Certain that Alvin is planning an attack on Berk, Stoick restricts the Dragon Academy from flying. Hiccup forms a secret “Dragon Flight Club”, in order to train the kids to defend against Alvin without his father knowing.
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 under the island, threatening the villagers. Also, a new, terrifying threat arrives from underneath Berk.
Hiccup must help Astrid face her fear of the terrifying Flightmare, a ghostly dragon that seemingly "froze" her uncle with fear ten years ago, shaming her family name. With Fishlegs' help, they discover that there's more to the Flightmare than meets the eye.
When wild dragons start overwhelming Berk, Hiccup and Toothless discover islands are disappearing throughout the archipelago. When Hiccup and Toothless investigate the cause, they then encounter the Screaming Death, who is now bigger, meaner, and smarter. "Not the greatest combination."
Barf and Belch are Tuffnut's only guard when he gets caught in an old dragon trap in the forest. Things go from bad to worse when Tuffnut is faced with not only a large Typhoomerang, but a deadly forest fire that soon threatens him and both the dragons.
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 must battle both the Skrill and Dagur’s Berserkers.
Hiccup, Ruffnut and Tuffnut must infiltrate Outcast Island and contend with the new alliance between Alvin and Dagur, who wield the fearsome power of their combined armada, along with the deadly Skrill.
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, Toothless and the dragon riders discover an injured Scauldron on the shores of Changewing Island. They must heal him and return him to the ocean before he falls victim to the elements, or the 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.
Hiccup must mend fences between Astrid and Snotlout when Stormfly and Hookfang start fighting to the death. Astrid and Snotlout must grudgingly work together to find the cause of the feud, if they want to help keep their dragons from killing each other.
While gathering medicine for an Eel Pox epidemic on Berk, Toothless accidentally eats an eel, which causes him to become paranoid, skittish and possibly dangerous. Stranded together on Eel Island, Hiccup must find a way to retrain his wayward dragon.
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 and Stoick, along with Toothless and Thornado, discover an orphaned trio of young Thunderdrums. The dragons follow them back to Berk, and it soon falls to Thornado to take the unruly dragons under his wing.
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 Treacherous is brought back to Berk, but his motives are unclear.
Hiccup cautiously accepts Alvin's help on a mission to rescue Stoick from Dagur's clutches. The dragon riders, Berserkers, and the Screaming Death all converge on Outcast Island, for a confrontation that may decide the fate of the archipelago.