The final season finds Bjorn now the king of Kattegat, while Ivar is a fugitive in Russia and Lagertha plans a peaceful retirement to a country farm.
It’s six months after the battle of Kattegat and Bjorn is now King. As he struggles with the responsibilities of kingship, he finds he can’t rely on his mother. Ivar falls into the hands of the Kievan Rus, and in their ruthless and unpredictable ruler, Prince Oleg, he may finally have met his match.
Messengers arrive in Kattegat with news that presents Bjorn with a dilemma. Prince Oleg of Kiev seems untroubled by his conscience. Lagertha has settled into her new, peaceful life, but danger lurks nearby.
Lagertha is forced to take action. In Kiev, although Oleg continues to be friendly, Ivar is aware of the threat which Oleg poses to the vulnerable young heir to the throne. Bjorn has answered the call and come to the aid of an old enemy.
Lagertha leads her village's response to the recent attacks, but despite her best efforts, the consequences are tragic. Olaf has a bold new plan for the future of Norway. Ivar discusses Oleg's ambitions for Scandinavia.
As Norway’s Kings and Jarls arrive at Harald’s territory for the election of the King of all Norway, Olaf is certain of the result, however, it may not go as smoothly as he believes. Lagertha’s village is anxious in anticipation of another attack and she is relieved when Gunnhild arrives with re-enforcements. In Kiev, Igor and Ivar engage in subterfuge and Ivar is stunned when he encounters a ghost from his past.
Bjorn is forced to act quickly in the aftermath of the election for the king of all Norway. The Bandits attack Lagertha’s village again confident of an easy victory. However, when the defense boils down to single combat with Lagertha, victory seems less likely. In Kattegat, haunted and paranoid, Hvitserk continues to unravel.
Bjorn returns to Kattegat. Harald gains a measure of revenge on Olaf. In Kiev, interesting news reaches Ivar and Igor about Prince Dir.
Bjorn faces a difficult decision. Ubbe and Torvi leave Kattegat in search of new lands and perhaps old friends. Oleg's plans for the invasion of Scandinavia take shape. King Harald is baffled by the origin of a mysterious raiding party.
In Iceland, Ubbe and Torvi finally meet a mysterious wanderer. Bjorn is forced to re-think who his enemies are when Erik returns from a scouting mission with worrying information. Bjorn will need allies, but he may not be able to convince his old foe, King Harald, to join forces in the face of the new threat. Ivar is reunited with someone close to him.
Ivar and Igor may be plotting against Oleg, but they're still part of the force that departs Kiev to invade Scandinavia. King Harald and King Bjorn begin furious preparations for the invasion.
In Iceland, Ubbe learns the truth about Kjetill and has a difficult decision to make. Back in Norway, the battle against the Rus has had grave consequences. But re-enforcements have arrived, and Bjorn has an idea that may yet save the day.
Ubbe persuades the settlers in Iceland to embark on a voyage of exploration. The defeat of the Rus creates power vacuums in both Kattegat and Kiev.
The election for Kattegat's new ruler is held, but a surprise visitor disrupts the proceedings. Ivar puts his plan to overthrow Oleg into action.
As his enemies scheme for power, Harald prepares for his coronation and chooses a queen. Ubbe's party reaches shore — but not the one they're seeking.
Factions in Greenland spill blood, driving Ubbe's company away. Troops under Ivar and Dir seize Kiev for Prince Igor. Gunnhild makes a fateful choice.
Ubbe and his followers languish on the open sea. Ivar and Hvitserk receive a frosty reception in Kattegat, until a restless Ivar has a revelation.
Ivar and Hvitserk lead King Harald's army to England, where King Alfred of Wessex abandons his royal villa and marches to meet the sons of Ragnar.
King Alfred falls ill as the Viking army seizes the high ground. Ingrid curses Erik. Ubbe and his people find a new world — but they're not alone.
Ubbe and the settlers open relations with the indigenous people of the new land. Ivar's strategy lures Alfred's forces into a devastating battle.