Ragnar Lothbrok and his brother Rollo return from a battle in which Ragnar has visions of the god Odin and his valkyries. Home again, Ragnar takes his son Bjorn to Kattegat so that Bjorn may undergo his rite of passage. Left at home, Ragnar's wife Lagertha quickly dispatches two would-be rapists. In Kattegat, Ragnar convinces Rollo that raids to the west are worthwhile and possible thanks to new navigational tools but is rebuked by his ruler, Earl Haraldson, who orders raids into the Baltic as usual and has the upstart Ragnar watched. Bjorn and Ragnar visit Floki, a gifted shipwright and Ragnar's friend. He has been secretly building a new type of longship, which they successfully test. Meanwhile, back at home, Rollo makes unwelcome advances on Lagertha.
After gathering volunteers, Ragnar, Rollo, and Floki embark on an unauthorized raid to England. Lagertha unhappyly objects to Ragnar's refusal to take her along. Earl Haraldson has a man killed who attempts to seduce his wife Siggy, as well as the blacksmith who forged Ragnar's anchor. After a tense voyage, Ragnar's men land in England near the monastery of Lindisfarne, which they proceed to sack. They hug most of the monks and capture the rest to take back as slaves, including the young Athelstan, whom Ragnar protects from death at Rollo's hands.
Ragnar's warband returns in triumph to Kattegat, where the Earl confiscates the plundered riches except for one piece per man. Ragnar picks the distraught Athelstan and returns home. The monk's faith and his vow of chastity perplex Ragnar, but Ragnar gathers useful intelligence about the kingdom of Northumbria. On that basis, Earl Haraldson authorizes another raid on England. Now accompanied by Lagertha and the Earl's trustee Knut, Ragnar re-embarks post-haste, leaving Athelstan to mind the farm and the children. As the Vikings set foot on English soil, they are met by the local sheriff and a handful of armsmen, who invite the newly landed "traders" to meet King Aelle. Ragnar agrees, but his other warriors' distrust incites a battle, in which the Northumbrians are slaughtered.
The Vikings head back to England to see what other treasures this new world has to offer. Ragnar and his crew sail out with Earl Haraldson's permission….and also with Earl Haraldson's spy. After bloodshed on the beach, the Vikings choose the Sabbath to invade the town of Hexam and unleash a flood of violence and panic. In the midst of the terror, Ragnar and Lagertha (Katheryn Winnick) discover that there are consequences for every action and Earl Haraldson is always watching.
In the Great Hall of Kattegat, a seer reads Earl Haraldson's future and tells him that Ragnar Lothbrok searches for his death. The unanswerable question is whom do the gods favor more? As a man willing to defend his power and status to the death, Haraldson intends to take precautionary measures. Ragnar, his home and farm burned by the Earl is hurt but hidden safely away. He must now choose the path that will lead him back to his freedom.
Ragnar, weak and still hurt, must meet the Earl head-on after it comes to light that Rollo has been tortured on Haraldson's orders. The two men will come together face to face with a single outcome possible: Only one man will leave this fight alive.
Three Viking ships sail upriver towards the very heart of power in eastern England: the Royal Villa of King Aelle. Ragnar Lothbrok has come looking for ransom in exchange for peace. However, King Aelle has a different plan in mind for these northern heathens.
The traditional pilgrimage to Uppsalla to thank the gods brings a torrent of emotions for Ragnar, Lagertha, and Athelstan. Ragnar, pulling farther away from his wife, goes to make peace with the death of his unborn son. Lagertha, still reeling from her miscarriage, wants to find out from the gods if more sons are in her future. As the Vikings come together to sacrifice and give thanks to their gods, Athelstan discovers just how strong his Christian faith still is.
At the behest of King Horik, Ragnar assembles a small party to travel to Gotaland (modern day Sweden) to resolve a land dispute with the area's leader, Jarl Borg. Ragnar's renown precedes him and Jarl Borg is intrigued - has he found a new ally or is Ragnar just a puppet of the king? Meanwhile in Kattegat, with a plague taking its toll on the village, the people look to Lagertha to help appease the gods.