The film opens with Gobber’s house on fire and the Vikings and resident dragons getting it under control. Gobber is convinced that his old nemesis The Boneknapper Dragon, a giant, yet perfectly silent dragon (the only one with no roar) that clothes itself in bones for armor, is responsible. However no one else believes that the Boneknapper even exists, much less that it burned his house, especially when the evidence suggests that the fire was started by his underwear drying at his hearth and catching fire.

Undeterred, Gobber vows to hunt down the beast once and for all and Hiccup reluctantly decides that he cannot let his mentor go alone, so he convinces his Viking recruit comrades to join him as well. As the gang row to the island where Gobber believes the dragon lives, Gobber tells them of his past encounters with the monster, shown to the viewer in traditional animation. He apparently first met it when he discovered a group of frozen Vikings during a family outing and he extracted a small treasure chest from the ice. As the frozen Viking he stole it from resisted, the Boneknapper appeared and chased him until he landed back in the family boat. The contents of the chest turn out to be a small bone that Gobber has since used as a belt buckle.

The stories continue, becoming increasingly ridiculous as Gobber tells of later encounters being interfered with by hammerhead sharks, hammerhead whales, hammerhead yaks, and the Norse thunder god Thor in different combinations. Unfortunately, the group’s attention is so occupied that they allow their boat to run aground and sink. Taking the loss of their only way home in stride, Gobber sets up a trap using Fishlegs, disguised as himself, as bait. Unfortunately, the Boneknapper silently comes up from behind the group and Fishlegs’ attempts to warn them are ignored until it is in position to attack. The gang have to take shelter inside their own trap, and all seems lost until Hiccup sees an empty spot on the dragon’s armor that exactly resembles Gobber’s belt buckle bone. Hiccup realizes that the dragon may simply be seeking it to complete his armor.
However, Gobber refuses to listen his proteges’ advice to give up the bone to placate the creature and he is soon seized in the beast’s jaws. Finally, as he is thrown up into the air, Gobber decides to surrender the bone and throws it at the dragon. The bone lands perfectly in the empty spot and the dragon is finally able to triumphantly roar with his armor completed.

As it turns out, the satisfied Boneknapper is a friendly one with his quest completed, and it agrees to fly the adventurers back to Berk. Furthermore, Fishlegs observes that the dragon’s roar is a mating call, and Gobber notes that he will certainly be believed now, with multiple Boneknapper dragons following them home.