The stage has now been thoroughly re-set after the death of Haraldson, with Ragnar facing a new enemy which includes at long last his brother Rollo. Brave and smart, but not as smart or visionary as Ragnar, Rollo has all season been both loyal to his brother and chafing under the recognition that Ragnar has received but Rollo has not. Like all second-bests, Rollo seems unaware that it was Ragnar's vision, which Rollo often opposes, that propelled Ragnar's journeys to the west and their successful plundering of England.
But, significantly, Rollo is not the only one separating from Ragnar. The same is happening with Lagertha, although she definitely doesn't want this. But Lagertha's miscarriage has somehow given the superstitious Ragnar the idea that maybe the gods are not smiling on the possibility of more of his sons from Lagertha. So he is primed to look elsewhere, and Lagertha fully senses this.
Thus, for a variety of reasons, Ragnar can't resist sleeping with the beautiful princess he finds on his journey to the big ash tree. I can't recall if Ragnar slept with anyone other than Lagertha in the story before the finale - he refrained during the attacks on England, certainly the one in which Lagertha accompanied him - but given the Viking culture that we've seen, it seems a pretty safe bet that he has indeed partaken. On the other hand, son Bjorn who sees Ragnar and the princess making love - and is unsurprisingly not happy about it - warns Ragnar that "Lagertha would cut your balls off" if she knew about this, and gets Ragnar to promise that this will just be a starry one-night stand.
Surprisingly, Ragnar keeps that promise, at least once saying no to the princess when she wakes him the next evening. But her revelation the next time they meet that she's pregnant will change everything, assuming she's telling the truth and especially if she's carrying Ragnar's son. Depending upon how quickly the series moves through the years of Ragnar's life and exploits, we may well see some sort of conflict in the future between Bjorn and the son the princess bears.
But, first, Ragnar will have to deal with fighting his brother in the conflict between the king whom Ragnar supports and the earl that Rollo has now thrown his lot in - not to mention what will happen with Ragnar and Lagertha.
Lots of good viewing in the future of Vikings with such ground-changing developments below the ash, and I'm looking forward to the second season on the History Channel in 2014.