[Warning: this article contains discussion of season 4 episodes]
The season premiere brought us back to Kattegat and the time skip is pretty obvious when you take a look at Ragnar's sons. There's definitely been some time since the Lothbroks and company left Paris behind for Rollo to deal with. It seems we'll have three basic locations where plots will come from this season: Scandinavia (Kattegat and Hedeby), Wessex and Paris.
In Kattegat, we see a convalescing Ragnar, who's been denied entry to Valhalla in a dream-like allucination of sorts. (Nice symbology, by the way, with the golden door, the warriors laughing and the dog in the guardpost.) We get to see Ragnar's reasoning as to why he's not allowed to enter, in a succession of quick flashes: Athelstan' death, his constant fighting with Rollo, his kills. Later on, when he regains consciousness, his son Ubbe gets excited to see his father alive, to what Ragnar utters an "unfortunately".
While Ragnar was out of it, Bjorn takes matters into his own hands and orders the arrest of Floki for Athelstan's death, which doesn't please Ragnar in the least. He'll now be "forced to deal with his errant friend", which he'd clearly been avoiding deeply. Besides the situation with Floki, Ragnar also has to deal with his wife Aslaug, who seems not so happy that he's come back from the brim of death. Apparently, she's in it for the chance, to one day, be queen of Kattegat, which Ragnar clearly sees as her being untrustworthy. To make matters worse for Ragnar, Bjorn decides he wants to test his manhood and live in the wilderness, simply because Ragnar "doesn't think he can" make it.
In Hedeby, we get to see Lagertha getting along quite well with Earl Kalf, including his decision to share the Earldom with her, which doesn't please a lot of people. Kalf takes matters into his own hands and tricks Einar and his followers into stepping up and showing their disapproval of Lagertha. Like a good Viking, he proceedes to kill all of them and Lagertha, emboldened by this, cuts a pivotal body part of Einar, finishing him off (quite unecessarily, if you ask me).
Back in Paris, where we left off last season, Rollo gets married to a very unwilling princess Gisla and becomes a Duke, sealing the protection of Paris - and eventually Frankia - from his brother's future raids. On the wedding night, Gisla proves to be quite the wildcat and pulls a knife out on Rollo, who simply turns to his side and eventually falls asleep. (No doubt some of the best parts of this season will come from these two and Paris.)
But not everything is light in Paris and the Duke's life, so Rollo is faced with news that the remainder Viking army has split opinions on him being now a christian. He reassures Eirik he'll deal with it (and, oh boy, does he ever). Along with the Parisian soldiers, Rollo goes back to the Viking camp and kills every last one of them, even his friend Eirik. He doesn't get off easy though, and receives the absolute best line of the episode from a dying Eirik: "Ragnar is coming. Ragnar will avenge us." (I'm definitely looking forward to that.)
Overall, this first episode pretty much sets the tone for the season - or at least the first half of it. There'll be growth and discovery of selves, the clash of religions once again, a little loving from odd couplings, some forgiveness and, of course, a whole bunch of violence.