On the isolated island of Berk, the viking villagers sound the alarm for an incoming dragon attack. As the battle begins, small, clumsy teenager Hiccup leaves his house to join in despite all of the other vikings telling him to get inside and out of the way. He is caught by the chief of the tribe, the hulking viking Stoick the Vast, who sends him to the forge where he apprentices under the weapons master Gobber the Belch. Gobber puts him to work and keeps him from going out into the fray, as Hiccup's small stature makes him too weak to use weapons, and his reputation for accidentally causing havoc has made him a disgrace to the entire village. Nevertheless, Hiccup dreams of slaying a dragon, such as a tough, bulbous Grockle, a quick and agile Deadly Nadder, a two-headed Hideous Zippleback, or a self-igniting Monstrous Nightmare. However, the ultimate prize is the Night Fury, the rarest of all dragons and one so fast, stealthy, and accurate in its blasts that no one has ever actually seen it. Hiccup is determined to gain the other vikings' respect and the admiration of his crush, Astrid. She is captain of the Fire Brigade, a team made up of fellow teen vikings Snotlout, Fishlegs, Ruffnut, and Tuffnut. The fight intensifies as the dragons raid the village for sheep and other food, so Gobber leaves to join in and orders Hiccup to stay at the forge. However, Hiccup immediately takes his newest invention, a bola launcher, and brings it to the edge of the village hoping to catch a Night Fury. Hiccup sees it's shadow and blindly fires the bolas, which hit their mark and ensnare the Night Fury. The dragon crashes in the forest, but Hiccup is found and chased by a Monstrous Nightmare. Stoick intervenes just before he is immolated and fights it off with his bare hands. The structure Hiccup was hiding behind collapses and the destruction releases several trapped dragons, which retreat along with the rest along with a large number of the village's sheep. Stoick, who is Hiccup's father, chastises him for never following orders and staying out of the way and does not believe Hiccup's story that he shot down a Night Fury, having had many false alarms in the past. The entire village, including Astrid and the other teens, agree that Hiccup is nothing but a nuisance, and Stoick orders Gobber to take him home as the sun rises. On the way, Hiccup complains about hid dad's stubbornness and refusal to listen, though Gobber notes that stubbornness seems to run in the family. He is sympathetic towards Hiccup, but suggests that he give up trying to fit in. As soon as Gobber leaves, Hiccup sneaks out again to try and find the Night Fury.
In a meeting with the villagers in the Great Hall, Stoick tries to rally them to set sail and search for the dragons' nest, which lies somewhere in a mist-covered and highly dangerous portion of the archipelago. The other vikings are fed up looking, as the ships that go out never return again. Still, Stoick is able to rouse them and convince the tribe to take to the ships one more time before winter. He asks Gobber to stay behind and train the latest batch of young recruits, though Gobber points out that that would mean leaving Hiccup unattended. He suggests Stoick put Hiccup into training as well, but Stoick refuses, thinking his son is too weak and talentless. Stoick grew up as a perfect viking, strong, obedient, and tough, but Hiccup is none of those things. Gobber says he will never be ready if they don't give him a chance, and Stoick won't be there forever to protect him. Deep in the forest, Hiccup scours the area but finds no sign of the downed dragon. Just as he is about to give up, he finds evidence of a crash and spots the Night Fury, completely immobilized by the bolas, but still alive. Hiccup draws his knife to slaughter the dragon and prove himself to his tribe and his father, but when he looks into the Night Fury's eyes, he cannot bring himself to do it. Unwilling to leave it behind to suffer, he cuts the ropes binding it. The dragon immediately pounces but does not kill him, letting out a roar and trying to fly away, though it seems uncoordinated. Hiccup returns home and is greeted by Stoick, who has decided to allow Hiccup to begin training. Hiccup tries to explain that he no longer wants to fight dragons, but Stoick is insistent and makes it clear he expects him to start acting like a viking, before heading to the ships. The next day, as many of the adult warriors has set off to find the dragon's nest, Astrid, Snotlout, Fishlegs, and the Twins enter the arena to begin training, called the Trial of Flame. To their annoyance, they are joined by a reluctant Hiccup, who they assume is only allowed to train because he is the son of the chief. Gobber explains that they will learn to fight each of the main five types of dragons that they keep in the arena for training. At the end of the course, best recruit will earn the title of Top Slayer, and will be given the honor of killing their first dragon in front of the whole tribe. They start with a Gronckle, with Gobber simply releasing it into the arena as the teens scramble to grab shields. Gobber tells them to try to avoid being hit by its fireballs, and advises them to make clanging noises with their shields to disorient the dragon. One by one, all the recruits except for Astrid are eliminated when their shields are blasted, but Hiccup is cornered and nearly killed before Gobber redirects its last shot. Gobber tells them a dragon will always finish off its prey, and the lesson ends.
