Ragnar and Floki are still at odds; Rollo makes great attempts to win over his new bride, Princess Gisla; Bjorn's survival skills are put to the test; King Ecbert sets the task for his son Aethelwulf to rescue Queen Kwenthrith.
Ragnar tells Floki he has to pay a heavy price for his actions. Rollo finds an ally in Count Odo. Bjorn Ironside finds more than he bargained for in the wilderness.
Ragnar reveals his plan to raid Paris again, and although the fleet is storm-tossed as it sets out, the challenge lies in Frankia where Rollo awaits. Meanwhile, Ecbert dispatches Aethelwulf and Alfred on a pilgrimage to Rome.
Now aware that Rollo has betrayed him, Ragnar carries out his plan of attack, which has dire consequences. Ecbert is given an offer from Prince Wigstan of Mercia. Harbard causes a stir with the women of Kattegat.
After facing defeat on the river, Ragnar refuses to retreat, prompting him to change his methods of attacking Paris. Count Odo's plot against Charles and Rollo backfires. Ecbert now rules Mercia, which dissatisfies Kwenthrith.
The Vikings finally reach the river. Aethelwulf and Alfred reach Rome. Erlendur makes his move against Bjorn. Sigurd makes a shocking discovery in Kattegat. Ecbert is crowned king in Mercia.
A ferocious battle between the Vikings and the Franks eventually comes down to Ragnar against Rollo. The outcome will seal the fate of the two brothers.
Ragnar seeks to equip his voyage to Wessex, even while being warned that the gods have deserted him. Bjorn prepares to fulfill his dream of exploring the Mediterranean.
The Great Army is gathering in Kattegat, as Lagertha is saved by Bjorn Ironside from the hands of Ragnar's sons. Rollo is establishing the kingdom of Normandie as a safe-heaven for all vikings in search for land.
