Tensions mount between the sons of Ragnar Lothbrok as the Vikings continue to threaten the very heart of England. As the Great Army moves to take York, with King Aethelwulf and his family still in hiding, Heahmund, the warrior bishop, must rally the Saxons to defend the Realm.

As the Vikings establish their stronghold in York, Ivar is determined to lead the Great Army. In Kattegat, Lagertha misjudges King Harald and pays a heavy price. Floki arrives at a mysterious land.

Celebrations are cut short in the aftermath of the battle at York. Ragnar Lothbrok’s sons are pitted against each other as tensions reach an all-time high and each is forced to choose a side.

Guided by Bishop Heahmund's visions, the Saxons devise a battle plan. Ivar the Boneless strategizes on his own as the Vikings undergo increasing pressure. Bjorn Ironside finds himself in a new territory and must convince the local commander he is a trader and not a raider.

Ivar meets his match in Bishop Heahmund, while Floki returns to Kattegat and Bjorn receives a lavish welcome in North Africa.

Lagertha is betrayed and Bjorn must find a way to support the distraught queen. Floki and his settlers arrive in a new land but it is far from what they expected.

Bjorn returns to Kattegat to learn that an attack is to take place during the next full moon. Ivar must decide if he can place his trust in a former enemy on the battlefield.

The battle for Kattegat begins. As the two armies line up to fight, the Great Heathen Army must decide between a final plea for peace or all-out war. Floki faces discord amongst the settlers.

The army leaders consider their options in the aftermath of the battle. In Floki's camp, all hopes of binding the community together are dashed as tragedy unfolds.

A sense of doom looms over Kattegat as bloodshed ensues. As the defeated army flees in the face of the victors, a legendary warrior makes his way home.