Continuing the adventures of Hiccup and his friends from How To Train Your Dragon and the Dragons: Riders of Berk TV show! "Underworld": During a routine training exercise, Hiccup and his friends discover a huge, mysterious pit in a forest near Berk and decide to investigate. Meanwhile, a number of fishing vessels near Berk have gone missing, so Stoick decides to investigate - and bumps into an old enemy. Just how are Hiccup and Stoick's predicaments linked?