Full episode spoilers.
Lagertha's arrival gives Ragnar enough support to challenge Borg, but first they need to bring him out of Kattegat. Ragnar takes Bjorn into the occupied settlement to burn its grain stores while other men create a distraction. The raid succeeds at a cost to the attackers. Borg follows them into the countryside, where Ragnar, Lagertha and their allies meet his warriors in open battle and force him to retreat.
Bjorn fights beside his parents but loses his footing and weapon during the struggle, drawing criticism from Ragnar afterwards. The victorious force returns to Kattegat, where Ragnar gives Bjorn the task of killing a prisoner in the celebration of their victory. The son who once begged to sail with his father now has a direct place in his fighting household, including its acts of punishment and sacrifice.
In Wessex, Athelstan recovers from his crucifixion under Ecbert's protection, but religious visions disturb his attempts to resume Christian worship. In Kattegat, Ragnar hopes Lagertha might stay, despite the discomfort this causes Aslaug. Lagertha decides that Bjorn may remain with his father while she returns to Sigvard. She announces her decision before the household and asks Ragnar to look after their son, then rides away without him.