Tensions mount between the sons of Ragnar Lothbrok; with King Aethelwulf and his family still in hiding, Heahmund, the warrior bishop, rallies the Saxons to defend the Realm, as the Great Army moves to take York.
Ivar is determined to lead the Great Army as the Vikings establish their stronghold in York; 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 one 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; Floki returns to Kattegat; 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 will 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 among 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; the defeated army flees in the face of the victors, and a legendary warrior makes his way home.
Ivar is crowned the new king of Kattegat just in time to welcome Rollo home. Bjorn, Lagertha, and Ubbe must rely on unsteady alliances to survive.
Bjorn, Lagertha, and Ubbe face an uncertain fate as Heahmund's loyalty is tested. In Iceland, Floki fights to understand the will of the gods. In Kattegat, Ivar may have met his match.
Heahmund must try to convince King Alfred that his actions are in defense of the Crown. Ubbe and Torvi strengthen the position of the Vikings but not all in the Viking camp support their strategy. Another settler disappears in Iceland.
As the celebrations for Ivar continue in Kattegat, grief hits Iceland and Floki must now make a fated decision; Harald's army approaches Wessex; a conspiracy grows against King Alfred.
Bishop Heahmund is wracked with guilt as he fights to renounce his passions. Viking will clash with Saxon on a battlefield leaving a key figure lost in the balance.
Bjorn achieves one of Ragnar's dreams. Back in Kattegat, Ivar hatches a new plan while preparing for a divine arrival. In Iceland, a settler returns in a terrible state. King Alfred faces his greatest threat yet.
An unexpected turn amongst the settlers leaves Floki powerless. King Alfred confronts Judith. In York, Bjorn must strike a deal with Harald. Wessex is once again threatened by a Viking force, but who will lead the Saxon army to defend the Realm?.
Hvitserk is severely tested. Floki makes an amazing discovery. Freydis gives Ivar a surprise. Ubbe negotiates with the three Danish Kings that have massed their armies in Reading, but the negotiations may have a perilous outcome.
Ubbe is forced into hand-to-hand combat. Floki reaches new depths and is met by a shocking sight. Bjorn sets off for Scandinavia with an old rival.
A new battle for Kattegat is on and only the gods know who will emerge victorious.