King Ecbert proposes a deal to the Vikings in the Season 3 premiere. Later, Ragnar leads his forces into battle.
Lagertha and Athelstan help to establish the Viking settlement in Wessex. Harbard, a mysterious wanderer, arrives in Kattegat, realizing the forewarned dreams of Aslaug, Siggy and Helga.
King Ecbert visits the developing Viking settlement as the first harvest is sown and rebuffs criticism from his leading nobles. In Mercia, the Wessex/Viking forces charge at the battle at the Hill of the Ash. In Kattegat, Harbard arrives in to the Great Hall and his mysterious aura fascinates Aslaug and Helga but Siggy remains suspicious. There are strange and tragic happenings in Kattegat, and Siggy suspects Harbard is the cause.
The victorious Wessex/Viking forces return to Wessex but there is rancour in the Viking camp–Floki is angry over the alliance with Ecbert and resentful the influence that he feels Athelstan has over Ragnar. Princess Kwenthrith makes some calculating political moves following the battle at the Hill of the Ash in Mercia. Visitors from the past arrive in Hederby, at Kalf's invitation.
The fleet returns to Kattegat to find tragic circumstances await. Lagertha learns that her Earldom has been usurped and convinces Ragnar to travel to Hederby with her in an effort to reclaim her title. Ragnar causes great surprise when he announces the next raid. The Seer has interesting prophecies for Rollo.
Preparations for the Paris raid pick up pace, and Rollo ponders the Seer's prophecy about his destiny. Kalf arrives in Kattegat to join the raid and the Viking fleet sails up the Seine and catches sight of Paris for the first time. Porunn and Judith both deliver children, and with new life there is also death.
The appearance of the Viking fleet causes panic among the population of Paris. The Emperor Charles, urged by his daughter Gisla, announces in Court he will remain in the city. The defense of Paris is in the hands of its Commander, Count Odo, who hopes that a heroic victory against the Vikings will gain him Gisla's hand in marriage. The Vikings establish camp downriver from the city and plan their attack.
The Viking army embarks and Paris goes into lockdown as the army prepares the defense, overseen by Odo. The Vikings are determined to make it through the gates of Paris and will stop at nothing to succeed.
The Vikings go all out for a second assault on Paris. Recognizing the desperate state of affairs for his city, Emperor Charles must make a difficult decision. In Wessex, Judith is compelled to make a difficult decision of her own.
With one last chance to take Paris, Ragnar and his Viking troops take a daring chance. Ragnar asks Bjorn for a favor that could change the course of Viking history.