Hiccup goes back to the forest where he found the Night Fury wondering why it didn't finish him off, and finds a small overgrown entrance down into a secluded cove. He finds the Night Fury struggling to get out of the cliffs surrounding the cove, but as it tries to fly, it is unable to control itself or gain altitude. Hiccup sketches the dragon, and realizes that one of its tail fins has been ripped off by the bolas, and leaving it with no way to steer itself. Hiccup meets the other recruits and Gobber in the Great Hall for supper, most of them thoroughly unimpressed by his performance. Gobber gives them the Book of Dragons, containing information on every known breed of dragon, and orders them to learn them while they can. Snotlout and the Twins have no interest in reading, though Fishlegs is obsessed with dragons and has read it many times, leaving only Hiccup and Astrid. Hiccup suggests that perhaps dragons aren't as bad as they have been told, but Astrid finds this view naive and idiotic, saying that he has the luxury of such ideas being the sheltered son of Stoick. She on the other hand has fought hard to be the up-and-coming warrior she is, and has ambitions of even becoming chief one day, when Hiccup proves himself unable and unworthy. Hiccup agrees that Stoick would much prefer her over him anyway, and she warns him not to get in her way. Hiccup reads through the Book of Dragons, finding that every single one bares instructions to kill on sight except for the Night Fury, which has no information, only the advice to run from the supposedly evil monster. Across the sea, the Viking boats make it to the edge of Helheim's Gate, a deadly section of the ocean completely covered in thick fog. Stoick orders the ships to enter, and they attempt to carefully navigate the sea stacks and jagged rocks. However, they are soon ambushed by countless dragons, and the ships are torn apart.
At training the next day against a Deadly Nadder, Gobber teaches them to outmaneuver the dragon and find its blind spot, which every dragon should have. Hiccup pesters Gobber for more information on Night Furies, but Gobber says there is none. The Twins' brainless bickering causes them to get eliminated when the dragon finds them, and Fishlegs is scared away. Snotlout's reckless bravado to try impress Astrid leads him to being chased out of the lesson as the course crumbles, and Astrid saves Hiccup as the dragon charges, subduing it. Astrid chastises all of them for treating training like a game instead of as practice for war. Hiccup returns to the cove with a fish, and runs into the Night Fury. He drops his knife and shield to show he isn't a threat and gives him the fish which the now placid dragon accepts, and Hiccup realizes that it has retractable teeth. Hiccup tries to touch the dragon, but it is still untrusting and does it's best to ignore him. Hiccup spends the rest of the day following and observing the Night Fury, which shows intelligence much higher than other dragon breeds, trying to copy Hiccup's smile and even attempting to imitate him drawing in the sand. Hiccup tries again to touch him but the dragon warns him off, so Hiccup turns away to show his own trust. The Night Fury gently touches his head to Hiccup's hand, leaving Hiccup amazed. That night, Hiccup and the other teens listen to Gobber tell the story of his hand and leg being eaten by dragons, with Snotlout vowing to avenge him by taking dragon legs as trophies. Gobber tells them to instead take legs and tails, as a dragon who cannot fly is as good as dead. Astrid notices Hiccup slip away from the group, as he goes to the forge. He designs and constructs a mechanical tail fin, and the next day brings it to the cove along with food for the Night Fury, which he has named Toothless. After giving him the basket of fish, Hiccup discovers that Toothless is terrified of eels, and removes from the pile. While the dragon is eating, Hiccup straps the mechanical fin to his tail and, sensing the difference, Toothless takes off and tries to fly. Hiccup, clinging on, manually holds it open, finding he can influence the dragon's direction. Toothless bucks Hiccup off, but without him the tail retracts and Toothless crashes back into the cove. Hiccup, thrilled with the tail's success, sets to work making more equipment to help the dragon fly again.
