Following the end of Berk's war with the dragons under the Red Death, Hiccup, son of Chief Stoick, had started to train more dragons. (How to Train Your Dragon) The Hooligan tribe had few to no conventional weapons after Mildew blew up their armory in "We Are Family, Part 1."
In "Twinsanity", the new Berserker Chief, Dagur the Deranged, traveled to the Isle of Berk to sign a peace treaty with the Hairy Hooligan chief, Stoick the Vast. Dagur also heard rumors about Hiccups victory over the Red Death and how he had been training dragons. Dagur threatened Stoick with war if the rumors were true. At first, Stoick was able to convince Dagur that the rumors were false until Dagur demanded the treaty be signed in dragon blood. Stoick also allowed Dagur to hunt down the Hideous Zippleback, Barf and Belch in order to appease. However, Hiccup, his riders, and their dragons were then able to fool Dagur by orchestrating a dragon attack on them. This forced Dagur and his Berserkers to retreat and to consider the treaty signed.
Following these events, Dagur ventured to Dragon Island in "The Night and the Fury", where he learned about the Dragons. It was here that he also encountered Hiccup and his riders "hunting" dragons. At first, Hiccup cooperated with Dagur until he revealed that Berk does not hunt dragons anymore, but ride them now.
Feeling betrayed by Stoick's and Hiccup's lies, Dagur attacked Hiccup and tried to kill Toothless. However, Hiccup was able to defeat Dagur and left the Island with the other riders and their dragons. Dagur rejoined his men and his Captain, Vorg, said he would skin Hiccup for lying to them. However, Dagur said he would do it to both Hiccup and his Night Fury.
Sometime after these events, in "A View to a Skrill, Part 1", Dagur's herald, Captain Vorg, and his men then found a Skrill frozen in ice on an iceberg. There, they dug at until two fishermen, Bucket and Mulch arrived, believing that they hit an iceberg. However, Vorg and his men attacked, trying to claim to their ship. Unfortunately, Vorg and his men were forced to retreat without the Skrill when Hiccup and riders drove them away.
Hiccup and his riders then took the Skrill back to Berk, hoping to keep it away from Dagur. Unfortunately, Snotlout and the twins inadvertently thawed the Skrill and it escaped. Hiccup and his riders then searched for it. Dagur and Vorg also searched for the Skrill. Dagur's and Hiccup's forces then found the Skrill and then fought each other for it. However, Dagur was forced to fall back when he was under attack by the riders. The Skrill was then shot down and crashed in the sea. It was then captured by Alvin and his Outcasts.
in "A View to a Skrill, Part 2", Dagur made an alliance with Alvin and his Outcasts for the Skrill. However, Dagur double-crossed Alvin, took the Skrill, and then "killed" Alvin with the Skrill. Dagur then absorbed the Outcasts, now under Alvin's 2nd in command, Savage, into his force. Unfortunately, Dagur lost the Skrill when he tried to fight Hiccup and Toothless with the Skrill. Despite this setback, Dagur continued his obsession with killing Hiccup and capturing Toothless.
In "The Flight Stuff, Dagur gave Trader Johann information about a secret weapon he was testing. In truth, he was setting a trap for Hiccup and his riders. The riders flew in to destroy the "secret weapon" but instead were ambushed by Dagur. At least Hiccup would have been captured if Gustav hadn't been there to help.
In "Cast Out, Part 1", Dagur and his men sneaked onto Berk and planted Dragon Root at the Dragon Academy. This was an attempt to separate the riders from their dragons and destroy them. It ended with the capture of Stoick the Vast by the Berserkers, and the alliance of the Berkians and the Outcasts.
In "Cast Out, Part 2", Dagur attempts to ransom Stoick for Toothless, who he wants to kill. Hiccup and Toothless, however, use Alvin's knowledge of Outcast Island to sneak into the arena where Stoick is held. Hiccup and Toothless are captured, but Alvin releases Whispering Deaths into the arena and takes out the Berserker Guards. Several Outcasts switch sides to Alvin, and a huge battle ensues. They still might have lost, if Fishlegs hadn't used the Dragon Root to bring the Screaming Death to Outcast Island, creating massive chaos. Snotlout defeats Dagur, and then Alvin captures him. The Outcasts go back to being under Alvin's leadership, and the Berserkers are jailed.
Dagur, his Berserkers and a few Outcasts then spent three years in prison on Outcast Island as revealed in "Dragon Eye of the Beholder, Part 1". According to what's mentioned during Berk Council meeting in "Imperfect Harmony" these three years of peace were among the best years the people of Berk had seen.