While Ragnar must deal with the aftermath of the fatal storm, Bjorn goes to meet his uncle. Meanwhile, Lagertha returns to Kattegat.
While the great and the good descend into Kattegat in preparations for Bjorn Ironside's voyage to Rome, Ragnar must try to convince the remainder to join his raiding party to England.
In the 10th episode (mid-season finale), victory proves costly in the battle between the Vikings and the French as Ragnar squares off against Rollo, sealing the fates of the two brothers.
As they near Paris, the Vikings relaunch their boats, but this time they bear interesting structures made by Floki; Aethelwulf and Alfred arrive in Rome, where honors are bestowed by Pope Leo IV, but in Wessex, relations are less than cordial between King Ecbert and a disgruntled King Aelle.
Ragnar launches the attack towards Paris. Rollo is however prepared for his fellow Vikings. In Wessex, King Ecbert sets out to meet Prince Wigstan who then gives an tempting offer to the King. In Kattegatt, Harbard meets the women of the village.
It's Christmas time. Ragnar spends some alone time with the new slave Yidu. Bjorn has an unexpected encounter in the forest. Ragnar come home to find his chair in the great hall occupied by another king.
Both Ragnar and King Ecbert gets a visit by Athelstan. Aethelwulf comes home after a successful rescue mission. Rollo tries to learn the language in Paris which proves difficult.
Floki has escaped and the search for him is on. In Wessex the King gets some bad news about his allies. Aethelwulf is off on a rescue misson.