Hiccup is the son of the chief.
Well, he's supposed to be, anyway. But try telling that to the acting-chief, Magnus, who has some serious abusive tendancies, is despised and feared by all, and treats Hiccup more like a prisoner than future leader. His father is gone, unintentionally stranded, and doesn't show any signs of returning any time soon. Worse (or perhaps better), there are rumors floating around Berk of a rebellion soon to come. The plan: Usurp Magnus and put Hiccup in his place as the rightful chief. Simple, right?
But there's just one problem. Hiccup has been exiled to a remote island, Magnus' way of putting down the rebellion. Now, Hiccup has no home and no way to travel. Without him, Berk's chances of escaping Magnus' cruel tyranny seem much diminished. And if that wasn't bad enough, Hiccup finds that he is not alone on the island. Someone else lives here, too. Someone whose kind has been warring with Hiccup's for centuries.
His worst enemy.