King Bagsecg, portrayed by James Cosmo, leads his beleaguered Viking army against the Saxon forces in the year 871 AD. As the brutality of battle intensifies, Bagsecg urgently summons reinforcements, including his son, Prince Steinar, played by Charlie Bewley. Steinar journeys with a loyal group of warriors, such as Hagen, portrayed by Clive Standen, Grim played by Michael Jibson, and Jokul, brought to life by Guy Flanagan. When Steinar finally reaches his father, he discovers the devastating outcome of their struggles: the army has been vanquished, and Bagsecg lies grievously injured.
While Steinar yearns to seize the title of king, Bagsecg implores him to seek out Hakan, played by Elliot Cowan, his firstborn son, whom he deems worthy of the crown—unlike Steinar, whom he believes is unfit. Further complicating matters, Bagsecg compels Steinar to eliminate Vali, played by Theo Barklem Biggs, his stepson, who has refused to take up arms. However, Steinar adamantly declines to harm his half-brother.
Hakan had previously been exiled by Bagsecg due to his instability, but Bagsecg now asserts that enough time has passed, and Hakan should return. The companions unite in their mission to find Hakan, necessitating a journey to the black mountains to locate Ivar, played by Ivan Kaye, an old hunting partner of Hakan’s. Upon Hakan's banishment, Ivar too found himself exiled.
Meanwhile, Steinar's elder brother, Harald, embodies fury over the king's decision and, in Steinar's absence, conspires with the Saxon king, plotting to ascend as the King of Vikings upon Bagsecg's demise. This treachery is revealed to Steinar by Vali, who joins him on the perilous quest to find Ivar, as abandoning Vali with Harald would risk his life.
As they venture deeper into the woods, Steinar ponders why Bagsecg had once deemed Hakan unfit for leadership yet now wishes for his return. They leave Vali with their horses at the fringes of the Black Mountains and venture onward, eventually confronting Ivar. In a game of arm wrestling, Steinar bests Ivar, who subsequently agrees to guide them to Hakan, accompanied by his servant Agnes, a role played by Alexandra Dowling.
Upon returning to Vali, they shockingly discover that all the horses have perished, a fact Vali blames on thieves, although he seems unscathed. Their journey takes a dire turn as they face an ambush by well-trained Saxons, leading to Grim's demise in battle. Hagen later discovers a Saxon coin that suggests Vali might have been leaving signals for the enemy, a betrayal that causes division within the group. Steinar struggles to reconcile Vali's alleged treachery, born of his own Saxon heritage.
The tensions escalate as the group is captured by the Saxons, where their leader attempts to sway Steinar to ally with the Saxons for power in exchange for his loyalty to their Church. Fortunately, Agnes intervenes to free Steinar, who then rescues his crew. Tragically, Ivar succumbs to his injuries but provides valuable direction towards Hakan's location.
Amidst their reprisals, Hagen imprisons the captured Saxons in a church and sets it ablaze. Despite pressure from Hagen and Jokul to eliminate Vali to garner the gods' favor, Steinar remains steadfast, ultimately leading to a fatal confrontation with Hagen when he refuses to comply.
Steinar finally reaches the forest designated by Ivar and finds Hakan in a cave, now a savage ruler of a band of outcasts. In a gruesome twist, Hakan kills and devours Jokul before brutally murdering Vali. Steinar uncovers the disturbing truth about Hakan's exile, which ultimately involved a taboo relationship with their mother, leading to her residence in Hakan's lair.
Refusing to yield to Hakan’s demand for submission, Steinar engages in a perilous duel which culminates in Hakan’s defeat and death. Steinar returns with Hakan's severed head and Agnes, reclaiming his position as a leader. With the defeat of Harald, Steinar ascends as the new King of Vikings, ready to march his people into battle against the Saxons.
