It all takes place in the isle of Berk, "twelve days north of hopeless, and a few degrees south of freezing to death" and "located solidly on the meridian of misery," as Hiccup once described it. It's where Vikings once fought dragons for centuries, braving through battle to protect their own despite the endless war of the so-called vile creatures terrorizing their land. That is, until Hiccup takes it upon himself to actually befriend one of these dragons instead of killing it (even giving him the name Toothless to just showcase that growing companionship), choosing to find more about their habits and ways of life which in turn teaches him that everything they once believed about them had been completely wrong. However, if there's one thing that Vikings are known for, it's their stubbornness. So even when he tries to convince the village, he gets pushed away, shunned for siding with their long sworned enemy, forced to hear his father, Stoick (who just so happens to also be the chief), utter to him: "You're not a Viking. You're not my son." In the end, it takes battling against the large terrorizing queen of the dragons' nest which threatens the lives of Viking and dragon alike to change that line of thinking. The battle costs Hiccup the loss of one of his legs (similar to Toothless' injury in missing a tail fin), but forces the Vikings to reconsider their war history and join together in living alongside dragons.

Five years pass and the lifestyle of Berk completely transforms from a constantly dreary warzone to something more positive as Hiccup's changes his tone to announcing that "life [there] is amazing, just not for the faint of heart." Because at this point, dragons have been integrated into the village's culture, now filled with energy and excitement as they enjoy things such as dragon racing and having these dragons living alongside them in their homes as pets. At this point, the village and Stoick have placed such a strong trust in Hiccup for transforming their world that he's been secured as next in line to become Chief of Berk. Except the one-legged Viking has been escaping his responsibilities, instead venturing the world to discover uncharted lands with Toothless. It's through his travels that he learns about Drago trying to bring together a dragon army and a mysterious dragon rider that has been trying to stop him. Wanting to try talking peace with Drago, Hiccup begins to clash heads with his father who insists they prepare for war. Yet habit has Hiccup ignoring the chief's orders, attempting to seek passage to see the dragon army's commander before once again getting stopped by Stoick who warns him of Drago's unrelenting need to take command of all dragons, willing to destroy anyone who steps in his path. Still not convinced, Hiccup escapes once again, where he meets the mysterious dragon rider who turns out to be his mother who has been living with dragons for 20 years after being kidnapped. Learning that his close connection to dragon stems from her side, Hiccup is thrilled to have her around, convinced that they could talk to Drago together before she protests just as his father did, insisting they instead had to do everything to protect the dragons. Yet, soon enough, Stoick appears to bring his son back home, reuniting with his estranged wife, allowing for a few moments of happiness where the family comes together, leaving Hiccup filled with a strong hope for the future.

But that carefree happiness is short lasting when they discover Drago's army has arrived. Going out into the battlefield, Hiccup guides Toothless into destroying as many dragon traps as they come across while the rest of their friends arrive to help. Still convinced he could end this war with talk, Hiccup finally confronts Drago, but his words fail to get across and instead backfire as Drago uses his Bewilderbeast, the new commanding Alpha of dragons, to put Toothless under his control. Having lost his free will, the Night Fury is directed to shoot an attack at Hiccup, but Stoick arrives just in time to protect his son from the blast, losing his life in the process. Completely distraught, Hiccup pushes Toothless away in anger before immediately regretting his actions when Drago uses the Night Fury to ride off to Berk. Afterwards, they hold a funeral for Stoick where Hiccup expresses his long lasting fear of never being able to have been as great of a chief, as great of a person, as his father, but regains his resolve when his mother tells him that Stoick had always believed in him since he was born. Deciding to use the dragon babies, the only dragons that cannot be controlled by the Alpha, to head back to Berk, Hiccup uses his words to get Toothless to come back to his senses, telling the dragon that he understands the attack hadn't been his fight, that the two still remained best friends. The depth of that friendship is enough to fight off the Alpha's control, leaving Hiccup prepared to fight off Drago and the Bewilderbeast. Using the strengths of their flying and training, they managed to separate their two enemies just before the Alpha entraps them in a blast of ice. However, resolved to protect his best friend, Toothless gains new stronger abilities, destroying the ice and stepping up to find the giant Bewilderbeast on his own, slowly convincing the controlled dragons to join in his battle. Proving himself as the new Alpha, Toothless harms and shames the large dragon just enough to have him give up and return to the waters, as Hiccup reminds Drago that the "alpha protects them all". With Berk cheering over their victory, a ritual proclaims Hiccup officially as the new Chief of Berk, which he accepts with a meek smile. As the Vikings are seen repairing the village back together, Hiccup speaks in a voice over of their stronger determination, that "[they] are the voice of peace. And bit by bit, [they] will change this world".