Full episode spoilers.
Horik brings his family to Kattegat while preparing to destroy Ragnar's household. He treats the earl's growing influence as a threat to his own rule and his children's future. Floki appears willing to help him, but Horik demands a killing as proof. Torstein becomes the apparent victim of poisoning, convincing Horik that the shipbuilder has finally crossed a line from resentment into action against Ragnar's trusted companions.
Once satisfied, Horik reveals the wider plan. Floki is to kill Bjorn, and Siggy receives a knife to murder Ragnar's younger sons. Both seem to offer Horik access to the people he cannot safely attack in the open. Meanwhile, Ragnar and Athelstan pray together, and Horik's warriors approach Kattegat. The household's ordinary encounters conceal preparations that Horik fails to understand while he concentrates on carrying out his own attack.
When the killing begins, Horik discovers that his supposed allies have deceived him. Floki protects Bjorn, Siggy does not kill the children, and Torstein is alive. Lagertha fights Horik's wife as Ragnar's supporters turn against the intruders. Horik reaches the hall expecting victory and instead finds the people he relied on standing with Ragnar. Ragnar kills him, ending the king's attempt to wipe out his family. Horik's death leaves Ragnar as king, with his household intact and Floki and Siggy's loyalty confirmed.