The movie begins with Hiccup (Jay Baruchel), a young Viking, talking about his island village, Berk. Berk is located in a terrible part of the world. Everything, from the people to the weather, is tough and crude. However, the worst part of the region is the local pests: Dragons.
Dragons begin attacking Berg, and Hiccup runs through the rest of the village to get to his post as the blacksmith's apprentice. Along the way, everyone yells at him to get inside, as he is absolutely useless on the battlefield. Gobber (Craig Ferguson), the blacksmith, orders Hiccup to remain at the smithy to repair weapons while he joins the battle. Hiccup momentarily loses himself after seeing Astrid (America Ferrea), one of the best young Vikings and Hiccup's crush. Meanwhile, Stoik (Gerald Butler), the Viking Chief and Hiccup's father, is battling the various species of dragons that are overrunning the village.
The main species of dragons are: The Deadly Nadder, which shoots spikes from its tail as well as fire from its mouth, the Gronckle, an ugly and lazy dragon, the giant Monsterous Nightmare, which coats its body in fire before attacking, the two-headed Hideous Zippleback, which has one head that blows a flammable gas and another that lights the gas.
Stoik and the other Vikings are doing all right, when the last, and most deadly dragon shows up: The Night Fury. Pitch-black, the Night Fury only attacks at night and moves at incredible speeds. No one has ever seen it and lived to tell about it.
Hiccup wants more than anything to kill a dragon to win the admiration of his fellow Vikings, so he sneaks away from his post with one of his inventions, a giant mechanical crossbow. By pure luck, Hiccup manages to hit the Night Fury with a bola, and the dragon crashes somewhere on the island. However, no one sees Hiccup do this. To make matters worse, a Nightmare shows up and is about to kill Hiccup when Stoik saves his son. However, Hiccup accidentally frees all the dragons that had been trapped during the battle, and they escape with much of the Viking's food.
No one believes Hiccup about the Night Fury, and he is sent home in shame. Stoik and Gobber talk about how much of a disappoint Hiccup is (A running gag is that an experienced Viking will tell Hiccup "You just need to lose this" while gesturing towards his entire body.) Gobber suggests enrolling Hiccup in his dragon-fighting training, which Stoik reluctantly agrees to. Also, Stoik plans to launch an expedition to hunt down the dragon's nest, in hope of driving them off once and for all.
The next day, after the ships have left on the mission, Hiccup makes an utter fool of himself at dragon-training. He is almost killed, and throws everyone else off. He leaves the arena in shame, and decides to go looking for the Night Fury. After searching most of the island, Hiccup is ready to give up when he notices a tree that's been knocked over, as well as several large skid marks on the ground. He follows the trail, and soon discovers the still alive Night Fury, although it's still restrained by the bola from earlier. Hiccup walks up to it, pulls out his knife, but makes the mistake of looking into the Fury's eyes. As it turns out, the dragon is quite intelligent, and seems to understand that Hiccup is going to kill it. It stops struggling and gives up. However, Hiccup can't do it. He instead cuts the ropes and releases the dragon, who quickly pins Hiccup to the ground. The Night Fury roars in Hiccup's face, but then flies off, crashing into several trees. Hiccup follows the dragon and finds it trapped in a large ditch, unable to fly high enough to escape. It turns out that Hiccup's shot knocked off one of the Fury's tail fins, and it is unable to fly.
The dragon seems to give up and tries to catch some fish in a nearby pond, but is unable to. Hiccup catches one and tries to give it to the Night Fury (Scene from the trailer) The dragon eats half of the fish, and wants Hiccup to take a bite as well. He does so, and the dragon seems to consider themselves friends now. Hiccup returns home, slightly disturbed by his betrayal of Viking principles.
The next day, Hiccup does even worse at dragon-fighting. But he also finds time to design and build a replacement fin for the Night Fury (he is the blacksmith's apprentice). He brings the fin and a basket full of fish to the ditch where Toothless (Hiccup names the Night Fury Toothless because of his retractable teeth) is still stuck. Hiccup finds out that dragons HATE smoked eel, and also manages to attach the fin. The replacement works, much to Toothless' joy, but there's a catch: The fin has to be flexed in order to be used for flight, which means Hiccup has to control it from Toothless' back.
