Berk is getting its yearly visit from the Berserkers to re-sign a peace treaty. Stoick is expecting Oswald the Agreeable, but his son, Dagur the Deranged turns out to be the new chief of the Berserkers.
Certain that Alvin is planning an attack on Berk, Stoick institutes flight restrictions on the dragon academy. Hiccup, certain that Alvin will be using his newfound dragon-training knowledge to bolster his attack, forms a secret "Dragon Flight Club" in order to train the kids to defend Berk against Alvin's dragon army.
Hiccup must help Astrid face her fear of the terrifying Flightmare, a ghostly dragon that seemingly "froze" her uncle with fear, shaming her family name. With Fishlegs' help, they discover there's more to the Flightmare than meets the eye.
When the dragon riders discover a frozen Skrill, a lightning-powered dragon revered by the Berserkers, they scramble to get it off Berk. But when the Skrill is thawed and unleashed, Hiccup and Toothless are faced with a battle between both the Skrill and Dagur’s Berserker armada, which could prove to be more than Hiccup and his Night Fury can handle.. The episode is the first of another two part story.
When Alvin the Treacherous captures the Skrill, Hiccup, Ruffnut, and Tuffnut go undercover to discover that a truce is in the process of being formed between the Outcasts and the Berserkers to invade Berk with the Skrill as their weapon. The trio must now flee to release the Skrill from their control before the truce can be settled.
Snotlout becomes convinced he is dying, and leaves Hookfang to young Gustav. But when Snotlout "miraculously" recovers, he takes his dragon back, and Gustav decides to get a dragon of his own (named Fanghook) and join the academy… whether they want him or not.
Hiccup, Toothless and the dragon riders discover an injured Scauldron on the shores of Changewing Island. When the Scauldron takes a shine to Ruffnut, the kids see a chance to heal the dragon and return him safely to the ocean, before he falls victim to the elements, as well as the territorial Changewings.
Hiccup and Toothless return from a mission to find Berk mysteriously empty. They discover that the village has been overrun by Speed Stingers, a venomous non-flying dragon that has trekked across the frozen sea to Berk and driven everyone out of town. Hiccup must find a way to drive the Speed Stinger horde away from Berk.
Fed up with Snotlout's disrespect, Hiccup suspends him from the academy. When Snotlout leaves Berk, he encounters both the Screaming Death… and Alvin! The former Outcast chief rescues Snotlout, but his motives are unclear. On top of it all, Dagur unleashes a plan that could wipe out the academy dragons.
Hiccup cautiously accepts Alvin's help on a mission to rescue Stoick from Dagur's clutches. The dragon riders, the Berserkers and the Screaming Death all converge on Outcast Island, for a confrontation that may decide the fate of the archipelago.