Ragnar's difficulties raising a crew for the voyage to Wessex leaves Kattegat locals admonishing him that the gods have deserted him. Meanwhile, Bjorn's dream to explore the Mediterranean is within reach.
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, but in Wessex, relations are less than cordial between King Ecbert and a disgruntled King Aelle.
Ragnar's leadership is called into question after their defeat, evacuation and retreat down-river, although he does have a plan that he keeps under wraps. Rollo and Gisla have information that will strengthen Rollo's position at the French court, while Count Odo is slow to identify his true enemies, which could have serious consequences.
In England, King Ecbert schemes to secure the crown of Mercia, while outside Paris, a considerable force of Vikings attack Rollo's defenses. Also, the mysterious wanderer Harbard returns to Kattegat and causes a stir among the women.
At the Thing in Kattegat, Ragnar reveals his plan to raid Paris again, and, although the fleet is storm-tossed as it sets out, the challenge lies in France, where Rollo awaits. Meanwhile, Ecbert dispatches Aethelwulf and Alfred on a pilgrimage to Rome.
Betrayal is in the air at the Court in Paris and Wessex as trusted people plot against Odo, while Ecbert agrees to throw his support to Kwenthrith in Mercia. Meanwhile, Ragnar reveals his deepest secret to Yidu; pregnancy brings happiness to Lagertha and Kalf; and an arranged marriage doesn't come on as planned.
A Norwegian king and Bjorn, who is accompanied by a new partner, are welcomed to the Great Hall in Kattegat; King Aelle arrives at Wessex for Christmas and is unimpressed with the development of family relations. Meanwhile, in Paris, Rollo's future depends on whether he can win over his new bride, as a papal delegation arrives to arrange a divorce.
Floki remains a prisoner and Ragnar reveals to him the heavy price that has been paid for his actions; a mysterious visitor counsels mercy to Ragnar; intrigue in the French Court includes Rollo finding an unlikely ally in Count Odo. Also, Bjorn battles a fierce opponent in the wilderness.
In the Season 4 premiere, a dangerously ill Ragnar returns to Kattegat, where his suspected death kicks off a scramble to succeed him. Meanwhile, Lagertha engages in a power struggle with her calculating, former second in command, Kalf; Rollo betrays his heritage and remains in Frankia; and Floki pays the price for his brutality against the Christian priest.