When Ruffnut and Tuffnut have a falling out, Hiccup and the kids must reunite them before their wayward Zippleback is discovered by Dagur the Deranged, the visiting chief from the fearsome Berserker tribe.
With Bork Day, the annual holiday celebrating the village's history, fast approaching, Stoick bestows Hiccup with all of Bork's notes and studies on dragons.
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.