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.
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Complete story — includes the ending
When 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. The Screaming Death happened to be digging shafts underneath the islands 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 reaches Berk.
Sources and content license
Source and revision history · Adapted from Wikipedia contributors, List of DreamWorks Dragons episodes. HTML formatting and reference markers removed. · Content license