In the aftermath of the movie, the kids are enjoying their new lives with dragons, but the rest of the villagers aren't so sure. Hiccup and the other kids must find a way that the dragons can help the village, or risk having them get banished forever.
With the winter storms approaching, the dragon riders are forced to find a way to help the Berk farm animals (yaks, sheep, etc.) overcome their fear of the dragons. If they can't, Berk will starve from lack of eggs, milk, wool, etc.
When Hiccup discovers an injured baby dragon who he names Torch, Toothless takes a fierce dislike to the energetic and mischievous beast. But unbeknownst the riders, Torch's much bigger and angrier mother is looking for him, and is headed their way…
After losing their armory and their dragons, Berk finds itself facing an attack by Alvin the Treacherous and the Outcasts, their sworn enemies. Hiccup and the other dragon riders must ward off the Outcasts long enough to get their dragons back.
Hiccup is dismayed when he sees his and Stoick's father-son portrait has been altered to make Hiccup look more muscular. Desperate to prove himself to his father, Hiccup sets out to find a fabled long-lost treasure.
When Mildew plants Blue Oleander flower throughout Berk, the dragons fall deathly ill one-by-one. Hiccup and Stoick must venture out to find the Scauldron, an aquatic dragon whose venom is the only cure for the flower's poison.
When a young girl named Heather washes up on the shores of Berk, seemingly attacked by pirates, Astrid is suspicious of her motives, especially when Heather takes a vested interest in the workings of the dragon academy. The episode is the first of another two part story.
Heather reveals that Alvin is holding her parents captive, blackmailing her into working for him. Astrid embarks on a mission to Outcast Island, disguised as Heather, to retrieve the lost Book of Dragons and rescue Heather's parents.
When dragon events are included in the annual Thaw Festival games, Hiccup sees a chance to finally beat Snotlout, the winner for the past several years. But when Hiccup's competitiveness gets the better of him, he risks alienating his friends.
Mildew leads the village to near revolt when Toothless appears to be attracting a vicious lightning storm, claiming that Thor is punishing the village for taking him in, and that the Night Fury must be banished.
When the Whispering Death, a vicious tunneling species of dragon, comes to Berk, it becomes clear that he and Toothless have a long-term grudge to settle. The only problem is that Toothless won't let Hiccup help him, and thus he's unable to fly.
A falling out between Ruffnut and Tuffnut leaves their Zippleback, Barf-Belch out of control at the worst possible time, as Berk is visited by Dagur the Deranged, the deranged chief of the warlike Berserker tribe.
While arguing during a flight, Hiccup and an insubordinate Snotlout get stranded on Outcast Island. Hiccup must evade wild dragons and repair Toothless's tail before Alvin discovers he's on the island.
When Trader Johann goes missing, Hiccup and the kids venture out to find him. They discover that Johann's ship has been taken by the mysterious creatures of Breakneck Bog, along with his cargo: a gift from Hiccup's late mother.
When Fishlegs discovers a supposed Stone of Good Fortune, the kids soon realize that the stones are in fact Changewing eggs. Soon enough, the mother Changewings overrun Berk in search of their missing babies.
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.