A dangerously ill Ragnar returns to Kattegat, where his suspected death kicks off a scramble to succeed him. Meanwhile, Lagertha engages in a power struggle with her calculating, former second in command, Kalf. Rollo betrays his heritage and remains in Frankia. Floki pays the price for his brutality against the Christian priest.

As Ragnar and Floki remain at odds, Rollo makes great efforts in Paris to win over his new bride, Princess Gisla. Meanwhile, King Ecbert sets the task for his son Aethelwulf to rescue Queen Kwenthrith.

Floki learns of the heavy price he has paid for his actions. Rollo finds an unlikely ally in Count Odo. Bjorn has to fight a fierce opponent in the wilderness.

King Aelle goes to Wessex for Christmas and is not impressed with how family relations are developing. Rollo's future hangs in the balance.

There's an air of betrayal as those whom Odo trusts plot against him. Ecbert agrees to support Kwenthrith in Mercia. Pregnancy brings happiness to Lagertha and Kalf and a marriage is arranged.

Ragnar declares another raid on Paris at the Thing in Kattegat. Ecbert also has a journey in mind as he dispatches Aethelwulf and Alfred on a pilgrimage to Rome.

In England, King Ecbert's true ambitions are revealed as he plots to gain the crown of Mercia while outside Paris. The Vikings attack Paris with considerable force, but will Rollo's defenses be too much for them to overcome? Meanwhile, Harbard, the mysterious wanderer, returns and causes a stir amongst the women of Kattegat.

Defeat for the Vikings calls Ragnar's leadership into question as they evacuate their camp and move back down river. Ragnar remains inscrutable until he orders the fleet to beach at a cliff face and unveils an ingenious plan. Rollo and Gisla have news that strengthens Rollo's position at the French Court.

The labors of the Vikings eventually bear fruit as, within sight of Paris, they re-launch their boats but this time along with some interesting structures that Floki has built. Aethelwulf and Alfred eventually arrive in Rome and honors are conferred by Pope Leo but relations are not so cordial in Wessex between King Ecbert and the disgruntled King Aelle.

A ferocious battle between the Vikings and the French eventually comes down to Ragnar against Rollo. The outcome will seal the fate of the two brothers.

Ragnar returns to Kattegat and formulates a plan to head back to Wessex, where he intends to right past wrongs.. Meanwhile, Lagertha plans a power play in Kattegat and Bjorn prepares to fulfill his long-held dream to explore the Mediterranean in a sleek new boat built for him by Floki.

The Kattegat locals chide Ragnar that the gods have deserted him as he struggles to crew his voyage to Wessex. It's Bjorn who has charisma now as he prepares to fulfill his long-held dream to explore the Mediterranean in a sleek new boat built for him by Floki. By contrast, Ragnar must accept weathered boats and a crew he has bribed but when Aslaug foresees that the fleet will be lost in a vicious storm her warnings go un-heeded by Ragnar and her precious son, Ivar.

Ragnar's fleet is shipwrecked and washes up on the Wessex coast, but without enough survivors to form a credible force, Ragnar and Ivar have a difficult decision to make. Rollo meets his Viking comrades again and can't resist the urge to join Bjorn's voyage, risking his marriage and his new Frankish identity. Ragnar too is taking a risk as he carries Ivar towards the Royal Villa in Wessex – throwing himself and his son upon the mercy of King Ecbert.

Lagertha attempts to fulfill a long-held ambition. Ragnar negotiates an unexpected concession from King Ecbert.

Ragnar and Ivar plot against the Saxons and reach a new level of understanding.

Bjorn's fleet sails onward and launches a surprise attack. Back in Kattegat, Lagertha continues in her quest for power.

The Viking army begins to gather in Kattegat. Lagertha must remain vigilant in the absence of her son, Bjorn.

Ragnar's plan comes to fruition. The Viking army lands on the coast of Northumbria.

The Viking army causes panic in the English countryside as King Ecbert and Aethelwulf plan the defense of the realm.

Prince Aethelwulf finds himself subject to the Vikings' battle master plan. Ecbert remains behind with a plan of his own.

Tensions mount between the sons of Ragnar Lothbrok as the Vikings continue to threaten the very heart of England. As the Great Army moves to take York, with King Aethelwulf and his family still in hiding, Heahmund—the warrior bishop—must rally the Saxons to defend the Realm.

Tensions mount between the sons of Ragnar Lothbrok as the Vikings continue to threaten the very heart of England. As the Great Army moves to take York, with King Aethelwulf and his family still in hiding, Heahmund—the warrior bishop—must rally the Saxons to defend the Realm.

Celebrations are cut short in the aftermath of the battle at York. Ragnar Lothbrok's sons are pitted against each other as tensions reach an all-time high, and each is forced to choose a side.

Guided by Heahmund's visions, the Saxons devise a battle plan. Ivar the Boneless strategises as the Vikings undergo increasing pressure. Bjorn Ironside finds himself in a new territory and must convince the local commander he is a trader.

Ivar meets his match in Heahmund. Floki returns to Kattegat. Bjorn receives a lavish welcome in North Africa.

Lagertha is betrayed, and Bjorn must find a way to support the distraught queen. Floki and his settlers arrive in a new land, but it is far from what they expected.

Bjorn returns to Kattegat to learn that an attack will take place during the next full moon. Ivar must decide if he can place his trust in a former enemy on the battlefield.

The battle for Kattegat begins and, as the two armies line up to fight, the Great Heathen Army must decide between a final plea for peace or all-out war. Floki faces discord among the settlers.

The army leaders consider their options in the aftermath of the battle. In Floki's camp, all hopes of binding the community together are dashed as tragedy unfolds.

A sense of doom looms over Kattegat as bloodshed ensues. The defeated army flees in the face of the victors. A legendary warrior makes his way home.

Ivar is crowned the new king of Kattegat just in time to welcome Rollo home. Bjorn, Lagertha and Ubbe must rely on unsteady alliances to survive.

Bjorn, Lagertha and Ubbe face an uncertain fate as Heahmund's loyalty is tested. Floki fights to understand the will of the gods. Ivar may have met his match.

Heahmund must try to convince King Alfred that his actions are in defense of the Crown. Ubbe and Torvi strengthen the position of the Vikings but not all in the Viking camp support their strategy. Another settler disappears in Iceland.

As the celebrations for Ivar continue in Kattegat, grief hits Iceland and Floki must now make a fated decision. Harald's army approaches Wessex. A conspiracy grows against King Alfred.

Bishop Heahmund is wracked with guilt as he fights to renounce his passions. Viking will clash with Saxon on the battlefield leaving a key figure lost in the balance.

Bjorn achieves one of Ragnar's dreams. Ivar hatches a new plan while preparing for a divine arrival. In Iceland, a settler returns in a terrible state. King Alfred faces his greatest threat yet.

An unexpected turn amongst the settlers leaves Floki powerless. King Alfred confronts Judith. In York, Bjorn must strike a deal with Harald. Wessex is once again threatened by a Viking force, but who will lead the Saxon army to defend the Realm?

Hvitserk is severely tested. Floki makes an amazing discovery. Freydis gives Ivar a surprise. Ubbe negotiates with the three Danish Kings that have massed their armies in Reading, but the negotiations may have a perilous outcome.

Ubbe is forced into hand-to-hand combat. Floki reaches new depths and is met by a shocking sight. Bjorn sets off for Scandinavia with an old rival.

A new battle for Kattegat is on and only the gods know who will emerge victorious.