A dangerously ill Ragnar returns to Kattegat, where his suspected death kicks off a scramble to succeed him. Meanwhile, Lagertha engages in a power struggle with her calculating, former second in command, Kalf. Rollo betrays his heritage and remains in Frankia. Floki pays the price for his brutality against the Christian priest.
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.
Floki learns of the 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 plots to gain the crown of Mercia. The Vikings attack Paris with considerable force, but will Rollo's defenses be too much for them to overcome? 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.
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.
