Hiccup convinces Stoick to ride a dragon to aid him in his duties as chief, but he quickly insists on riding Toothless on a regular basis, to both Toothless and Hiccup's dismay. Meanwhile, a mysterious dragon has been attacking the tribe's fishing ships.

Immediately after the visit of the renowned Trader Johann, all dragons on the Isle of Berk are mysteriously falling ill. When Mildew is discovered responsible, he is forced to help Hiccup, Stoick, and Gobber find and capture a ferocious water dragon called the Scauldron in order to concoct an antidote before it is too late (after the Scauldron bites Mildew's butt).