It’s no secret that Ragnar (Travis Fimmel) and Floki (Gustaf Skarsgard) have had one of the most rollercoaster relationships over the past three seasons of Vikings. When the History series returns for a fourth season on Thursday, however, it seems as though their long-standing friendship may finally be coming to an end.

The season premiere, “A Good Treason,” picks up several weeks following the raid on Paris, with the Norsemen returning home to Kattegat to deal with the repercussions of battle. That includes a weakened Ragnar, who is still healing from that precarious fight, and a more mature Bjorn (Alexander Ludwig) who is forced to speak to their people in his father’s absence. But when he arrests Floki for the murder of Athelstan (George Blagden) however, he unknowingly forces his father’s hand, spinning the shipbuilder and the king into a do-or-die kind of situation.

“Obviously one of the many burdens that Ragnar has to carry is Floki and Floki’s murder of Athelstan,” creator and showrunner Michael Hirst tells The Hollywood Reporter. “Athelstan meant something to Ragnar but so does Floki. So it’s very, very pivotal and difficult for Ragnar to deal with Floki. He finds it soul-destroying to have to decide this man’s life and death. Frankly, he can’t do it.”

In the exclusive season-four premiere video above, it seems as though Ragnar may have no other choice now that his former friend has been so publicly accused. But with Rollo (Clive Standen) plotting to battle his brother when he returns to France, Aslaug (Alyssa Sutherland) beginning to eye the throne for her own sons, and Lagertha (Katheryn Winnick) dealing with her own political problems, there aren’t many friends left in Ragnar’s corner. Sending one of the last men standing to Valhalla may not be in Ragnar’s best interests.

“He needs some unexpected intervention. Floki and Ragnar are too close and I think you feel that,” Hirst says. “These are incredible scenes. Gustaf is just amazing in these scenes when he’s tied up and expecting the worst. I don’t think there’s any way that Ragnar can deal with this except for some unexpected intervention.”

Meanwhile, things are picking up steam in Paris, when Rollo finally makes good on his promise to join the other side by marrying a devastated Princess Gisla (Morgane Polanski), effectively turning his back on Ragnar for good. Call it a divine intervention of sorts, as this season sets up the ultimate showdown between brothers.

“Rollo needs to know whether the gods really do think it’s his destiny to rule in Frankia and achieve great fame in,” Hirst says. “He doesn’t know the answer, and he has to suffer a lot of humiliation and social pride to find out whether that’s true.”

Vikings returns on Thursday at 10 p.m. on History Channel.

How do you think things will shake out with Floki and Ragnar? Sound off in the comments below.

Twitter: @amber_dowling