Heather's betrayal to Berk caused the Book of Dragons to fall into Alvin the Treacherous' hands, and Heather's imprisonment on Berk. In order to get the book back from Alvin, Astrid disguises herself as Heather and goes into Outcast Territory. Towards the end of her mission, Astrid learns and confirms Heather's true motives for helping the Outcasts-freeing her parents.

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.