Over the course of the next few weeks, Hiccup secretly comes up with a saddle design that allows him to control the fin perfectly, and also uses tricks he learns from Toothless in his class. He is able to use the smoked eel to drive off a dragon, finds a plant that has a catnip effect on dragons, and learns that they enjoy chasing reflected light (Like a cat with a laser pointer) He even finds a place that, if scratched, will cause a dragon to go into a bliss-induced stupor (Like the spot on a dog's chest.) All of these tricks soon make him the most popular dragon fighter in the village, even though he's never killed one, much to the ire of Astrid.
Hiccup and Toothless finally have a fully successful flight and landing, after which they share a meal of fish. Some small dragons, about two feet in length, try to steal some, but Toothless scares them off by shooting fire into one of the dragon's mouth, knocking it unconscious. Hiccup gives another dragon a fish, and the dragon curls up next to him. Hiccup muses about how everything the Vikings think they know about dragons is wrong.
The next day, Stoik and the expedition return, utterly defeated by the dragons without finding the nest. However, Stoik is happy to hear that his son has become a great dragon fighter. He even gives Hiccup the honor of killing his first dragon the next day. Astrid is enraged by this, as she wanted to have that privilege, and follows Hiccup as he goes to meet Toothless. She discovers the dragon, and vows to reveal Hiccup's secret, when Toothless snatches her. After Hiccup and Toothless take her on a flight, Astrid begins to accept that there are advantages to teaming up with dragons. However, Toothless accidentally gets caught up in a huge swarm of dragons, and they all fly back to the dragon nest, where it is revealed that a massive dragon forces all the others to collect food for it.
The trio get back to Berk, and Hiccup convinces Astrid to give him a chance before she tells Stoik about the nest. The next day, the whole village turns up to see Hiccup kill a Monsterous Nightmare, and are shocked when he instantly befriends it. However, Stoik, in a massive rage, jumps into the ring and enrages the Nightmare. Hiccup screams for help, which Toothless hears. Toothless manages to escape the ditch and travels to the arena to rescue Hiccup. But, the Vikings manage to restrain Toothless.
Hiccup pleads with his father to release Toothless, accidentally revealing that Toothless can lead the Vikings to the dragon nest. Stoik ignores his son's pleads, and plans to use Toothless to find the nest and vanquish the dragons once and for all. When Hiccup further protests, Stoik disowns him.
After the adult Vikings leave, Hiccup is ready to give up, when Astrid and the other young Vikings arrive to help him. Hiccup gives them a crash-course in dragon-training, and the group uses the dragons from Gobber's class to fly to the dragon nest. At the nest, the Vikings have attracted the attention of the king dragon, which destroys all of ships, one of which Toothless is still trapped on. Just then, the cavalry arrives. While the rest of the young Vikings distract the king dragon, Hiccup tries to save his dragon, but almost drowns in the attempt. However, Stoik saves both Hiccup and Toothless. Hiccup, determined to free both the dragons and Vikings from the wrath of the king dragon, convinces it to follow him and Toothless into an aerial battle, where Toothless manages to kill the giant. However, the fin Hiccup built is burnt, causing Toothless to lose control and crash.
Stoik is devastated by the loss of his son, and Astrid is crying as well. The whole village mourns the loss of Hiccup, but it turns out that both Toothless and Hiccup survived the crash. Well, most of Hiccup did.
Several days later, Hiccup wakes up in his home with Toothless by his bedside. As it turns out, Hiccup lost one of his feet in the crash, which has been replaced much like Toothless' fin. After going outside, Hiccup discovers that almost every Viking now has a pet dragon, and is presented with a new saddle and fin for Toothless by Gobber. Stoik also tells his son how proud he is.
The movie ends with Hiccup and Astrid flying around the island (Astrid got her own dragon). Hiccup again complains about his village, but says there is one upside.
"Most places have pets like ponies or parrots. Not Berk. Here, we have Dragons."
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