The battle begins between Ragnar and King Horik's forces against Jarl Borg; Princess Aslaug makes her way to Kattegat.

The time has come for an unlikely alliance to band together for a new raid on England; a storm pushes the Viking fleet to a new destination; Ragnar and his men may have met their match.

King Ecbert finds himself facing an entirely new kind of foe; Ragnar races to dominate in the west; Jarl Borg has his own plans in store for the future of Kattegat.

Ragnar and Ecbert come face to face; Jarl Borg rules Kattegat with an iron fist; Rollo must now become the leader his people need in Ragnar's absence.

Lagertha and Ragnar unite to try to win Kattegat back from Jarl Borg; Aslaug must face the truth of her prophecies; Athelstan struggles to define his faith; Bjorn must once again make a choice.

King Horik returns to Kattegat with a surprising proposition for Ragnar; Lagertha runs into a less than enthusiastic homecoming from her new husband; Athelstan becomes confidant to King Ecbert.

Ragnar and King Horik clash over how to dispense justice to Jarl Borg. In Wessex, King Aelle arrives and Ecbert has an eye on a new alliance.

Princess Aslaug's latest prophecy comes to fruition as she readies to give birth once more. Ragnar, Horik and their new ally Lagertha set sail for Wessex - Ragnar and Horik have very different ideas about the true purpose of this trip.

Ragnar and King Horik return to Kattegat; Ragnar places his trust in those who have stood by him.