In the aftermath of the events of the movie, the kids are enjoying their new lives with dragons, but the rest of the villagers aren't so sure. Hiccup and his friends have to find a way to train the dragons to live in peace with the rest of Berk.
Now that the dragons have assimilated into Berk, just about everyone seems to be better off. Except for Gobber, who made a career of building weapons to fight the dragons. Hiccup takes it upon himself to find a new role for Gobber in the new dragon-friendly village.
As the dragons become more comfortable as a part of Berk society, the livestock are becoming less comfortable - fire-breathing predators are, after all, around every corner. As a result, Berk is running low on eggs, milk and other food supplies. With a heavy winter storm on the way, the kids must train not just the dragons, but also the livestock, if there's any hope of surviving the winter.
While out flying on their dragons, the kids stumble upon a small dragon with an injured leg. They bring it back to Berk - no one has ever seen this species before, and everyone loves the new dragon, even Stoick, who names him Torch. Everyone but Toothless, that is, who knows that Torch is the offspring of the deadly Typhoomerang, who is heading towards Berk in search of her missing baby.
When the dragons are blamed for a series of damages around the village, Hiccup and the kids form the Dragon United Monitoring Brigade (aka DUMB). But when the Great Hall is ravaged, Hiccup suspects that someone else might be framing the dragons, in order to get rid of them.
After the dragons are banished from the island, Hiccup searches for evidence to prove their innocence. But when Alvin, the villainous leader of Outcast Island and Berk's greatest enemy, decides to attack the island, Hiccup must protect his friends and prevent Alvin from learning the secrets of dragon training.
A rogue sea dragon is attacking Berk's fishing ships, but Stoick and his men have been unable to pursue the dragon by sailboat. Hiccup tries to convince Stoick that he needs a dragon of his own for just such occasions. But when Stoick decides to use Toothless, Hiccup needs to find a new dragon for Stoick in order to get his own dragon back.
When Stoick has a portrait of himself and Hiccup painted for the collection in the Great Hall, Hiccup ends up depicted as a strong, burly Viking, not his usual skinny self. To prove he's the Viking his father wishes he was, Hiccup embarks on an epic quest to find the lost treasure of Hamish the First, Berk's first chief.
Trader Johann comes to Berk, bringing with him new and exotic goods that the people of Berk hurry to trade for. But soon, the dragons begin sneezing and getting sick. Hiccup, with the help of the others, determines that the dragons have been poisoned from eating the Blue Oleander flowers that have been growing throughout the village. The only cure: the venom of the Scauldron, a massive and vicious sea dragon.
A mysterious girl, Heather, washes up on the shores of Berk. Proving herself a valuable addition to Berk, she wins the affection of everyone except Astrid, who notices Heather is particularly interested in flying dragons and in the Book of Dragons. Hiccup refuses to believe she could be lying about anything...
After Heather's betrayal leaves the Book of Dragons in the hands of the Outcasts, the kids devise a plan for Astrid to disguise herself as Heather, sneak onto Outcast Island, and retrieve the book from Alvin and the Outcasts.
For his entire life, Snotlout has been beating Hiccup and the others in the Thawfest games. But this year, the games include dragon-based events, and Hiccup is determined to finally win. Hiccup and Snotlout both train excessively, bringing out Hiccup's competitive side. In his fierce quest to win, Hiccup risks humiliating Snotlout in front of the entire village and alienating his friends.
When a series of lightning storms hit Berk, Mildew claims Thor is angry at the village for allowing Toothless, a Night Fury, to live among them. Hiccup must prove Toothless's innocence as he and Stoick face an uprising against Toothless.
A rogue dragon - a Whispering Death - shows up in Berk, tearing up the village. He's on a mission - to find Toothless and settle a score.
The gang meet a pretty stranger who has a secret.
Hiccup and Snotlout face difficulties getting home from Outcast Island.
The gang visit a haunted bog to retrieve a lost gift for Hiccup.
Fishlegs discovers a gem that is of great value to a flock of dangerous Changewings.
Hiccup and Toothless look for Night Furies.
Conclusion. Hiccup and Toothless hatch an escape plan after being trapped on Outcast Island.