Sudden lightning storms are endangering the residents of Berk and it is believed that Thor is angry at the island. Mildew leads the superstitious villagers into blaming Toothless, "the unholy offspring of [lightning] and death itself", for the lightning, leaving Hiccup to find the true answers for Thor's sudden destructive anger.

Dagur the Deranged visits Berk to renew a peace treaty between the Hooligan Tribe of Berk and the Berserkers, and the dragons are put into hiding to dispel rumours that Berk is training a dragon army, which will make Dagur cancel the treaty. But when Ruffnut and Tuffnut begin to fight, their dragon, Barf and Belch, is left out of control. The Viking teenagers must bring the twins back together to control their dragon and avoid provoking Dagur to war.