Five years after the Viking inhabitants of Berk established peace with the dragons, Hiccup and his Night Fury, Toothless, embark on a journey to chart uncharted territories. While his father, Stoick the Vast, hopes for Hiccup to take the reins as chieftain, Hiccup grapples with his own uncertainties about being prepared for such a role.
While exploring a charred forest, Hiccup and Astrid stumble upon the remnants of an icy fort and encounter a band of dragon trappers led by the villainous Eret, who seeks to seize their dragons on behalf of the nefarious Drago Bludvist. Drago has sinister plans to capture and enslave all dragons to use them as his soldiers. Escaping Eret's clutches, Hiccup and Astrid race back to warn Stoick. During this tense meeting, Stoick recounts his past encounter with Drago at a gathering of chieftains, revealing how Drago offered protection from dragons in exchange for loyalty. When they refused, Drago retaliated by sending his dragons to attack, leaving Stoick as the lone survivor of that fateful day.
Refusing to accept that war is unavoidable, Hiccup sets off with Toothless to confront Drago directly. However, instead of finding him, they meet a mysterious dragon rider who turns out to be Hiccup's long-lost mother, Valka. Presumed dead after being taken by a dragon, Valka reveals that she has dedicated her life to rescuing dragons from Drago, bringing them to a unique island sanctuary protected by an immense alpha dragon known as a Bewilderbeast. This magnificent creature can command smaller dragons through enchanting sound waves. Stoick and his lieutenant, Gobber, track Hiccup to the sanctuary, where Stoick is overwhelmed to find Valka alive. Meanwhile, Astrid and the other dragon riders force Eret to lead them to Drago’s lair, but they are soon captured along with their dragons, prompting Drago to launch an assault on the dragon sanctuary. Tensions rise as Astrid's dragon, Stormfly, boldly rescues Eret from execution.
A fierce battle erupts between the dragon riders, Valka's dragons, and Drago's forces. During the chaos, Drago reveals his own Bewilderbeast to confront Valka's alpha. The titanic clash results in Drago’s Bewilderbeast emerging victorious, killing the competing alpha and taking command of all dragons. In a desperate attempt to stave off disaster, Hiccup reaches out to Drago for peace, but Drago retaliates by ordering his Bewilderbeast to make Toothless turn against him. Under mind control, Toothless prepares a lethal shot aimed at Hiccup, only for Stoick to heroically step in, sacrificing himself to protect his son. This moment of tragedy causes a brief disruption in the Bewilderbeast's control, but overcome with grief and anger, Hiccup drives Toothless away.
Left stranded on the island, Hiccup, along with the others, is abandoned by Drago and his forces. Following a solemn Viking funeral for Stoick, Hiccup, now grappling with the loss of both his father and dragon, feels lost. However, Valka’s encouragement reveals that it is his destiny to unite humans and dragons. Drawing inspiration from her words and the legacy of his father, Hiccup and his companions return to Berk determined to confront Drago, riding on baby dragons that remain resistant to the Bewilderbeast's sway.
Upon their return, they find Berk under attack by Drago's forces, who have taken control of the dragons. Hiccup manages to liberate Toothless from the Bewilderbeast's influence and confronts Drago, but unfortunately, they are encased in ice. Toothless then displays incredible determination, shattering the ice and entering a radiant, super-powered state, immune to the Bewilderbeast's control. He rises to the challenge, defending Hiccup and driving the Bewilderbeast into retreat after delivering a powerful blast that breaks its grip over the other dragons, who rally behind Toothless.
With victory won, the Vikings and their dragons rejoice in their success, and Hiccup is officially named the new chieftain of Berk. In a symbolic gesture of unity, the dragons from both Berk and Valka's sanctuary acknowledge Toothless as their new leader. As Berk begins to recover from the recent turmoil, Hiccup stands tall, assured that they will uphold the peace they forged with the dragons.
