Stoick fears that Alvin is planning an attack.
Meatlug ingests magnetic rocks, causing destruction throughout the town.
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 a water shortage in Berk has been caused by the Outcasts' plan to hide Whispering Death eggs under Berk.
The gang look for a cure for Hookfang after he loses his ability to catch fire.
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.
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.
While searching for traps, Tuffnut, Barf and Belch get trapped themselves... and a forest fire is approaching.
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 out and unleashed, Hiccup and Toothless are faced with a battle between both the Skrill and Dagur the Deranged's Berserker armada, which could prove to be more than Hiccup and Toothless can handle.
When Alvin the Treacherous captures the Skrill, Hiccup, Ruffnut, and Tuffnut go undercover to discover that a truce is in the process of being formed between the Outcasts and the Berserkers to invade Berk with the Skrill as their weapon. The trio must now flee to release the Skrill from their control before the truce can be settled.
Gustav tries to fill in for Snotlout at the Dragon Academy.
The gang must heal and return a dragon to the ocean before it falls victim to the elements.
When Speed Stingers infest Berk, the residents go into hiding.
Hookfang and Stormfly quarrel while the town prepares for the harvest.
Berk has been plagued with a bad case of the Eel Pox. Hiccup, Toothless and the gang, along with Gothi, go gather the ingredients for the cure. There's just one problem; the final ingredient is an eel from Eel Island.
A rogue species of dragon steals the town's metal.
Adolescent Thunderdrums menace Berk.
Snotlout is expelled from the academy; Alvin returns to Berk to make amends.
Alvin reveals he trained the Screaming Death to attack Berk and destroy the island's inhabitants and only Snotlout and Hookfang can save Berk before it's too late. Meanwhile Hiccup gets ready for a final fight against Dagur to save Toothless and end the barbarians invasion of Berk once and for all.