The island of Berk, a Viking settlement, suffers continuous attacks from dragons that pilfer their livestock. Among the villagers is Hiccup (played by Jay Baruchel), the somewhat clumsy son of the village chief, Stoick the Vast (portrayed by Gerard Butler). Unlike the other Vikings, Hiccup struggles to wield conventional weapons against these menacing creatures; however, he utilizes his ingenuity and the guidance of Gobber (Craig Ferguson), the local blacksmith, to devise mechanical contraptions for dragon defense.
During a dragon raid, Hiccup believes he has taken down a Night Fury, a fierce dragon that has never been seen before. He soon finds the dragon ensnared in his bolas in a nearby forest. Although he initially intends to kill the beast, he finds himself unable to do so and decides to free it instead. The Night Fury lets out a roar before vanishing into the woods. As Stoick organizes a dragon-hunting expedition, Hiccup is placed in a dragon-fighting class run by Gobber, where he shares a rivalry with his crush, Astrid (America Ferrera).
Determined to understand the creature that he freed, Hiccup discovers the dragon's injured tail, which hinders its ability to fly. He gains its trust and names the dragon Toothless (Randy Thom) due to its retractable teeth. With creativity, Hiccup constructs a custom tail piece that enables him to assist Toothless in soaring through the skies. His bond with Toothless helps him excel in his dragon-fighting class, impressing his peers—including Astrid—despite stirring some jealousy.
As Hiccup grows more confident, he faces a pivotal moment—a chance to kill a dragon before the entire village. This weighs heavily on him. Meanwhile, Stoick returns home from an unsuccessful dragon hunt invigorated by Hiccup's progress. Astrid discovers Hiccup training with Toothless and, rather than report him, joins Hiccup for a ride on Toothless, which leads them to the terrifying dragon nest and a colossal dragon known as the Red Death. The Red Death relies on the smaller dragons for sustenance, and Hiccup feels compelled to keep this secret to safeguard Toothless.
Hiccup's final exam requires him to confront a Monstrous Nightmare, but when he attempts to showcase the dragon's true nature, Stoick intervenes, jeopardizing Hiccup's safety. In a moment of desperation, Toothless swoops in, saving Hiccup but falling into captivity. Hiccup's unintended revelation about the dragons’ lair puts everyone in peril. His attempts to alert Stoick go unheeded, resulting in Hiccup being estranged from his father.
After a heartfelt conversation with Astrid, Hiccup devises a plan to rescue both the Vikings and their dragons. When they rush to the scene, they find the Red Death wreaking havoc, having incinerated the Viking ships and threatening Stoick and Gobber. Hiccup's friends distract the beast while he strives to liberate Toothless. They face danger as they nearly drown when the ship is destroyed, yet Stoick comes to their aid. In a thrilling climax, Hiccup and Toothless manage to outmaneuver the Red Death, ultimately leading to its demise in a massive explosion.
Hiccup wakes to find himself back in Berk, but with one leg amputated, replaced by a prosthetic crafted by Gobber. Yet instead of despair, he feels joy as he witnesses the villagers and dragons collaborating to restore their home. In a touching moment, Astrid rushes to him and kisses him, signaling a newfound harmony. The story culminates in the emergence of a new peace between Vikings and dragons, with Hiccup and his friends joyfully racing alongside their airborne companions.
