Savage and the Berserkers have finished scavenging all materials worth of value from the Reaper, in which they resurfaced it, however, Dagur points out that they are still missing one valuable object, the Dragon Eye.
At Dragon's Edge, while experimenting and observing the Dragon Eye's mechanics, Astrid arrives at Hiccup's hut informing Hiccup that they have a worse problem than Dagur: Gustav and Fanghook visiting, as they crash through Hiccup's hut. Gustav is now 16 years old, but still acts reckless and immature such as when he stares at the Dragon Eye's projecting light causing his vision to loosen making it hard for him to see his way. Snotlout asks Gustav when is he leaving, but to their surprise, Gustav decides to stay and move in with them. Hiccup asks Gustav there must be an explanation, Gustav answers that by reminding them that they promised Gustav he can join the Riders when he is ready, so today he claims that he is and asks to move in.
A while later, Hiccup favors Astrid by keeping an eye on Gustav as he is working through some major breakthroughs with the Dragon Eye, Astrid reluctantly accepts, and Hiccup reassures him that he can't be that reckless, however, Gustav and Fanghook are already causing trouble such as tiring and harassing out Smidvarg and the rest of the Night Terrors, causing Astrid to settle him down.
At the Arena, Astrid assigns Gustav to help her do target practice, where he must throw a target barrel when Astrid gives the signal, for Stormfly to shoot. However, Gustav daydreams due to being romantically obsessed and attracted to Astrid causing Gustav oblivious to Astrid's signal. As a result, Stormfly fires her spines at the barrel where Gustav is still holding, but fortunately Astrid uses her axe to prevent the spines from hitting Gustav. She also has to dive in and tackle Gustav out of the way where one spine almost hits him, but Gustav continues to be in love at her, creeping Astrid out. At the Twins' hut, the Twins spot Gustav resting near their hut, where he explains Astrid drop him off and help the Twins around with their place, by filling up their 'big hole' under their hut with rocks. However, Gustav doesn't know that it's actually the Twins' Boar Pit that Gustav covered up, causing Tuffnut to be dearly upset while Ruffnut comforts him.
At Fishlegs's wing, Fishlegs invites Gustav and Fanghook for a quiet and calm meditation at a hot spring, to show Gustav to be centered. However, when Fishlegs tells them to breathe in and out, Fanghook breathes fire at Fishlegs's topiaries, ultimately burning it.
A few hours later, Snotlout has had enough of Gustav and tells Gustav to stay in Hiccup's hut, before he leaves, Gustav tries to inform Snotlout his head is caught on fire, but disbelieves it and carries on and shortly after, finally realizes it painfully. As they wander throughout Hiccup's hut, they find Hiccup and Toothless absent, so they play around with Hiccup's stuff, while imitating Hiccup by putting on his prosthetic leg, he painfully steps on Fanghook's tail causing him to burst fire at one of Hiccup's tail designs of Toothless. But he quickly gets distracted, when he spots the Dragon Eye present, projecting a map where Gustav thinks it leads to treasure.
So, they journey off to find the location of the island where they claim it holds treasure. They draw the map in paper to find directions of where the treasure is kept. When they enter a cave, Gustav lets Fanghook brighten the place by flaming up his body, but when the cave suddenly shakes, their map gets caught on fire. Luckily, Gustav reveals they brought the Dragon Eye with them. As they continue to venture through, the ground quickly becomes unstable, creating a huge hole. Fanghook manages to reel in Gustav from falling, but he notices the Dragon Eye is on the ground rolling through the hole. Gustav manages to grab it, but his grip on the edge of the hole is weakening, he almost falls to his death but luckily someone saves him. Where it is revealed to be Hiccup and Toothless, who have followed Gustav.
At Dragon's Edge, Hiccup sermons Gustav saying that all of his irresponsible and reckless actions he has done, going through that island was his worst. Hiccup scolds Gustav saying that what if Dagur got his hands on the artifact, Gustav reasons that if he found the treasure in that island, the Riders could finally accept him. However, Hiccup informs him that there is no treasure in that island as they've already been there and warns him that the island is honeycombed with the most unstable caverns, saying that he could've been killed. Gustav apologizes for his actions saying he only wanted to prove himself worthy, but Hiccup humiliates him saying he has proven himself to be irresponsible, immature and not trustworthy enough to be one of them.
When the scolding ends, Hiccup tells him to get some rest and continue the talk about his actions in the morning, as he leaves Gustav and Fanghook alone. At night, while feeding Fanghook, Gustav bickers after being scolded, and contends with himself feeling betrayed, as he has done everything the Riders told him for 2 years, from cleaning out their stables and shoveling dragon feces, to only be repaid by saying he is not good enough. So, Gustav reassures himself that the Riders will someday realize that he is good enough, but it'll be too late as Gustav and Fanghook decides to abandon them.
While flying out at night, Gustav claims the Riders will be begging him to rejoin when they realize his absence. However, a net immobilizes Fanghook causing them to plummet into the ocean. It is revealed the Beserkers were the ones who captured them and they reel them in on deck, Dagur gives a look hoping it is one of the Riders but finds Gustav and Fanghook instead and asks their identities. Gustav only reveals his name refusing to release anymore information, but when Dagur orders him to be fed from the eels, Gustav finally talks revealing his cooperation with the Riders. Dagur asks what he has to offer, Gustav offers Dagur that he can get the Dragon Eye for Dagur.
