Set in the late ninth century among Norse settlements in Britain and the Scottish islands, the story follows Bjarni Sigurdson, a 16-year-old Norse settler banished from his community who spends five years wandering the seaways of the Viking world.
Bjarni stands before Rafn Cedricson, chief of Rafnglas, a Norse settlement in northwest England. He accidentally drowned a Christian holy man in the horse-pond after the man kicked his dog. Rafn reveals why this is grave: He swore an oath on the Hammer of Thor that followers of the White Christ would be safe in his territory, for the sake of his foster-brother who became a monk. Bjarni's act has made Rafn an oath-breaker. Rather than bring Bjarni before the Thing, the settlement's law gathering, Rafn gives him a sword and banishes him for five years. His brother Gram gives him a silver arm-ring for journey money. Before leaving, Bjarni buries a blue glass dolphin in a side glen as a promise to return and secures passage aboard the merchant ship Sea Cow.
In Dublin, Bjarni tries to join King Halfdan's bodyguard but is mocked for being too young. A cutpurse steals his money, but a half-starved black cur he befriended in an ale-house attacks the thief and drives him off. Bjarni names the dog Hugin, after one of Odin's ravens. Penniless, he returns to Heriolf the merchant, who agrees to take him north to Belfast Lough.
At a Norse fleet base in Northern Ireland, Bjarni is captivated by the longship Sea Witch. Its captain is Onund Treefoot, a fox-haired ship chief who lost a leg fighting King Harald Finehair and walks on a wooden leg. When a warrior breaks his sword arm in a drunken fall, leaving Onund short a man, Bjarni formally offers his sword-service, and Onund accepts.
Over two summers on Barra in the Outer Hebrides, Bjarni joins merchant runs and a land campaign against Irish kings. When pirate brothers Vigibjord and Vestnor, who drowned Onund's younger brother, are reported raiding the coast, Onund rallies the fleet. In the Kyles of Bute, he lures the raiders beneath a ruined tower where stone throwers rain destruction on the pirate ships. When Onund's wooden leg is splintered by a throwing axe, Bjarni swims to his lord's side. With Bjarni's left-handed swordsmanship complementing Onund's need for support, they sweep a pirate ship from stem to stern as "the two-sworded beast." Onund kills Vigibjord in single combat and lets the surviving brother escape, declaring the blood-debt paid.
Onund marries Aesa, the Hall Chieftain's daughter, bonding the fleet together. At the wedding feast, Thara, daughter of Asmund the Odin Priest, tries to pull Bjarni into the ring-dance; he pushes her away and dances with another girl, humiliating her. Thara begs her father for revenge, and that night Asmund arrives in a drugged frenzy, demanding Hugin as a sacrifice to Thor. Bjarni fights to protect his dog but is restrained. Onund ends the standoff by cutting two toes from Hugin's forepaw, rendering him unfit for sacrifice since a maimed beast cannot be offered to the gods. He then dismisses Bjarni, returning his sword-service "with honour." Heriolf explains that Onund saved both dog and man: After defying the priest, Bjarni would not have survived on Barra.
On Mull, Bjarni brings the feverish Hugin to the Lady Aud, mother of the warlord Thorstein the Red, known as Aud the Deep-Minded. Her bower woman Muirgoed, a freed former queen of Argyll, tends the dog. Bjarni offers his sword to Thorstein and befriends Muirgoed's son Erp Mac Meldin, who still wears the iron thrall-ring marking an enslaved servant. In spring, Aud includes Bjarni in an Easter pilgrimage to Iona, the holy island of the Celtic monks. There he meets Brother Gisli, Rafn's foster-brother, the very man for whose sake the oath was sworn. Gisli tells Bjarni he forgives Rafn. After days of reflection, Bjarni asks to be prime-signed, a preliminary Christian rite accepting him into the faith's community without full baptism.
At a council on Orkney a year and a half later, Onund Treefoot tricks Jarl Sigurd into foster-kinship by placing his infant son on the Jarl's knee, a bond that cannot be refused under Norse custom. Bjarni walks Onund to the harbor; Onund invites him to Iceland, but Bjarni declines, his sword still pledged to Thorstein.
Thorstein and Jarl Sigurd campaign into Caithness, and the Lady Aud arranges her granddaughter Groa's marriage to Dungadr, a Pictish chief, to cement the alliance. The peace is fragile: Jarl Sigurd dies of a festering wound, and at his funeral pyre Melbrigda Tusk's young son tries to assassinate the new Jarl but is cut down, with Thorstein taking a small stab wound. After peace is renegotiated, Thorstein is struck by an arrow on the ride north, killed by the dead youth's twin in a Blood Feud, a cycle of retaliatory killing. Bjarni pursues and kills the bowman.
The Lady Aud overrules warriors demanding vengeance, arguing the debt is already paid, and announces she will sail for Iceland in the spring. Through the winter, she oversees the building of her ship Seal Maiden. Bjarni saves Brother Ninian, Aud's chaplain, from a bear attack, suffering severe claw wounds. Recovering, he reflects that at either end of his five years stands a holy man: one dead because of him, one alive. Erp, whose thrall-ring has been removed, advises him to return to Rafnglas. When Seal Maiden sails, Aud gives Bjarni fair pay and an ancient pattern-forged sword with an amber pommel, telling him one is for his hand and one for his son.
Bjarni journeys to Mull, reunites with Hugin, and boards Sea Cow for the voyage to Rafnglas. Near the Lakeland coast, a violent storm sweeps Hugin overboard. Bjarni leaps after him and, swimming one-armed, is washed onto a sandy Welsh shore.
He stumbles inland to a derelict farm where a young Welsh herb-woman named Angharad takes him in. Angharad learned advanced healing from an elderly nun, but the villagers fear her Latin incantations and the wine-colored mark on her neck, which they take for a witch mark. Behind their fear stands her kinsman Rhywallan, the King's Falconer, who covets her land and stokes their superstition to drive her out. Bjarni becomes her hired sword.
They harvest the barley and celebrate. That night Rhywallan sets his hounds on Hugin. Bjarni kills one; Angharad drives off the other. Old Gwyn, her paralyzed cattleman, dies at dawn. Returning from burying him, they find the farm ablaze, Rhywallan urging a mob screaming for the witch to burn. Bjarni pulls Angharad away, and they flee with the horse Swallow and Hugin.
At dawn, Bjarni proposes that Angharad come with him to Rafnglas. With her home destroyed and Bjarni the only familiar thing left, she accepts, saying her dowry is "a horse and a ring that cures warts."
They travel overland for weeks. At the boundary of Rafnglas, Bjarni takes Angharad up on horseback, riding home as a man returned from making his fortune. Gram does not recognize his bearded brother at first, then embraces him. Bjarni says he will make his own land-take further up the dale. Rafn tells him Heriolf survived the storm and left Bjarni's old sword. Bjarni delivers the message he has carried for three years: Brother Gisli forgives his foster-brother the oath-breaking. Rafn is deeply moved. He welcomes Bjarni home, calling his wandering "the Sword Song of Bjarni Sigurdson."