Five years after the Viking village of Berk and the dragons made peace, they now live together in harmony. Hiccup goes on adventures with his dragon, Toothless, as they 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 remains unsure if he is ready for this responsibility.
While investigating a burnt forest, Hiccup and Astrid discover the remains of a fort encased in ice and meet a group of dragon trappers led by Eret, who blames them for his fort’s destruction and attempts to capture their dragons for an insane conqueror named Drago Bludvist. The two dragon riders escape and return to Berk to warn Stoick about the dragon army that Drago is amassing. Stoick orders the villagers to fortify the island and prepare for battle. Hiccup, however, refuses to believe that war is inevitable. After Stoick interrupts Hiccup’s plan to get Eret to take him to Drago, Stoick explains that he once met Drago at a gathering of chiefs, where Drago, mocked after offering the chiefs his service in return for their servitude, murdered them all using armored dragons, with Stoick as the only survivor. Undeterred, Hiccup flies off with Toothless in search of Drago to try to reason with him.
They run into a dragon rider named Valka, who is revealed to be Hiccup’s long-lost mother. She explains that she, like her son, could not bring herself to kill dragons. After being carried off during a dragon raid, she spent 20 years rescuing dragons from Drago’s traps and bringing them to an island nest created out of ice by a gigantic Alpha dragon called a “Bewilderbeast”, which is able to control smaller dragons. Stoick tracks Hiccup to the nest, where he discovers that his wife is still alive. Meanwhile, Astrid and the other riders kidnap Eret to find Drago, but Drago captures them and learns of Berk’s dragons.