- The Catastrophic Quaken is introduced.
- Dark Deep, the ancestral home island of the Gronkles is found.
Hiccup and Toothless finish constructing a somewhat long-distance telescope in their Island to act as a scouting mechanism, just then Hiccup wonders how the Twins are doing on constructing the watch tower, where Astrid is certain it's going to be a disappointment. However, Tuffnut and Ruffnut actually build a fine watch tower and, for the first time, they have this feeling of 'accomplishment'. They decide to hurriedly fly back and 'rub it' on Hiccup's face before the feeling is gone. When they arrived, the Twins invite them to their 'accomplishment' of showing their watch tower, but when they arrived, the watch tower suddenly dissapears in plain sight. The Twins tries to tell them they did build a fine watch tower but most of the Riders are unconvinced.
Meanwhile, Fishlegs is feeding Meatlug different types of rocks that causes her to barf different colors of flames to experiment with the Dragon Eye, as the light of the different colored lava Meatlug spews are revealing and deciphering new directions to an island Fishlegs has been searching, Dark Deep. He finally unlocks all maps pointing to Dark Deep and calls Hiccup in excitement for his discovery. Hiccup is glad to see Fishlegs find all the maps pointing to the ancestral home of all Gronckles, Snotlout comes in and asks what's all the commotion about, Fishlegs explains to him but in the end, Snotlout gives no care or support whatsoever. Nonetheless, Fishlegs and Meatlug prepares to travel to the island tomorrow as he wants to be the first Berkian to ever set foot on the island, Hiccup wants to tag along which he accepts but tells him to not step on the island first.
The next day, Hiccup and Toothless prepare for their journey but pauses seeing the Twins all heavily tired as they tried to construct the watch tower again, they invite Hiccup to witness it, but again, the watch tower is gone. The Twins suspect that someone is stealing their 'watch tower' and leaves the scene bickering after all their hard work is in vain.
Afterwards, Hiccup and Fishlegs begin their journey, Fishlegs' excitement causes him to sing in joy throughout the journey and Hiccup has never seen him this happy since they were seven. Once they arrived, Fishlegs still continues to sing in great joy but is oblivious to see that the place is empty, Hiccup suggests that they are in the wrong island, but Fishlegs is certain this is the home of Meatlug's species. Meatlug later on finds a few Gronckles below ground and he attempts to greet them, but the Gronckles are a bit aggressive towards them. Suddenly, the ground shakes and they spot a huge boulder rolling quickly towards them, they flee and Toothless is unable to slow down the 'boulder'. When the 'boulder' misses them, it suddenly redirects and heads straight for them. Hiccup, Fishlegs and the dragons run for their lives they are nearly crushed, but luckily finds another route that causes the 'boulder' to miss them. They finally lost sight of the 'boulder' and attempts to escape quietly, but falls apart when Fishlegs' stomach rumbles after his decision to skip 'third breakfast' causing the 'boulder' to find them.
The 'boulder' reveals to be a new Boulder class dragon that is huge and powerful, the dragon uses it's tail to batter the rocks causing it to fall down to them, while in a dangerous moment, Fishlegs names it to be the 'Catastrophic Quaken', Hiccup tells him to name it later and focus on an escape. Toothless fends off the Catastrophic Quaken causing to go on protection by rolling up as a boulder, Hiccup sees it as their chance to escape, and they resurface. Hiccup and Toothless tells Fishlegs and Meatlug to retreat along with the stray Gronckles.
When Hiccup and Fishlegs bring in the stray Gronckles, the Riders complain as a lot of wild Gronckles, all from Dark Deep, have already decided to live on their island after their homeland has been threatened by the Catastrophic Quaken. Hiccup and Fishlegs informs them they can't relocate the Gronckles back to their home as the Catastrophic Quaken has chased them all off, Snotlout is dissapointed at them of having one dragon take the island of Meatlug. Hiccup proposes that they gather all the wild Dragons in order, where he assigns the Twins in charge of it after failing to build the watch tower. The Twins to struggle the Gronckles as there are too many of them to take care, Fishlegs and Meatlug watches them as they are upset about Meatlug's homeland taken over by one dragon and promises to take it back someday. Just then Snotlout interrupts offering Fishlegs help by training him to be ruthless, risk-taking, and aggressive. But Fishlegs is reluctant to do it, but Snotlout asks him that does he really wants to win over the island, Fishlegs answers 'yes' but does not want to do it with Snotlout's way. Fishlegs leaves but Snotlout shouts telling him that he will certainly return and take the offer.
