After finding her, the riders scour the beach and the wreck of the Reaper for clues on their whereabouts, but only find a new lens for the Dragon Eye that works with a Changewing's acid. One by one the riders cure their dragons using salt water, and agree to take vacations at home while Hiccup promises to lighten up on his Viggo obsession. Meanwhile, Snotlout, as a result of his saving Mala's life from the Hunters, undergoes The Trials, a series of tests to prove himself as King of the Defenders of the Wing. Meanwhile, Astrid, Heather, and Snotlout find the marble is being used to construct a dragon-proof fortress that will make it impossible for any captured dragons to escape. According to , executive producer for , the series was planned to be much darker and deeper than 's previous television series spin-offs, with a similar tone to the movie, and would follow after the events of the first movie. Hookfang has a habit of attacking Snotlout or abandoning him when Snotlout tries to assert dominance, and does not always pay attention to Snotlout.
Though the Hunters take the island, the riders manage to drive them off with the help of Mala and the Gronkles living on the Edge are recruited to safely return the Eruptodon home. In the commander's quarters of 'The Reaper', Hiccup finds a very mysterious device and after a struggle with Dagur all of the group, apart from Hiccup, are trapped in a cage. During the trip, they are ambushed by hunters, but the twins rescue Hiccup and Johann and deduce that the only way the hunters knew they'd be there is if there was a mole within their group. Soon they get a stomach ache and return to Berk to get Meatlug checked by Gobber. They deliver the axe late only to discover a fight broke out and the wedding was cancelled.
After much effort they finally get it open and Hiccup is able to retrieve the device from a distracted Dagur. They question why Johann would simply abandon the King of Dragons, but discover that the flame of the Dramillion dragon shows a different picture through the Dragon Eye and reveals the true King of Dragons, a Bewilderbeast, in the Berserker Island. All is well until suddenly the dragons begin to wildly revolt, and the others assume Hiccup's obsessive work regime is to blame. He later double-crosses Dagur returning the Dragon Eye to Hiccup and saving both of them in the process. The riders and Gothi travel to Glacier Island to retrieve the tooth. Archived from on September 6, 2012.
Meanwhile, the Wingmaidens entrust the egg with Valka, Stoick recovers from his injuries, Mala and Dagur are married, Fishlegs and Snotlout both begin courting Ruffnut, Hiccup decides to destroy both Dragon Eyes to prevent them from falling into the wrong hands and the Riders leave the Edge to return to Berk. Despite their competition through the three challenges, Ruffnut decides to save Tuffnut instead of completing the final challenge, which Gruffnut reveals was the secret, fourth challenge: putting family above everything else. The teens must drive the Screaming Death and the Whispering Death hatchlings away from the village before they destroy Berk. Hiccup uses his flight suit to win the trust of the wild dragons and leads them in a counterattack that fends off Ryker and his group. Despite Gobber and Hiccup's insistence, he is unwilling to trust any new traders that come to Berk. Hiccup and Toothless decide to face against it one on one and later lead it into a trap, but the Skrill anticipates their plan and traps them instead. Hiccup must find Toothless and reason with him before the rivalry turns fatal.
The three trust the Fire Terrors to take the Eruptodon egg and it safely reaches the nesting ground where it is bathed in the lava. Meanwhile Fishlegs teaches the twins about Viking marriage and Tuffnut, believed to be certified, weds Fishlegs and Ruffnut. Dagur is giving Alvin the support of his vast fleet in exchange for the Skrill, but the truce is uneasy. Feeling he may be denying his sister a chance at lifelong happiness, Tuffnut uses a visiting Changewing on migration to fake his death and allow Ruffnut to go with Throk, but Ruffnut refuses to move on by painting her brother's image on the belly of a Changewing she names Snuffnut and Throk refuses to leave without her and attempts to tame Snuffnut on his own. Heather refuses to abandon the baby dragon, convincing the riders to leave it on Melody Island to be cared for by the elder Death Song.
Feeling responsible, Hiccup tries to help find a new job for Gobber, however Gobber's slightly eccentric and overenthusiastic personality makes this difficult. Gobber believes it to be the Lycanwing, a mythic dragon whose bite can turn humans into vicious dragon hybrids, and Snotlout preys on Tuffnut's paranoia making him believe he will become a Lycanwing by the next full moon. Heather, Fishlegs, and Dagur face difficulty saving them due to Viggo's dragon-proof ships until Heather and Dagur work together to take down the fleet. Toothless appears to be the only exception until he too begins to rebel, but the riders discover to be the true cause to be freshwater-dwelling parasitic dragons called Grimoras. It's dangerous behavior forces the other Gronckles to leave and seek refuge on Dragons' Edge, much to everyone's dismay. The fourth season was released on February 17, 2017. Hiccup faces Johann alone while Toothless defeats Krogan and recovers the egg.
Though Ryker attempts to break them by showing their dragons imprisoned, the riders hold their silence. Toothless also has the unique ability to navigate in dark places using. The group mostly welcomes her back with open arms, but Hiccup is suspicious of her new, more aggressive behavior. They identify the rider's dragon with the Dragon Eye as a Razorwhip and pursue it to a remote island. They launch their attack and rescue the others, and both sides prepare for their next encounter. Alvin and the Outcasts then attack Berk, and while the riders are defending against the attack, Savage plants Whispering Death eggs in the caverns under Berk. Ruffnut, however, is suspicious of Gruffnut's intentions, especially when he tries to fly off with each of the riders' dragons.
While Hiccup and the other teens openly welcome the newcomer, Astrid's jealousy turns to suspicion when she notices Heather is taking an unusual interest in the Book of Dragons. To make things worse, the ingredients needed to cure the illness are in Toothless' satchel and all but Fishlegs are caught sick. When the bucket grows tighter, the locals regard this as an early storm warning. Snotlout in turn is nearly killed by the aggressive female Razorwhips but is saved by the Riders and Wingmaidens who in the end celebrate their newly formed alliance, and Snotlout learns a lesson in etiquette. It is up to Hiccup and his friends to convince him otherwise. Following the events of How To Train Your Dragon, the Vikings of Berk and their former enemies, the dragons, are living together in peace, but this co-existence is not as harmonious as one may think. Following him, the riders find that he has mated with a female Monstrous Nightmare.