At the Trial of Flame, Gobber pairs the teens up and sets them against the Hideous Zippleback, telling them that one of it's two heads spews flammable gas, and the other sparks it, instructing them to work out which head is the ignition head, and douse it with buckets of water to prevent it lighting the gas. As the arena is filled with the thick smoke, Snotlout and Tuffnut waste their water when they mistake Astrid and Ruffnut for the dragon, and Astrid's bucket is knocked from her hands as the dragon sweeps her with it's tail. A terrified Fishlegs accidentally douses the wrong head and flees, and Gobber rushes into to ring to help the isloated Hiccup again. However, Hiccup intimidates the Zippleback back into it's cage before it can spark using the eel he took from the fish basket. The rest of the group are stunned, but Hiccup leaves before they can question this feat. Over the next days, Hiccup makes a saddle for Toothless so he can stay on and operate the tail fin, inventing a complex peddle system that allows him to help the dragon maintain his flight. As they learn to work together and fly, Hiccup finds out several useful dragon traits that he uses in the Trial of Flame to help him subdue the training dragons without violence. His reputation in the ring grows, as does his popularity among the vikings and other teens, all except for Astrid, who is suspicious of him making excuses to leave all the time and frustrated that he is outshining her. She spots him in the forest one day, but loses track of him when she tries to follow. Eventually, Stoick and the rest of the surviving vikings make it back to Berk, having failed to find the nest. The villagers congratulate Stoick on Hiccup's recent success in training, with Gobber explaining that Hiccup has a strange way with dragons. Meanwhile, Hiccup and Toothless are attempting their first full flight, Hiccup having written an instruction sheet to keep track of the tail positions. As they soar around the coast, they begin to improve and try more difficult maneuvers. Toothless climbs high into the sky, but Hiccup slips off the saddle and they both go into a free fall. Hiccup is able to grab on and get back into place just in time to open the tail again and, working perfectly in sync with eachother, they are able to navigate a dangerous set of coastal rocks at high speed, making it through unscathed. Hiccup goes back to the workshop and is surprised by Stoick, who is thrilled to hear that Hiccup has finally become a real dragon fighter and is doing well in training. He is confident that Hiccup could even become Top Slayer, as he himself did, and gives him a helmet as a gift for his final test before the determination. Hiccup awkwardly accepts, and the next day, goes in for his last lesson. Astrid orders him to keep clear while she takes care of the dragon, determined to win. He promises to do so, but before she can get to it, the dragon attacks him, and unseen by her, Hiccup is able to pacify it. Astrid is furious as Hiccup tries to leave again, but it is time for Top Slayer to be announced. Elder Gothi confirms Hiccup to be Top Slayer over Astrid, to both of their dismay. Hiccup goes to the cove, planning to run away with Toothless, but finds Astrid waiting for him. She demands to know what secret he's been keeping that has been helping him in the ring, certain that he is a fraud. Hiccup makes up a lie, but Astrid spots Toothless and she and the dragon try to attack eachother before Hiccup gets between them. He introduces Toothless to Astrid, but she bolts. Hiccup and Toothless chase and catch her, flying her to the top of a cliff to force her to hear Hiccup out. She refuses to listen to what he has to say, but agrees to get onto Toothless so he can get her down. Hiccup tells Toothless to bring them back to the ground, but Toothless instead launches into the air, flying as rough as possible to scare Astrid. Hiccup insists he is not in control, so Astrid apologizes to Toothless, who levels out and flies smoothly. Astrid and Hiccup go on a beautiful ride through the skies as the sun goes down, and Astrid admits that there is more to Toothless than she first thought, but she questions how Hiccup plans to complete the Trial of Flame without killing the dragon. Hiccup wants to try and change the vikings' minds about the creatures, but Astrid is skeptical that he could ever get through to them after the death and destruction the dragons have caused, including taking Hiccup's own mother when he was young. Toothless senses something and suddenly dives into the mist below, joining with a massive flock of dragons returning to their nest with the night's spoils. The two teens find themselves flying into a massive cavern in the heart of a mountainous island, where they see all of the dragons dropping their food into the smoke filled pit below. One dragon does not bring back enough, and an enormous head comes out of the darkness and eats it whole. Toothless flies Hiccup and Astrid out as the panicked dragons flee the angry giant. Back in the cove, Astrid wants to tell the chief about the nest, but Hiccup says they would have to tell them about Toothless as well, which would certainly get him killed. Astrid agrees to keep the secret, and Hiccup says he will think of something before the trial's end the next day. Astrid punches Hiccup for kidnapping her, then kisses him for sharing the experience with her.
