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.moreless
As a second assault on Paris is being prepared, Emperor Charles must make a difficult decision. Meanwhile, in Wessex, Judith must make her own difficult choice.
The Viking army attacks Paris as the city musters its forces for defense.
The populace of Paris is in a panic after the appearance of the Viking fleet, and Emperor Charles, at the behest of his daughter, announces he'll remain in the city.
While Rollo worries about the Seer's prophecy, the Vikings make final preparations for their raid on Paris.
Tragic circumstances greet the fleet on its return to Kattegat; Lagertha is usurped and convinces Ragnar to help her reclaim her title.
An alliance angers Floki; Princess Kwenthrith makes political moves.
As the new Viking settlement prepare for their first harvest, King Ecbert pays them a visit.
A mysterious wanderer arrives in Kattegat; Lagertha and Athelstan establish a Viking settlement.
King Ecbert proposes a deal to the Vikings in the Season 3 premiere. Later, Ragnar leads his forces into battle.
When safely back at Kattegat with King Horik, Ragnar only trusts those who stood by him.
Ragnar's invading party are met with extreme violence when they march on King Ecbert's villa.
Horik and Ragnar travel to Wessex, but their true purpose is disputed. Meanwhile, Princess Aslaug's prepares to give birth.
Justice is dealt to Jarl Borg, and allies are made on both sides of the sea.
Ragnar receives an unexpected offer from King Horik and Lagertha's new husband doesn't seem overly pleased to see her. Meanwhile, Athelstan becomes King Ecbert's trusted aide.
Lagertha is once again reunited with Ragnar and Bjorn make another choice. Meanwhile, Athelstan finds it difficult to define his faith and Aslaug has to accept the consequences of her prophecies.
While Ragnar meets Ecbert in search of a compromise, Rollo must lead his people. Meanwhile, Jarl Borg imposes a strict rule on Kattegat.
While the raids on Wessex continue, Ragnar attempts to rule in the west. Meanwhile, King Ecbert faces a new kind of enemy and Jarl Borg makes his own plans for Kattegat.
Four years have passed and Ragnar leads a new series of raids on England.
As Princess Aslaug journeys to Kattegat, Ragnar allies himself with King Horik in the battle with Jarl Borg.
While a plague ravages the village of Kattegat, Ragnar sets off towards Götaland, where he must settle land dispute with Jarl Borg.