This comic contains a mini-comic at the end related to the events in the main story, called "Smeltdown".
During a routine training exercise, Hiccup and his friends discover a huge, mysterious pit in a forest near Berk . . . They 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 . . . And just how are Hiccup and Stoick’s predicaments linked?
— "Underworld" - Back Cover
While out training on Berk with Typhoomerang dragons in snowy weather, the Dragon Riders detect unusual activity near the Caves of Jotunn. Against Stoick's wishes, the Dragon Riders fly into the caves and discover a secret mining operation run by a former Hairy Hooligan Tribe member named Fiske and his crew. Fiske intends to weaken Berk and cause its collapse because of an old grudge. Meanwhile, out in the waters near Berk, Stoick runs afoul of a Submaripper with a grudge. This particular Submaripper has run into Stoick in the past, as well as Fiske. Astrid escapes from Fiske, finds Stoick, and eventually leads the angry Rogue Submaripper back to Fiske and his crew. The cave collapses, trapping Fiske and the Dragon to presumed death, while Fiske's crew and the other Dragon Riders escape.