Caught in a waterspout, Toothless' prosthetic tail is damaged, rendering him unable to fly and thus leaving him, Hiccup and Snotlout stranded on Outcast Island. The three must work together to repair the damage, overcome the rivalry between Hiccup and Snotlout and return home to Berk before they are captured by Alvin and the Outcasts.

When Snotlout and the villagers mistake color-changing dragon eggs for gems that bring good fortune, Fishlegs, who is responsible for the eggs' discovery, must take a stand against Snotlout,who is trying to sell the eggs, and reunite them with their Changewing mothers before they destroy Berk.