Spoilers for this episode.
Bjorn, Hvitserk, Harald and Olaf attack Kattegat, but Ivar's defenses defeat their first assault. Fire, missiles and traps turn the walls into a killing ground. Hvitserk and Harald scale the defenses before being forced back, and Magnus dies from an arrow during the retreat. Bjorn appeals to the people he grew up among, asking them to reject Ivar's rule. When archers refuse to shoot him, Ivar punishes their disobedience with executions.
Freydis visits the attackers and reveals a way into Kattegat. She also tells Hvitserk that Ivar had Thora killed and explains what happened to Baldur. The second attack breaches the town with her help. Ivar realizes that she betrayed him and strangles her, leaving her body behind as his rule collapses. Harald saves Bjorn during the fighting and is wounded in the process. Bjorn and Hvitserk reach the hall and find Freydis dead, while Ivar has already escaped.
Ubbe, Torvi and Lagertha return as Kattegat celebrates the victory. Lagertha presents Bjorn with the sword of kings, and the crowd acclaims him as its ruler. His triumph gives way to images of the responsibility he has inherited, including a bloodied battlefield and a conversation with the Seer. Ragnar's remembered words also confront him with the dangers of power. Ivar survives the defeat and is shown leaving in disguise on a cart, ending the struggle with Bjorn crowned and his youngest brother still alive.