|“||Hiccup reluctantly allows the Dragon Riders to go to a deserted island for a much-needed vacation, but the getaway has an odd effect on their dragons.||”|
At the Edge, the Riders are hard at work producing Gronckle Iron to improve their defenses. With the exception of Hiccup, the Riders and their dragons are tired and restless from their work, and Snotlout suggests they could use a vacation. Hiccup refuses as he knows Viggo will come looking for the key to the Dragon Eye and they have to be prepared and not leave The Edge.
Fishlegs and Astrid interrupt Hiccup working on his map to object to Hiccup's decision and come up with a way to benefit them both. Because of their work, they've exhausted the use of sandstone on the edge and tell Hiccup they could go to a specific deserted island to restock on sandstone and take a break.
Hiccup recruits Stoick and Gobber to protect the Edge while they're gone, and Tuffnut trusts Gobber to look after his pet chicken. The Riders and dragons (with the exception of Toothless) have fun in a lagoon when they arrive and start relaxing. Hiccup suggests they start working again, but Astrid reminds him of their limits and thinks they could use this opportunity to act more like people than soldiers. Hiccup agrees that they deserve this.
At night, the Twins prepare a dinner of heavily salted sea bass that disgusts everyone, even the dragons as they sleep next to the campfire. Fishlegs wakes up in the middle of the night and notices Meatlug's gone missing and leaves the rest to look for her. He finds her with a demented and drooling face while growling wildly. Fishlegs thinks that she's been overworked as Meatlug attacks him and chases him into the forest, where he hides in a tree stump. When he arrives back at the campfire, he notices the other dragons are awake and scratching and behaving wildly like Meatlug has. Fishlegs wakes up the other Riders as their dragons chew their saddles off and start attacking their riders. Snotlout accuses Hiccup of leading them to rebel by overworking them. Toothless is the only dragon unaffected and helps defend the Riders from their crazed dragons. Hiccup orders the Riders to split up and escape as he and Toothless stay behind to hold off their dragons. While fighting them, the two are knocked into the lagoon and fly off to escape.
Back at the Edge, Gobber's looking for Chicken and notices Skullcrusher cough up some bones. He assumes Skullcrusher ate Chicken and nervously tries to hide the bones from Stoick. He drops one of them as he walks off, and Stoick assumes Gobber ate Tuffnut's pet and starts to think of a plan.
At the island, Toothless tries to use his echolocation to try to find the other Riders, but instead finds the dragons flying above them. They find a nervous Ruffnut hiding in a bush, and the three leave to find the other Riders. Fishlegs and Tuffnut encounter Meatlug, and Fishlegs almost alerts her of their presence when he tries to reason with her. Elsewhere, Snotlout and Astrid hide in some bushes while reflecting on the situation.
Back at the Edge, Gobber takes one of his chickens to paint it up to resemble Tuffnut's, while Stoick plans to use his chiefly status to order Tuffnut to accept his chicken.
The Riders reunite only to be surrounded by all of their dragons. Toothless scares off the other dragons with his plasma blasts, allowing the group to hide in a cave, where Toothless then brings rocks down to cover the entrance. While thinking of a way to escape, the Riders blame Hiccup for overworking them and their dragons and pushing their beasts past the brink of insanity. Toothless then starts to behave like the other dragons and prepares to attack them. Tuffnut tries defending them by tossing his gear at Toothless and manages to snap him out of it by tossing leftover salt on him. They see tiny parasites hop off of Toothless, which Fishlegs identifies as the Grimora, which is the true cause of the dragons' behavior. The dragons picked them up when they went swimming in the lagoon since it has fresh water, also explaining why Toothless didn't act hostile immediately, as he didn't swim in it. While Hiccup's satisfied that it isn't his fault, he admits that he overworked the Riders and their dragons. When Tuffnut's out of salt, they determine the best way to save their dragons is to lead them into the salt water.
Fishlegs manages to help Meatlug by throwing her favorite meal, pyrite, into saltwater, freeing her of the parasites and making her act friendly towards him again. Astrid frees Stormfly by touching her in her soft spot and bathing her in saltwater. Hookfang chases Snotlout off a small cliff into the water and nearly eats him before the Grimora comes off. The Twins manage to confuse Barf and Belch and throw buckets of salt water on them at the right opportunity. Unfortunately, Toothless gets attacked by the parasites again when Hiccup is riding him, Hiccup tries to steer him towards the water before getting knocked off. Hiccup knocks Toothless and himself off a large cliff into the water, with the mad Toothless firing off one last plasma blast as they go in. The Riders are worried for their safety, but they soon emerge out of the water just fine.
The Riders return to the Edge and decide to take breaks closer to home rather than uncharted territory while Hiccup promises to lighten up when it comes to ordering them to work. Gobber and Stoick fail to convince Tuffnut to accept their chickens, and Tuffnut finds his real chicken right behind them. Tuffnut goes off to talk to his chicken about their adventure as Stoick and Gobber argue with each other.
- This is the first episode of Hiccup's map is shown.
- A parasite called Grimora is introduced.
- The spot Astrid scratches under Stormfly's chin is the same one where Hiccup scratched in the first movie.
- This indicates that although Astrid promises not to tell anyone about Stormfly's "soft spot", Hiccup knew about it even before she did.
- Both Stoick and Gobber tried to pass off a rooster as Chicken.
- The Grimora is the first fictional animal introduced that isn't a dragon. (Gobber's Hammerhead yak/whale don't count since they haven't been proven to exist).
- Jay Baruchel as Hiccup Horrendous Haddock III
- America Ferrera as Astrid Hofferson
- Nolan North as Stoick the Vast
- Chris Edgerly as Gobber the Belch
- Christopher Mintz-Plasse as Fishlegs Ingerman
- T.J. Miller as Tuffnut Thorston
- Andrée Vermeulen as Ruffnut Thorston
- Zack Pearlman as Snotlout Jorgenson
- Dragon Hunter War (mentioned)