The island of Berk has been plagued by attacks from dragons that steal their livestock and burn their homes. Hiccup, the awkward son of the village chieftain, Stoick the Vast, uses a cannon contraption he invented and shoots down a dragon he thinks may be a Night Fury, a rare dragon nobody has ever actually seen. Hiccup locates the dragon in the forest, but finds he is unable to kill the helpless animal. Hiccup instead cuts it free, and the dragon disappears into the forest. Meanwhile, Stoick assembles a fleet to seek out the dragons' nest, but before he leaves, he places Hiccup in dragon-killing classes taught by Gobber, the village blacksmith.
Searching the forest, Hiccup finds the dragon trapped in a shallow glade; the dragon's tail was injured, preventing it from flying normally. By trial and error, Hiccup discovers a way to earn the dragon's trust and begins to care for it. He names the Night Fury "Toothless," for its retractable teeth. Soon after, Hiccup fashions a makeshift harness and prosthetic tail that allows him to guide the dragon in free flight. Hiccup is able to transfer his knowledge of dragons to the other species of dragons at school, appearing to conquer each one in battle and becoming the star pupil, much to the dismay of Astrid, a girl in dragon training whom Hiccup has a crush on. Hiccup wins the class and gets the chance to kill a dragon in front of the entire village. The battered Viking fleet arrives home and Stoick learns of his son's exploits in school.
Later, Astrid follows Hiccup, suspicious of his sudden skill, and is shocked to discover Toothless. She attempts to escape to tell the rest of the village, but Hiccup coerces Astrid into flying with Toothless. At first, Astrid is terrified, but then begins to enjoy the excursion. Toothless unexpectedly joins a flock of dragons and takes the pair straight into the dragon's nest, where the truth behind the dragons' constant raids of their village is revealed – the Red Death, a giant Seadragonus Giganticus Maximus dragon, lives off the food they bring back, else it feeds on the dragons themselves. Astrid wants to tell the village of the nest, but Hiccup wants to keep it a secret to protect Toothless. Astrid, who admires his determination, agrees. Before she heads back, she punches his arm for kidnapping her, and then, kisses him on the cheek for "everything else".
Hiccup is put to his final exam the next day by fighting a Monstrous Nightmare, but when he tries to show the village the dragon's true nature, Stoick stops the fight, inadvertently angering the dragon and endangering Hiccup. Toothless hears Hiccup's cries and flies in to save him, but is captured himself. Hiccup, attempting to explain his actions, reveals how to find the dragons' nest. He tries to warn his father of the danger, but Stoick refuses to listen to his son, disowns him, and sets off with another fleet, using a restrained Toothless as their guide. Astrid talks with Hiccup about why he didn't kill Toothless before. Hiccup reluctantly admits that he didn't kill him because he looked as frightened as he was. Hiccup, then, concocts a plan to save the Vikings with the help of the school's captive dragons and his classmates.
The teens arrive after the Viking fleet has already uncovered the Red Death. Hiccup's classmates distract it while Hiccup goes to save Toothless. Hiccup and Toothless almost drown, but both are saved by a now-repentant Stoick. Hiccup and Toothless lure the Red Death into flight, ultimately damaging its wings and then forcing it into an inescapable dive back to earth, killing it in a massive explosion. While attempting to flee from the crashing dragon, Hiccup falls off Toothless, and Toothless dives into the flames after him.
After the explosion has settled, Stoick finds Toothless, who reveals an unconscious Hiccup safely wrapped in his wings. Hiccup wakes up back on Berk, finding an eager Toothless greeting him. As he leaves his bed, he discovers part of his left leg has been replaced by a prosthetic leg made by Gobber. His grief is turned to joy, however, as he steps outside to find the Vikings and dragons working together to rebuild their village. He is greeted by everyone, and Astrid rushes to kiss him. The film ends with the war between Vikings and Dragons finally over with Hiccup and his friends racing their dragons.
Thank you wikipedia