The next morning, Gustav and Fanghook are well-treated and well-fed after offering a great deal for Dagur. While enjoying their meals, Dagur proposes they possess the Dragon Eye by attacking the Riders with full force, but Gustav concludes that plan will fail as he suggests his plan of getting it that he claims will surely have Hiccup hand the artifact over, Dagur delightfully accepts.
At Dragon's Edge, the Riders are finally aware of Gustav's absence, though some of the Riders aren't worried as they finally have relief of Gustav's recklessness but Hiccup tells them that they can't celebrate his absence if they are certain he's safe. Shortly after, Fanghook arrives seemingly upset, Hiccup and Fishlegs notices a note attach to him. The note informs them that Dagur has Gustav captive and will only release him if Hiccup gives up the Dragon Eye. As Hiccup prepares to depart, the Riders are worried and proposes they support him but Hiccup tells them they can't as Dagur wants them alone, they leave promising he'll be cautious.
When Hiccup arrives at Dagur's ship, Gustav plays in his act by seemingly worried when Fanghook left. Hiccup wants to get this over with, but Dagur demands that Hiccup gives up the Dragon Eye first and then he'll let the boy go. When the trade is done, Hiccup tells Gustav to come forth, however, Gustav reveals that he has now taken alliance with Dagur, betraying Hiccup, saying that he has been treated better here than Hiccup and the Riders on their island. Hiccup tries to tell him he has no idea what he's getting into with Dagur, but Gustav tells him that if he gave Gustav a chance he wouldn't have done this and Gustav reassures Hiccup he knows exactly what he's doing.
Dagur decides to spare Hiccup of being double-crossed by his men as in return Gustav will use the Dragon Eye to help Dagur take him to the 'treasure'. Hiccup gestures strangely after hearing that but leaves Gustav on his own by saying that he will regret this. After a job well done, Gustav and Fanghook celebrates unbeknownst to them, that Dagur plans to execute him after they find the treasure.
The next day, the treasure Gustav promised to Dagur is actually located at the island they already visited prior. Before they enter, Gustav persuades Dagur to leaves his men outside as they might take the 'treasure' for their own, so Gustav, Fanghook and Dagur enter the cavern alone. As Gustav lets Fanghook light up the map, Dagur is impressed and continues to venture inside. Dagur notices that the cavern is beginning to shake violently, but Gustav tells him to get use to it. When Gustav informs Dagur they are getting very close to the 'treasure', Dagur begins his attempt to double-cross him. However, Gustav was expecting it and blinds Dagur with the Dragon Eye's projecting light, causing his vision to blur just like Gustav's prior. Gustav makes a break for it and hides, Dagur easily spots him but before he can attack him, Hiccup and Toothless threatens him not to by surprise.
This reveals that all of Gustav did was a ploy to get Dagur in trouble, but Dagur snatches the artifact from Gustav. Hiccup and Toothless tells him to hand it over, as he has nowhere to go due to the deep gaps created in between him. But Dagur lets go of the Dragon Eye into the pit, and Gustav immediately dives in to reclaim it, causing Dagur to escape. As Hiccup and Toothless dive in to rescue him, Gustav retrieves the Dragon Eye and he finally gets aboard with Toothless. However, a boulder hits Toothless causing Hiccup and Gustav to scatter, Hiccup barely manages to mount up with Toothless and get him to fly again, as for Gustav he gets rescued by Fanghook and they escape.
Gustav apologizes for his actions but Hiccup also apologizes also for mistreating him, because although he can be still irresponsible, immature and reckless he proved to Hiccup that he can be a Dragon Rider someday after the way he handled Dagur, Gustav gladly accepts. Before they leave the cavern, Hiccup notices a chest, he opens it up and is surprised to see it's content, but Hiccup has to check it out later as the cavern quickly becomes more unstable, and they leave.
Back on Dragon's Edge, Gustav prepares to head back to Berk because his mother would 'kill' him after noticing his absence for three days and still romantically says goodbye to Astrid as he flies off, but promises he'll be back soon. Hiccup then shows the Riders what was inside the chest, a new lens for the Dragon Eye. Concluding that there was treasure in that unstable island as they weren't just looking in the right place. Fishlegs is excited as he is certain there must be more of it out there.
Astrid points out they never would've grab hold of that if it weren't for Gustav, Hiccup is great to hear that and plans to tell that to Gustav. However, the Riders beg Hiccup not to as Gustav might return and stay with them even longer, so they just wave goodbye for Gustav and Fanghook's departure.
- Third time someone is seen imitating Hiccup. (Free Scauldy, How to Train Your Dragon 2)
- Dagur is given his new outfit.
- In one scene, the list on Dagur's arm is gone.
- The Dragon Eye also reveals locations of other lenses.
- Gustav returns and inadvertently helps discover a new lens for the Dragon Eye.
- Gustav's age is confirmed to be 16, though he still looks younger than all the riders did in How to Train Your Dragon when they're suppose to be 14/15. And even at 16 he's apparently still too young and immature to become a dragon rider.
- Jay Baruchel as Hiccup Horrendous Haddock III
- America Ferrera as Astrid Hofferson
- Christopher Mintz-Plasse as Fishlegs Ingerman
- Andree Vermeulen as Ruffnut Thorston
- T.J. Miller as Tuffnut Thorston
- Zack Pearlman as Snotlout Jorgenson
- David Faustino as Dagur the Deranged
- Lucas Grabeel as Gustav Larson
- Paul Rugg as Savage