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.

When three baby Thunderdrums follow Hiccup and Stoick back to Berk, the trio of Dragons prove to be a handful for the Dragon Academy to train. When the baby dragons are forced to live on Dragon Island after nearly destroying Berk, the group learns they can't survive without a parent, so Stoick releases Thornado so he can take care of them, the two parting reluctantly.