The next day, Fishlegs visits Hiccup while he is fixing Toothless' tail rod, Fishlegs asks Hiccup does he have any plans at all to reclaim Dark Deep for the Gronckles. Hiccup recommends not to fight the Catastrophic Quaken and leave the island to him, as he observes it to be just what happens in nature where something big eventually takes over an island, and then something else bigger will take place and so on. Fishlegs is dissapointed.
At night, Fishlegs gets a wake-up call after having a nightmare where the Catastrophic Quaken chases him and some Gronckles on Dark Deep while 'dream Snotlout' tells him that he would return and take the offer. Terrified by the nightmare, Fishlegs, though hesitant to say it, asks Snotlout how to fight the Catastrophic Quaken. Snotlout is glad to have heard that, and begins training but in secrecy, as Fishlegs does not want Hiccup to find out. While training for hours, they try to cover up their training from Hiccup as best the can, as Hiccup begins to become suspicious of their doings lately. Just as Snotlout tells them they are ready to take on the Quaken, Hiccup discovers them and asks Fishlegs the truth, Fishlegs confesses as he reasons to Hiccup that they can't just sit by and watch the Quaken live life by himself on Dark Deep as he knows that the island is the Gronckles' true home. Hiccup still pleads him not to battle the Quaken as he tells him that this is not the 'Fishlegs' he knows and he cannot listen to Snotlout's suggestions as he points out that his actions leads to disaster. Hiccup does accepts Fishlegs' pleas to have the Gronckles reclaim their home but not involving fighting the Quaken. Hiccup and Toothless leaves in hopes that Fishlegs' realize his lesson, just then the Twins interrupt asking them if they have seen any 20 Gronckles pass by them after realizing their mistake of building a wall of rocks to contain them.
Snotlout request a ride home from the Twins, but Fishlegs instead offers him a ride but on one condition. Fishlegs decides to face the Catastrophic Quaken with Snotlout despite Hiccup's pleas. Fishlegs feels bad but Snotlout cheers him up saying that he did the right thing, however, when the Quaken emerges, Snotlout did not realized the sheer size of the dragon and asks why Fishlegs did not describe the dragon exactly. Snotlout flees in fear followed by Fishlegs, Snotlout and Hookfang leaves but Fishlegs and Meatlug stay behind deciding to fight the Quaken. Hiccup and Toothless and Astrid and Stormfly arrives to help Fishlegs get away from the Quaken. In the ensuing mayhem, Fishlegs and Meatlug are pushed back down below inside the Catastrophic Quaken's cave where Fishlegs sees that he's been living in the island all by himself.
As Fishlegs and Meatlug run for the exit, they are confronted by the Catastrophic Quaken. Fishlegs stands his ground in bravery, where the Riders think that what he's doing is suicide. The Catastrophic Quaken rolls down quickly towards them, and then it gets launched and seems to plummet and crush Fishlegs to death. But at the last moment, Fishlegs lives but the Quaken faces them. Meatlug tries to communicate with it and discovers that the Quaken isn't actually aggressive at all, and befriends it along with other Gronckles also.
Hiccup is so impressed with Fishlegs after facing the large dragon, which he describes to be the craziest (mostly) and bravest actions he has ever witnessed. Fishlegs tells him he was right with him, that 'fighting dirty' was and is not his character after seeing the Catastrophic Quaken was only being defensive because it was actually scared by itself, and they mistaken him to be aggressive or angry, but now the Quaken shares the island to the rest of the Gronckles.
Snotlout congratulates Fishlegs after what he did today, claiming it's what he exactly trained him to do. Fishlegs asks him more about his training, but reveals to be only sarcasm and even owns him by saying 'Oh, that's right! I just remembered, I don't care.', the quote he also said to Fishlegs earlier. The episode ends with Snotlout claiming he 'taught him too well.'.
- Dark Deep bears a strong resemblance to Outcast Island.
- At the end of the episode, Snotlout breaks the fourth wall by smiling sheepishly at the audience, after he said he taught Fishlegs too well.
- 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