As Ragnar lies in his sick bed in Kattegat, events unfold beyond his control.

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 gets to know the new slave woman while Rollo reveals his ambitions to his wife. Bjorn meets an unexpected warrior on his way home.

Halfdan the Black arrives in Kattegat. Floki reveals a secret to Finehair. Ragnar and Yidu become closer. Meanwhile Lagertha settles a score.

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 has returned to Kattegat where he devises a plan to return to Wessex to right past wrongs.

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.

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

King Aelle can't hide his pleasure when Ragnar arrives in a cage and shows him no mercy.

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

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.

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 Kingdom of Wessex.

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