Hiccup Horrendous Haddock III is a young VIking who can't seem to do anything right. One night, he shoots down a Night Fury dragon with a powerful net, and tries to explain to the village that he really accomplished something. No one believes him, so he rushes to find the dragon.

He finds it and is determined to bring its heart back to the village to prove he really is a Viking after all. But out of pity, he can't bring himself to kill the dragon, and instead sets it free. The dragon, in a similar act of mercy, refuses to kill Hiccup and instead howls in his face, leaving.

Hiccup is put into dragon training--the field where he has to learn to kill a dragon--along with several other Vikings around his age. He is taught several things about dragons, but he keeps going back to the Night Fury--whom he names Toothless due to its retractable teeth--and finds out that all he's being taught about dragons is wrong. He uses his skills learned with Toothless to manipulate the dragons back to their cages without harming them, which ends up crowning him as the top of his class.

As he is about to run away from home, his friend Astrid happens upon him interacting with Toothless. He takes her on a ride on Toothless and they end up going to the Dragon Nest, which his father Stoick has been looking for for years. He decides not to run away, but instead show the Vikings the dragons' true nature.

However, his plan goes bad, and it ends with Toothless being captured and Stoick leading the Vikings on a trip to the nest. Hiccup is forced to teach his classmates how to ride dragons, and they fly off to save the Vikings and defeat the dragon "queen". Once they get there, Hiccup saves Toothless (with some help from his change-of-heart father) and manages to defeat the oversized dragon--though he loses his left foot in the process.

Hiccup and Toothless began living together in Berk, having united dragons and Vikings once and for all.