Five years after the Viking villagers of Berk and the dragons made peace, they live together in harmony. Hiccup and his dragon, Toothless the Night Fury the last of his kind, discover and map unexplored lands. Now 20 years old, he is being pressed by his father, Stoick the Vast, to succeed him as chieftain, although Hiccup feels unsure he is ready.

While investigating a burnt forest, Hiccup and Astrid discover the remains of a fort encased in ice and meet a dangerous group of dragon-trappers. One of the trappers, Eret, blames the two for his fort's destruction and attempts to capture their dragons for the trappers' leader, Drago Bludvist, who plots to capture and brainwash all of the dragons and make them his pets and army. Hiccup and Astrid escape and warn Stoick about the dragon army Drago is amassing. Stoick orders the villagers to fortify the island and prepare for battle. Hiccup, however, refuses to believe war is inevitable, and flies off to talk to Drago. Stoick stops him, explaining that he once met Drago at a gathering of chiefs, where Drago had offered to protect them from dragons if they pledged to serve him; when they refused, he had his dragons attack them, with Stoick the sole survivor. Undeterred, Hiccup flies off with Toothless in search of Drago to try to reason with him...