The next day, Stoick addresses the entire tribe that has assembled at the arena for the dragon slaying, expressing both his shock and his pride for his newly warrior-like son. Before entering the ring, Hiccup tells Astrid he is going to try and end the conflict, but asks her to keep her distance if things go wrong to preserve her reputation. He goes into the arena and is locked in as the Monstrous Nightmare is released. The angry dragon is explosively violent, but Hiccup, to the shock of the assembled crowd, drops his dagger, shield, and helmet, calming the beast. Stoick orders the fight to be stopped, but Hiccup tells them that dragons are not mindless monsters and they do not have to kill them, but Stoick shouts for a halt and the noise spooks the Monstrous Nightmare, which goes on the attack. Hiccup, trapped in the arena, does his best to avoid being killed as Astrid pries open the gate enough to slip through and help. In the cove, Toothless hears Hiccup in danger and desperately tries to climb out, eventually able to pull himself to the top. Stoick is able to lift the gate for both of the teens to escape, but the Nightmare cuts Hiccup off and pins him to the ground. Before he can kill him, Toothless blasts his way into the arena and battles the Nightmare. Toothless is able to drive the dragon away, but the vikings in the stands charge into the ring to kill him. Toothless fights back as Stoick himself enters the fray, trapping the chief and nearly blasting him before Hicucp yells for him to stop. The vikings take advantage of this to subdue and capture Toothless, offering the kill to Stoick. The chief instead orders him to be put with the other captive dragons, before taking Hiccup to the great hall and confronting him in rage. Hiccup admits he should have found a way of telling him of his activities sooner, and begs him not to hurt Toothless. Stoick is furious that Hiccup cheated in the ring and has become friends with their mortal enemies, but Hiccup argues that the vikings have killed far more dragons than dragons have killed vikings, and in any case they are forced to steal food by the leader of their nest. Stoick realizes Hiccup has found the dragon's lair and demands to know where it is, but Hiccup confesses that Toothless was the one who brought them there, using a strange instinct only a dragon has. Realizing what Stoick intends to do, Hiccup desperately tells him that the queen of the nest is far too large and powerful and that the vikings wouldn't stand a chance. Stoick ignores him, telling him he has chosen his side, and disowning him before ordering the ships to be made ready once more.
Toothless is restrained aboard Stoick's ship as Hiccup watches helplessly from a distance, and the Berkian warriors board the armada and sail away to find the dragon's nest. Astrid finds Hiccup, wishing he had just killed Toothless instead of putting him through this and sending his people to their deaths. Astrid asks why Hiccup wouldn't do what all the rest of them would have done and finished him off, and Hiccup admits that he saw himself in the dragon, just as frightened as he was. She tells him Toothless is even more afraid now, and asks what he plans to do about it. Suddenly inspired, Hiccup hatches an insane plan. The viking boats once again enter Helheim's Gate, and as they navigate they are once again surrounded by dragons, who hide instead of attacking. Toothless starts to shudder as if being drawn to something, and Stoick has the ships follow through the deadly rocks and all the way to a desolate island, the dragon's nest. Back on Berk, Astrid gathers the rest of the teens to meet Hiccup at the arena. He releases the training dragons and shows the others how to earn their trust and tame them. Snotlout bonds with the Monstrous Nightmare, Fishlegs to the Gronckle, Ruffnut and Tuffnut to the Zippleback, and Astrid to the Nadder. Hiccup gets to work fashioning makeshift reins to help them hold on. As day breaks, the vikings have dug in around the base of the dragons' mountain, and are ready for battle. Stoick gives a speech, and orders the catapults to fire. The stones smash through into the mountain, revealing hundreds and hundreds of dragons, who flee as the vikings attack, flying away without a fight. However, as the vikings declare victory, the mountain breaks open as the gargantuan dragon queen emerges; the Red Death. The vikings are immediately routed as the giant dragon attacks and unleashes a firestorm on the entire armada, endangering Toothless, who is still chained aboard. Now trapped, Stoick orders the warriors to retreat to the other side of the island while he distracts the hulking beast. Gobber decides to go with him, but they don't get far before the dragon turns it's attention on them.
