Bjorn decides he wants to explore and plunder the Mediterranean. Bjorn takes a bunch of men, including Hvitserk, his younger brother, and Floki.
Ragnar and Ivar head back to England.
Ragnar is executed (in a pit of snakes) by an English king.
Meanwhile, Lagertha, Ragnar's first wife, invades Kattegat and takes it over, apparently in revenge for Aslaug, Ragnar's second wife, seducing Ragnar and stealing her kingdom.
Ubbe and Sigurd are pretty mad, especially since Lagertha killed Aslaug in the process.
Ivar returns from England, is angry that that his mother has been killed and that his brothers haven't killed Lagertha yet.
Word comes to all of Ragnar's sons, through visions of Odin, that Ragnar is dead.
Ivar is angry and wants to kill Lagertha to avenge his mother. Ubbe, Sigurd and Ivar all challenge her in the throne room before Bjorn returns.
They all decide that they will band together to seek revenge in England for the death of Ragnar and then maybe deal with the other issue later.