Follows the further adventures and misadventures of Hiccup Horrendous Haddock the Third as his Viking training continues and his father leads a stranger and the Hairy Hooligans to the Isle of Skullions in search of a pirate's treasure.

What happens when someone says 'Do Not Open This Box'? Hiccup Horrendous Haddock the Third, now in pirate training, finds out when the box is opened not once, not twice, but three times!