Full episode spoilers.
Horik pushes the Vikings into battle against the English despite Ragnar's caution. Ecbert has used Roman military writings with Athelstan's help, and his forces draw the attackers into a trap. The Viking formation cannot withstand the cavalry attack. Ragnar, Lagertha and Bjorn survive the defeat, but Rollo is trampled and left behind. Athelstan recognises him on the battlefield and persuades Ecbert to have him taken for treatment rather than killed.
Ecbert then sends Athelstan to negotiate with the survivors. Ragnar and Lagertha are ready to listen, while Horik resists another agreement. Ragnar accompanies Athelstan out of the camp and protects his departure. With Aelle held as security, the Viking leaders attend talks. Ecbert offers treasure, farming land and Rollo's return; some Viking warriors will remain to serve as paid fighters in Kwenthrith's campaign for Mercia. The leaders ultimately accept the terms.
In Kattegat, Porunn trains to become a shield maiden, taking Lagertha as her example. Aslaug grants her freedom, changing the terms on which she can approach a future with Bjorn. Athelstan makes his own choice when Ragnar invites him to return with the departing ships. He leaves Ecbert and sails back to Kattegat with Ragnar and the wounded, although Rollo and Floki remain suspicious of where his loyalties lie.