Entry to Valhalla eludes Ragnar, but as he lies in his sick bed in Kattegat events unfold beyond his control. Bjorn orders the arrest of Floki for Athelstan's hugging while in Paris, Rollo betrays the last of his Viking supporters to strengthen his position in the French court. In Hedeby, Kalf announces joint earldom with Lagertha, but is power sharing Kalf's true intention and is Lagertha actually willing to share what once was hers?.
As Ragnar and Floki remain at odds, Rollo makes great efforts in Paris to win over his new bride, Princess Gisla. Meanwhile, King Ecbert sets the task for his son Aethelwulf to rescue Queen Kwenthrith.
Ragnar informs Floki of a heavy price he has paid for his actions; Rollo finds an unlikely ally in Count Odo; Bjorn has to fight a fierce opponent in the wilderness.
King Aelle goes to Wessex for Christmas and is not impressed with how family relations are developing; Rollo's future hangs in the balance.
There's an air of betrayal as those whom Odo trusts plot against him; Ecbert agrees to support Kwenthrith in Mercia; Pregnancy brings happiness to Lagertha and Kalf and a marriage is arranged.
Ragnar declares another raid on Paris at the Thing in Kattegat; Ecbert also has a journey in mind as he dispatches Aethelwulf and Alfred on a pilgrimage to Rome.
In England, King Ecbert's true ambitions are revealed as he plans to gain the crown of Mercia. While outside Paris, the Vikings attack with considerable force, but can they overcome Rollo's defenses? Meanwhile, Harbard, the mysterious wanderer, returns and causes a stir amongst the women of Kattegat.
Defeat for the Vikings calls Ragnar's leadership into question as they evacuate their camp and move back down river. Ragnar remains inscrutable until he orders the fleet to beach at a cliff face and unveils an ingenious plan. Rollo and Gisla have news that strengthens Rollo's position at the French Court whilst Count Odo fails to identify his true enemies, with fatal consequences.
The labors of the Vikings eventually bear fruit as, within sight of Paris, they re-launch their boats but this time along with some interesting structures that Floki has built. Aethelwulf and Alfred eventually arrive in Rome and honors are conferred by Pope Leo but relations are not so cordial in Wessex between King Ecbert and the disgruntled King Aelle.
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's reappearance in Kattegat triggers a chain of events no one - except the Seer - could have ever imagined. He unexpectedly arrives home to see what has become of his sons and to handle unfinished business in Wessex with King Ecbert. 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.