Full episode spoilers.
The Viking fleet meets Rollo's forces on the river, and Ragnar drives toward his brother through the fighting. Their confrontation becomes a savage hand-to-hand struggle while the battle continues around them. Lagertha is wounded, and Ragnar's companions struggle to reach him before the Franks overwhelm them. The Vikings pull him away against his will and retreat with their injured, leaving Rollo alive and Paris beyond their reach.
Charles has Roland and Therese strangled while Rollo fights for the city. The emperor then welcomes his battered son-in-law as the victorious defender of Paris, and Gisla receives her husband before the celebrating crowd. Ragnar has failed to defeat the brother who abandoned him. The story then moves forward several years to a larger Kattegat, where Aslaug rules and Ragnar has been absent long enough for his younger sons to grow into men.
A messenger brings news of Magnus and of the destroyed Viking settlement in Wessex, exposing Ragnar's decision to conceal the massacre. Bjorn tells his brothers, whose responses range from anger at their father's abandonment to Ivar's defence of him. Bjorn prepares to sail toward the Mediterranean with Floki and Helga. Ragnar then returns to Kattegat, walks among the crowd and confronts his sons, challenging anyone who wants to be king to kill him and take his place.