A ferocious battle between the Vikings and the French eventually comes down to Ragnar against Rollo. The outcome will seal the fate of the two brothers.

Ragnar returns to Kattegat and formulates a plan to head back to Wessex, where he intends to right past wrongs.. Meanwhile, Lagertha plans a power play in Kattegat and Bjorn prepares to fulfill his long-held dream to explore the Mediterranean in a sleek new boat built for him by Floki.

The Kattegat locals chide Ragnar that the gods have deserted him as he struggles to crew his voyage to Wessex. It's Bjorn who has charisma now as he prepares to fulfill his long-held dream to explore the Mediterranean in a sleek new boat built for him by Floki. By contrast, Ragnar must accept weathered boats and a crew he has bribed but when Aslaug foresees that the fleet will be lost in a vicious storm her warnings go un-heeded by Ragnar and her precious son, Ivar.

Ragnar's fleet is shipwrecked and washes up on the Wessex coast, but without enough survivors to form a credible force, Ragnar and Ivar have a difficult decision to make. Rollo meets his Viking comrades again and can't resist the urge to join Bjorn's voyage, risking his marriage and his new Frankish identity. Ragnar too is taking a risk as he carries Ivar towards the Royal Villa in Wessex – throwing himself and his son upon the mercy of King Ecbert.

Lagertha attempts to fulfill a long-held ambition. Ragnar negotiates an unexpected concession from King Ecbert.

Ragnar and Ivar plot against the Saxons and reach a new level of understanding.

Bjorn's fleet sails onward and launches a surprise attack. Back in Kattegat, Lagertha continues in her quest for power.

The Viking army begins to gather in Kattegat. Lagertha must remain vigilant in the absence of her son, Bjorn.

Ragnar's plan comes to fruition. The Viking army lands on the coast of Northumbria.

The Viking army causes panic in the English countryside as King Ecbert and Aethelwulf plan the defense of the realm.

Prince Aethelwulf finds himself subject to the Vikings' battle master plan. Ecbert remains behind with a plan of his own.

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.