It's been almost three decades since the rage virus escaped a biological weapons laboratory, and now, still in a ruthlessly enforced quarantine, some have found ways to exist amidst the infected. One such group of survivors lives on a small island connected to the mainland by a single, heavily-defended causeway. When one of the group leaves the island on a mission into the dark heart of the mainland, he discovers secrets, wonders, and horrors that have mutated not only the infected but other survivors as well.

On the rugged isle of Berk, where Vikings and dragons have been bitter enemies for generations, Hiccup stands apart, defying centuries of tradition when he befriends Toothless, a feared Night Fury dragon. Their unlikely bond reveals the true nature of dragons, challenging the very foundations of Viking society.