When the Flyers try to kill him because he's no longer useful, Viggo forces Hiccup to help him get revenge by stealing their Dragon Eye and lenses.
The gang investigates why Viggo's men are mysteriously shipping marble blocks from a quarry. Next, they try to save two dragons being used as slaves.
Hiccup grows wary when Gruffnut arrives on the twins' 19th birthday to administer a Thorston coming-of-age ritual involving three dangerous challenges.
When the twins fly to the Northern Markets to get Tuffnut's mace fixed, a chance turn of events lands them inside a secret Dragon Hunter operation.
After Hiccup and Toothless heroically save Barf and Belch, the grateful Zippleback tries to repay its life debt by abandoning riders Ruff and Tuff.
During a trip to Wingmaiden Island to find a Dragon Eye lens, Snotlout challenges Hiccup’s leadership. On another island, an unlikely romance blooms.
