When Meatlug accidentally ingests magnetic rocks, she becomes magnetic herself - causing destruction through the town, but ultimately helping the kids win a battle against the Outcasts.
The kids go on a dragon survival training mission to Dragon Island, where they encounter Dagur, the Berserker Chief, who has become obsessed with hunting the Night Fury.
Hiccup discovers that the town's water shortage has been caused by the Outcasts' plan to hide Whispering Death eggs under Berk, which soon hatch and tunnel underthe island, threatening the villagers.
Hookfang is losing his ability to catch on fire, and it's up to Snotlout, Hiccup and the others to find the lair of the Fireworm Queen, which may hold the only possible cure, before it's too late.
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 that there's more to the Flightmare than meets the eye.
A Terrible Terror training competition leads to a heated rivalry between Fishlegs and Snotlout, allowing Alvin to kidnap Meatlug. The kids must use their newly trained Terrors to rescue their friend.
Snotlout becomes convinced he is dying, and leaves Hookfang to Gustav. But when he "miraculously" recovers, Snotlout 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 and co. discover an injured Scauldron on the shores of Changewing Island. The kids must 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.
The mysterious theft of metal objects all over Berk reveals the presence of marauding Smokebreaths. Hiccup and Toothless soon discover that these dragons are the work of Dagur, in a plot to render Berk defenseless.
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 war that may decide the fate of the archipelago.