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.

Freya is the clan princess, pursued by Sven who wants only to become king after King Thorsson dies. Freya prefers Agnar, who was lost with a boatload of clansmen on a raid. The old king longs for one more grand adventure and takes a group to an island two days journey away. The island is cursed, being the realm of a great and brutal Beast - said to be protected by Odin. The King discovers that this is where Agnar and the others met their deaths. When the Beast attacks, most are quickly killed. The frail King is locked in battle with the Beast, and cowardly Sven runs away under the guise of helping a wounded Eric, leaving the King to die. Back in the village, Sven proclaims himself King and demands Freya wed him on the next full moon. Eric tells Freya the truth - that her Father might still be alive - and she sets out to free him, accompanied only be her friend Ingrid. They encounter the Beast and Freya exchanges herself for her wounded Father. Freya is left alone on the cursed island ...