Following explosive episode 3, History channel's historical drama "Vikings" season 6 is set to air episode 4 revealing how it all plays out for Bjorn after he learns about Olaf's plans for King Harald's kingdom. The next chapter will unravel Bjorn's next move as he attempts to rescue Harald. Here is everything we know so far about the upcoming segment.

[Spoiler alert! This article contains spoilers for "Vikings" Season 6 Episode 4. So, steer away immediately, if you don't want to learn more about it.]

While Bjorn is forced to retreat after Olaf defeats his army, Lagertha and her women are under attack. Forcing the queen to rise again in order to protect her neighbours and her grandchildren, Lagertha is back in action. "Vikings" season 6 episode 4 is titled "All the Prisoners" and it will deal with situations Bjorn and Lagertha are in.

The official synopsis suggests that horrors of battle have just begun. Fans must brace themselves for what is described as tragic consequences. According to the description, Lagertha is forced out of her retirement to take the lead in the wake of recent attacks.

In the last episode, we saw her dig out her sword that she so solemnly buried in the ground last week. It is certain, she was born to be a fighter and she continues to do so. However, as she takes the lead in protecting her village despite her best efforts, she fails in stopping the devastation. Will this be Lagertha's last battle?

Meanwhile, Olaf seems to be on the path of success, and he devises a new plan for the future of Norway. As Olaf basks in his triumph, Bjorn is suspected to be strategising his next move. Back in Kiev, Ivar probes into Oleg's plans for Scandinavia. However, his real interest lies in Igor, the young heir of Oleg's kingdom.

Vikings season 4
After fighting a wild grizzly bear, Bjorn will have an unexpected and life threatening visitor in the jungle History Channel

Meanwhile, the promo gives a glimpse at the battle Lagertha is leading back in the village. The trailer ends with Lagertha screeching in horror. Elsewhere, Ivar learns more about Oleg's plan who has ordered an invasion of Scandinavia and that he plans to use him as a "puppet ruler."

"Vikings" season 6 episode 4 airs Wednesday, December 18 on History.