When the nearby islands begin to disappear and the dragons that used to inhabit them begin migrating to Dragon Island, the teens discover that the cause of this is the Screaming Death, who has returned seeking vengeance. The Screaming Death happened to be digging shafts underneath the island that were large enough to cause the islands to collapse into the sea, forcing dragons to abandon their homes. The teens have to stop the Screaming Death before it causes Berk to collapse.

When the dragon riders discover a Skrill, a lightning-powered dragon revered by the Berserkers,that happens to be frozen in ice, they scramble to get it off Berk. But when the Skrill is thawed out and unleashed, Hiccup and Toothless are faced with a battle between both the Skrill and Dagur the Deranged's Berserker armada (also hunting the Skrill) which could prove to be more than Hiccup and Toothless can handle. During the battle, the twins witness Alvin in an Outcast ship capturing the Skrill...