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.
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.