Before it can kill them, it is fired upon by Hiccup, Astrid, and the other teens, all riding dragons. Hiccup appoints Astrid to direct the others while he finds Toothless. Astrid orders Snotlout and Fishlegs to find it's blind spot and disorient it while the Twins draw its fire. They hold the Red Death off long enough for Astrid and Hiccup to spot Toothless, and Astrid drops him off before rejoining the fray. Hiccup attempts to free his dragon, but the restraints are too strong. Snotlout's dragon crashes, stranding him on the queen's head. He is able to prevent it from eating Fishlegs by hitting it in it's eyes, but loses his grip and is in danger of falling. As the Red Death fights back, it smashes the ship that Hiccup and Toothless are on, plunging them into the water. Hiccup desperately tries to release Toothless, but almost drowns before Stoick pulls him out. The chief dives back in and breaks open the restraints, freeing Toothless, who returns to Hiccup's side. Hiccup mounts the Night Fury and gets ready to join the battle, and Stoick apologizes for his actions, telling Hiccup he is proud of him. With newfound motivation, Hiccup and Toothless take off. The Twins rescue Snotlout, but are knocked out of the sky in the process, along with Fishlegs. Astrid's dragon gets caught in the Red Death's jaws, and she jumps into it's mouth to free it before leaping out as it tries to crush her. Hiccup and Toothless catch her in time and bring her safely to the ground, then take back to the air to try and force the massive dragon to use it's wings. They succeed in provoking it to follow them high into the clouds, before using Toothless' stealth to vanish. As it searches for them in the darkness, they fire precision blasts targeting its huge wings. Enraged, it spews fire in every direction, igniting Toothless' prosthetic tail. Hiccup enacts the final stage of his plan, sending Toothless into a straight dive through the smoke with the Red Death on their heels. The dragon queen gains on them and gets ready to release another blast of fire and, as they reach terminal velocity, Hiccup tells Toothless to fire a blast into its mouth. It's gasses ignite, and burning it from the inside as the ground comes into view. The Night Fury pulls up, but the Red Death's damaged wings are unable to catch it and it crashes into the mountain, exploding in flames. Toothless and Hiccup attempt to fly back up and away from the massive fireball, but Toothless' tail fails completely and they lose control. They collide with the Red Death's tail, which knocks Hiccup from the saddle and sends him into the flames as Toothless tries to dive after him.
As the smoke clears, Stoick searches the destruction for Hiccup, but finds only an incapacitated Toothless, his saddle empty. Stoick, Astrid, and the other Berkians grieve the loss of Hiccup, and Stoick apologizes to him and his dragon. Toothless recovers, and unfurls his wings to reveal Hiccup in his grasp, unconscious and severely injured, but alive. The villagers celebrate, and Stoick thanks Toothless for saving his son's life. Days later, Hiccup awakens in his home and it greeted by Toothless. Confused, he gets up and finds that his left leg is gone, and has been replaced by a mechanical prosthetic. Toothless assists Hiccup outside, where he sees that the entire village has accepted dragons into their society, Stoick telling him that they have all been inspired by Hiccup's example. Astrid finds him, first punching him for scaring her, than kissing him for his bravery. Gobber, who made Hiccup's new leg, has copied Hiccup's designs and rebuilt Toothless' tail fin, along with a new saddle to accommodate his mechanical leg. Hiccup and the other Dragon Riders take to the sky on their dragons as a new era of peace dawns on Berk.
