place. Orm befriends Toke, a huge, good-humored oarsman. After failed raids, Krok's three ships head west. They rescue Solomon, a Jewish silversmith from Toledo. Solomon guides them to a fortress in León, and the night assault yields spectacular booty.
As the ships depart, seven Moorish warships attack. Nine surviving Vikings, including Krok, Orm, and Toke, are enslaved as galley rowers for the Caliph of Córdoba. For over two years they row in chains. Orm develops extraordinary left-handed strength at his larboard oar. Krok, consumed by grief, seizes the overseer and hurls him into a caldron of boiling pitch, dying under the guards' swords.
Orm learns Arabic from Khalid, a nobleman condemned to the galleys for blasphemous poetry. Upon release, Khalid contacts Solomon. The Northmen's liberation is arranged by Subaida, Toke's former captive from the fortress, now the chief wife of Almansur, the all-powerful regent of the Caliph's dominions. Subaida gives Orm and Toke each a magnificent sword. Orm names his Blue-Tongue.
For four years Orm's band serves in Almansur's bodyguard, campaigning across Spain. Orm saves Almansur's life during a night attack and receives a gold chain of red and green stones. When Almansur sacks Santiago de Compostela, Orm is assigned to transport its great bell by ship. They then avenge Krok's death, making themselves outlaws. They flee with the ship and the bell.
The bell's ringing guides them through fog to an Irish monastery, then on to Jellinge in Denmark. Brother Willibald, a fierce little physician, mixes medicine with holy water and dust from the bell, curing King Harald's toothache. At the Yule feast, a warrior named Sigtrygg, King Sven's kinsman, demands Orm's gold chain. They duel, and Orm beheads him. While recovering, Orm is tended by Ylva, King Harald's sharp-tongued daughter. He gives her the chain, believing he is dying. When he recovers, he asks for her hand. Harald demands proof of wealth. Toke then smuggles Mirah, the King's Moorish slave-girl, aboard in a chest, making both men outlaws and destroying Orm's chance of returning.
Orm reaches home to find his father dead. Wars consume Scandinavia: King Sven rebels, and King Harald dies. Orm joins the fleet of Thorkel the Tall, a Danish chieftain he had met at King Harald's Yule feast, and sails for England. They defeat Jarl Byrhtnoth's army at Maldon. Orm finds Brother Willibald among the survivors, and the priest reveals that Ylva lives at a convent in Westminster. Orm arranges his baptism, sails to Westminster, reunites with Ylva, and marries her. Brother Willibald accompanies them home as household priest.
They stop at Jellinge to retrieve the gold chain Ylva had hidden. King Sven rides out in pursuit, but Brother Willibald fells him with a hurled stone. They flee to Asa's inherited estate, Gröning, deep in the border forests. There Orm builds a homestead and church. Ylva bears several children, including twin daughters Oddny and Ludmilla, and sons Harald and Blackhair. When assassins hired by King Sven arrive disguised as peddlers, a boy warns Orm. He attacks first and forces the survivors to be baptized before releasing them.
At the Thing, a judicial assembly at the ancient Kraka Stone, Orm reunites with Toke, now a skin-trader among the Virds, one of the border peoples from Värend. He also befriends Olof Summerbird, a chieftain of the Finnvedings, a neighboring border people. When Orm's old enemy Östen ambushes him, Olof kills his own kinsman to protect his guest, sealing their friendship.
Years later, Orm's long-lost brother Are arrives, blind, tongueless, and missing his right hand. Using carved runes that Father Willibald transcribes, Are tells his story. He served in the Imperial bodyguard at Miklagard (Constantinople). His son Halvdan became the lover of a Byzantine princess, so Are transferred him to the navy for his safety. During the Emperor's war against the Bulgars, a corrupt treasurer stole a vast treasure. Are pursued and killed him but lost Halvdan in the fighting. He hid the treasure near the Dnieper's great weirs before being captured and maimed. He reveals the location to Orm, then walks into the river and drowns.
Orm assembles an expedition: Olof joins in exchange for Ludmilla's hand, and Toke signs on eagerly. They recruit the sons of Sone the Sharp-Sighted, a Göing chieftain from the Thing. An ancient seer prophesies that seven will return and four will die. They sail east, row up the Dvina, and drag the ship overland at the great portage. One of Sone's sons is killed at Gotland. At a village of the Dregovites, a local people whose log walls are defended by trained bees, three more men perish: two of Olof's men and another of Sone's sons. They descend to the Dnieper and reach the weirs.
Orm recovers four gold chests and silver sacks from the riverbed. At dawn, Patzinaks, nomadic steppe warriors, attack on horseback. Olof takes an arrow in the chest. Blackhair and Glad Ulf, Orm's foster-son, are captured while running for reinforcements. Toke drives off the attackers. Orm goes unarmed to the Patzinak camp and discovers that one of their chieftains is Felimid, one of King Harald's former Irish jesters. Felimid negotiates the ransom: four hats of silver for Glad Ulf, Blackhair's weight in silver. Orm pays.
On the homeward voyage, Olof is healed by local women. Orm divides one gold chest among all his men, keeping three for himself, Toke, and Olof. They reach home to find catastrophe. Magister Rainald, a priest living among the border people, has renounced God, gathered outlaws, and attacked Gröning. Rapp, one of Orm's longtime household men, was killed defending the gate. Harald was wounded, and Father Willibald was clubbed nearly to death. Ludmilla and the gold chain were taken. Olof vows to kill Rainald and become Christian if he recovers Ludmilla.
Orm assembles over a hundred men and marches for four days. They attack the outlaw village from three directions. Ludmilla strikes a guard and runs to safety. Blackhair fells the fleeing Rainald, and Olof drives his sword through him, fulfilling his vow. The gold chain is recovered.
Returning home, the hounds encounter wild oxen and drive several back to Gröning, fulfilling the ancient prophecy. Orm settles into contented old age. Olof marries Ludmilla. Blackhair later fights in King Canute's ship at the Battle of the Holy River and becomes a renowned chieftain. Toke moves near Gröning; neither he nor Mirah ever accept baptism. Orm and Toke live to a ripe old age, never tiring of telling stories of the years they rowed the Caliph's ship and served Almansur.