As everyone adjusts in the wake of Sigurd's death, a joint rule is established as Ivar plots a new conquest. Lagertha confronts Harald about his aspirations to take over the kingdom.
York falls to the Vikings. Lagertha crosses a red line with Harald. Floki explores the new land he arrived at.
As the Saxons try to retake York, Ivar stands against his brothers' wish for peace. With Ubbe lured into a trap, Ivar gains complete control of the Great Army.
Ubbe and Lagertha form an alliance against Ivar and Harald. Heahmund plots to breach York when the Vikings are at their weakest, but unbeknownst to him, Ivar has his own counter-plot.
As Ivar springs his trap on the Saxons, Heahmund is imprisoned and taken to Kattegat. Bjorn and his companions make a horrifying discovery at a banquet in Africa.
With Ivar and Harald preparing to attack Kattegat, Astrid's attempt to warn Lagertha bears tragic consequences for her. Bjorn sets a course for his return to Kattegat.
As Lagertha and her allies finalize their plan to throw off the impending invasion, Margrethe plots betrayal. The settlers clash with Floki as a new settlement starts to rise.
As the battle ensues, each side tries to get the upper hand. Ivar makes a pragmatic decision to protect his own forces, even if it means leaving an ally to his fate.
The battle's aftermath sees loyalties shifting and peace discussed. The tension between the settlers comes to head as fire breaks out and a tragic death occurs in the aftermath.
Floki resorts to drastic measures to keep the peace between the settlers. When Ivar's forces clash with Lagertha's one more time, brother faces brother on the battlefield.
With Ivar gaining control of Kattegat, he goes on the hunt for Lagertha and her companions. Lagertha makes a daring trip to lands belonging to old enemies.
Floki's faith is tested as he begins to doubt his visions. Ivar faces trouble from every side in Kattegat. Heahmund's homecoming proves to be anything but triumphant.
In Wessex, Lagertha's companions are divided by religious pursuits. Heahmund plots to eliminate Aethelred. Harald plans a betrayal.
As Ivar embraces his self-proclaimed godhood, Hvitserk grows concerned about their future. With Harald at their gates, Aethelred is forced to make a decision about his brother.
Heahmund is torn between his faith and his feelings for Lagertha. Alfred's victory lap is soured by the news of Aethelred's involvement in the conspiracy against him.
Bjorn plots a return to Kattegat, even if it means allying with an old enemy. Alfred's illness drives Judith to take extreme measures to protect the crown.
In the wake of a massacre, Floki is left powerless and despondent. New alliances are made as Wessex becomes under attack one more time, and Bjorn and Harald finalize their plans against Ivar.
Hvitserk's resolve earns him a surprising alliance. Judith locates the missing Lagertha. Ivar brutalizes his dissenters.
As multiple forces converge on Kattegat, Ivar decides to stand his ground. Floki makes a shocking discovery in the cave.
With Kattegat under attack, Freydis takes revenge on Ivar by turning on him. Bjorn's kingship is threatened as he receives a startling warning.