When Hiccup and Toothless discover an ice cave that is home to hundreds of new wild dragons and the mysterious Dragon Rider, the two friends find themselves at the center of a battle to protect the peace. It's been five years since Hiccup and Toothless successfully united dragons and vikings on the island of Berk. While Astrid, Snotlout and the rest of the gang are challenging each other to dragon races (the island's new favorite contact sport), the now inseparable pair journey through the skies, charting unmapped territories and exploring new worlds. When one of their adventures leads to the discovery of a secret ice cave that is home to hundreds of new wild dragons and the mysterious Dragon Rider, the two friends find themselves at the center of a battle to protect the peace. Now, Hiccup and Toothless must unite to stand up for what they believe while recognizing that only together do they have the power to change the future of both men and dragons.

As the village of Berk prepares for its winter holiday, the resident dragons all inexplicably fly away. The Viking town of Berk, now enjoying its new alliance with the dragons, is preparing for its annual winter holiday of Snoggletog. However, that spirit is threatened when all the dragons, except Toothless who needs Hiccup to help him, suddenly fly away for some reason. While Astrid ineffectually tries to salvage the occasion, Hiccup decides to build Toothless a new prosthetic to allow him to fly independently, only to have the dragon take off as well. However, as the holiday approaches, Hiccup finds himself swept up to learn the dragons' secret the hard way and to find a solution to bring them home early.

Hiccup and the gang join Gobber on a quest to find a legendary boneclad Dragon. When Gobber's house burns down, the old Viking is convinced that his nemesis, the legendary Boneknapper Dragon, is responsible. Bent on revenge, Gobber resolves to hunt it down once and for all, despite the fact that no one believes the dragon exists. Regardless, Hiccup and the gang decide to accompany him for his own good. On the sea voyage, Gobber tells the story of his rivalry with the beast, which gets sillier with each episode. When the gang find themselves shipwrecked on the suspected island of the beast, they are about to learn the truth about the dragon.

A hapless young Viking who aspires to hunt dragons becomes the unlikely friend of a young dragon himself, and learns there may be more to the creatures than he assumed. Long ago up North on the Island of Berk, the young Viking, Hiccup, wants to join his town's fight against the dragons that continually raid their town. However, his macho father and village leader, Stoik the Vast, will not allow his small, clumsy, but inventive son to do so. Regardless, Hiccup ventures out into battle and downs a mysterious Night Fury dragon with his invention, but can't bring himself to kill it. Instead, Hiccup and the dragon, whom he dubs Toothless, begin a friendship that would open up both their worlds as the observant boy learns that his people have misjudged the species. But even as the two each take flight in their own way, they find that they must fight the destructive ignorance plaguing their world.