Young Hiccup and his beloved dragon, Toothless, face a dark threat when they become the leaders of their communities. While searching for new islands and dragons, Hiccup, Astrid, Toothless and Stormfly discover Light Fury and a dragon underground world. Light Fury, a mysterious dragon female with rare physical attributes, aims to resist the wild side of Toothless, which has been numb with domestication. However, back in Arcapellago, Grimmel, a man who hunts each Night Fury and Light Fury requires Hiccup to teach Toothless and Light Fury. Refusing, he captures Light Fury and swears to destroy Hiccup and all his dear ones. When the darkest threat they have ever encountered detracts from their village, Hiccup and Toothless have to leave the only house they know and travel in a hidden corner of the world, which is believed to be just a myth . Hiccup, Toothless and his friends must confront this new threat and look for a new home where both they and the dragons can live in peace. As their true destinies are discovered, the dragon and rider will fight together to the end of the Earth to save and protect all of their loved ones.