After rescuing Trader Johann, Hiccup and Toothless return from a mission to find Berk mysteriously empty. They discover that the village has been overrun by Speed Stingers, a non-flying dragon that move at blindingly fast speeds and can paralyze their victims with their tails. The Speed Stingers have trekked across the frozen sea to Berk and driven everyone out of town. Now, Hiccup must find a way to drive the Speed Stinger horde away from Berk before their next raid.

While gathering an eel for medicine for an Eel Pox epidemic on Berk, Toothless eats a red eel attacking Hiccup after he falls into eel infested waters. This causes Toothless to come down with the dragons' version of Eel Pox, skittish and uncontrollable fire power. To make things worse the ingredients needed to cure the illness are in Toothless' satchel and all but Fishlegs has been caught sick. With little time to lose, Hiccup and Fishlegs must find a way to cure Toothless and get the stuff for the cure back to Berk.