The film begins aboard a dragon trapper ship, where Hiccup and his trusty dragon, Toothless, stealthily infiltrate to rescue captured dragons. Clad in protective dragon scale suits, they are joined by their friends: Astrid, Snotlout, Fishlegs, Ruffnut, and Tuffnut, racing in on their dragons to confront the trappers. Their daring mission succeeds as they free the dragons, but unbeknownst to them, one dragon remains behind.
Returning to Berk, the Dragon Riders find their village overflowing with the rescued dragons, causing chaos as new additions like a rambunctious dragon topple the towers. Leading the villagers as chief, Hiccup grapples with feelings of inadequacy compared to his late father, Stoick. Meanwhile, Valka trains dragons and Gobber urges Hiccup and Astrid to consider marriage, though both feel unprepared for such a commitment.
The tranquility is shattered by the arrival of the infamous dragon slayer, Grimmel the Grisly, who seeks out warlords capturing dragons. Surprised to learn from them that Hiccup still lives, Grimmel plans to use a white Night Fury—left behind during the rescue—as bait to lure Toothless, dismissing Hiccup as a mere child.
We flash back to a young Hiccup with his father, Stoick, who recounts legends of a hidden world of dragons—something he had dreamt of discovering to end the conflict between vikings and dragons. Meanwhile, Toothless ventures out at night and meets the elusive Light Fury, capturing his heart as he makes awkward but sincere attempts to impress her.
As the tension rises, Grimmel's traps come into play, and Hiccup discovers they are being hunted. When Grimmel invades Hiccup's home, he demands Toothless in exchange for peace, asserting that dragons and humans cannot coexist. During the commotion, Grimmel's Deathgrippers wreak havoc on their home, forcing the villagers to consider abandoning Berk. With some dissent among the Berkians, Hiccup stands firm, reminding them that their spirit lives on wherever they might go.
Seeking refuge on another island with the dragons, the adventure continues as the beautiful Light Fury appears, only to vanish upon mistaking Hiccup for a threat. Toothless’s attempts at dance and charm fail to impress her, leading Hiccup to improve his friend’s fin to help him soar to the skies once more. Valka returns to ensure safety, only to find themselves pursued by Grimmel and his crew.
As Hiccup's crew attempts to confront Grimmel, they realize they've fallen into his trap. However, they manage to escape with quick thinking, though not without leaving Ruffnut behind. Hiccup, pressured to come up with a plan, reflects on Toothless’s newfound freedom to be with the Light Fury.
Ruffnut inadvertently spills the beans about their location, and with Grimmel hot on their trail, it's a race against time. Hiccup and Astrid embark on a journey to locate Toothless, diving into a breathtaking hidden dragon world where Hiccup finally realizes the truth—dragons might need to live far from humans if they are to thrive.
Things take a turn when Grimmel attacks Berk, successfully capturing Toothless and the Light Fury, forcing the other dragons to obey him. Feeling defeated, Hiccup laments his inability to protect his loved ones. Encouraged by Astrid, who believes in him steadfastly, Hiccup rallies his friends to are gather once more and confront the looming threat of Grimmel.
The final showdown sees the Berkians battling with full force, freeing their dragons from Grimmel’s clutches. As the action crescendos, a series of breathtaking aerial maneuvers leads to Hiccup confronting Grimmel directly. Toothless’s electrifying moment of power enables them to escape danger just in time. In a final act of bravery, Hiccup and the Light Fury share a poignant moment that saves Toothless from demise as the two young heroes fall from the sky.
Ultimately, the Berkians celebrate their victory, but Hiccup knows it's time to let Toothless go. The heart-wrenching farewell sees them embrace and part, with the dragons destined for their hidden sanctuary.
Years later, we find Hiccup and Astrid happily married, donning their new roles as chief and chieftess. During a family boat trip to the hidden world, they spot Toothless and the Light Fury, now with their own hatchlings. Toothless's delayed recognition is met with a joyful reunion, as he and Hiccup renew their longtime bond. Together, the family takes to the skies, flying with their dragons as a symbol of enduring friendship. Hiccup reflects that while the world may believe the dragons have vanished, the Berkians know their secret and will keep it safe until dragons can once again roam free.
