Legend has it that an ancient dragon - a giant Seadragonus Giganticus Maximus - will rise from the ocean one day and end the world- and this is known as Ragnorok.
And as Berk experiences minor earthquakes and earthquakes - which are disturbing the dragons' behavior - Stoick orders all sea traffic to remain at anchor.
Hiccup, meanwhile, is trying to train a Changewing for tactical purposes. But every time he makes progress it responds by hypnotizing him with its mesmeric gaze (as it does its prey) - with comedic results.
Meanwhile, Alvin the Treacherous plans to take advantage of the earthquakes and attack Berk... However, something huge from the sea destroys his ships.
And then the Seadragonus Giganticus Maximus approaches Berk and it is seemingly unstoppable! Even Alvin tries to help the citizens of Berk.
Hiccup thinks quickly and uses the Changewing to hypnotise the SGM and it is defeated!