After a fateful encounter with Prince Oleg, the ruler of Kievan Rus, Ivar finds an unlikely ally in the prince. Bjorn is drawn into a conflict as an old friend seeks to collect on a previous favor.
Bjorn prepares to ride to Harald's aid. Oleg displays powers that help escape punishment after making a move against one of his brothers.
As Oleg's actions start to trouble Ivar, the past comes back to haunt Hvitserk. Lagertha is forced to step in to aid her neighbors to fend off raids in Bjorn's absence.
As Lagertha faces a devastating raid from White Hair and his men, Bjorn faces his own troubles as his army is besieged by Olaf. Ivar turns on Oleg.
Lagertha receives much-needed help from Gunnhild and her shield-maidens. Hvitserk continues to be haunted by horrific visions. The kings of Norway come together for an election.
As Harald and Kjetill plot together, Kjetill comes to realize that Harald has dangerous plans for Bjorn. A hallucination causes Hvitserk to commit a drastic act.
As Kattegat deals with the aftermath of Lagertha's death, Bjorn swears vengeance. Ivar makes plans against Oleg. Harald turns on Olaf.
Hvitserk faces the fallout of his murder of Lagertha. Tensions rise between Oleg and Igor after the latter issues a challenge. Harald prepares for the impending invasion by Oleg's men.
Ubbe gets information about Floki's disappearance. Bjorn asks for aid as the Rus's soldiers approach. A secret plot takes shape as Oleg tries to put an end to paganism.
As Oleg launches his attack, the kings of Norway face the possibility of defeat as political intrigue puts their alliance on the line. A betrayal threatens to bring Bjorn down.
With Kattegat being next in line for the invasion, Hakon and Hrolf try to figure out a war plan as Oleg executes one of the prisoners.
Erik manipulates Gunnhild and Ingrid to turn them against each other. Oleg resorts to violent measures in the wake of his defeat. Katia gets Ivar to reveal sensitive information.
Ingrid and Gunnhild's plans are interrupted by Harald's unexpected arrival. Hvitserk and Ivar clash over the former's loyalty to Oleg. A daring plan is set into motion in preparation for Dir's attack on Oleg.
Gunnhild struggles to make a decision after receiving an unexpected proposal. Ivar and Igor reunite with Dir as the latter rallies men to his cause.
Oleg's reign comes to an end as Dir arrives with his forces, with Ivar pushing Igor to make a decision about Oleg's fate. Gunnhild resorts to drastic measures as a response to Harald's proposal.
Death looms over Ubbe's expedition as they run out of food and water. Ivar and Hvitserk face backlash from their people when they return to Kattegat.
As Harald and Ivar leave for their planned invasion, Erik makes a power play to seize the throne. War breaks out in England as the Norwegians launch their invasion.
As Ubbe's expedition finds new land, their hoped-for salvation soon takes a turn. Erik clashes with Ingrid as she imposes her control over Kattegat.
As Ubbe makes peace with the native warriors, he comes to learn of a shocking arrival that preceded his own. Erik's plans to have Ingrid killed backfires.
The peace between Ubbe's people and the natives is threatened by an assault by one of the settlers. Ivar makes a sacrifice to end the hostilities in England and save his brother's life.