Full spoilers for this episode.
Ragnar travels as Horik's emissary to Jarl Borg, whose claim to disputed land leaves little room for an easy settlement. Borg proposes alternatives, but Ragnar lacks authority to agree and sends Floki back to the king. Rollo stays behind as security while Ragnar and the others visit a great ash tree. Borg uses the time to talk about betrayal between brothers and to question why Rollo remains unknown beside Ragnar's growing reputation.
During the journey, Ragnar meets Aslaug after his men encounter her bathing. She answers the unusual conditions of his invitation and joins the party. Their attraction leads to sex, upsetting Bjorn, who confronts his father about betraying Lagertha. Ragnar promises that it will not happen again. Aslaug later tells him that she is carrying his child. Bjorn has seen the affair himself, despite Ragnar's insistence that he still loves his wife.
In Kattegat, Lagertha asks the Seer about troubling dreams before a deadly illness spreads through the community. Athelstan and Siggy become ill, and Gyda and Thyri die, leaving both mothers grieving. Floki returns with Horik's refusal to compromise, ending Ragnar's mission without peace. Ragnar will not abandon his oath to the king, so Borg prepares for war. In the closing exchange, Rollo agrees to fight alongside Borg against his brother, turning his resentment into an explicit betrayal.