|“||Uh, what happened to Barf and Belch? Why is the island sunk? And where did those two Quakens come from? I feel like we missed a lot.||”|
|“||The gang investigates why Viggo's men are mysteriously shipping marble blocks from a quarry. Next, they try to save two dragons being used as slaves.||”|
The Riders attempt to ambush a Dragon Hunter convoy likely containing stolen dragons. Hiccup uses his new sleek, tail fin invention on Toothless, to allow the two to sneak up and attack them, signaling the other Riders to come. They tie up the shipmates, but they find no dragons inside the ships, just marble blocks. They then notice a large dragon stampede flying from a nearby island. Hiccup leads them towards the island to find the source of the problem, and the Twins crash Barf and Belch on the way down, injuring them in the process.
The island has more ships going out just like the ones the Riders ambushed. They capture a group of Dragon Hunters. Snotlout gets one of them to start talking, but before he can say anything important, he's knocked out by Tuffnut. Hiccup sends Astrid, Heather, and Snotlout to find the other convoy, while he, Fishlegs, and the Twins will scout the island for more answers.
Hiccup, Fishlegs, and the Twins come across a large marble mining operation. Hiccup prepares a plan of attack, but Fishlegs stops him, recognizing that the wrecking balls being used to carve the marble are actually Catastrophic Quakens, causing miniature earthquakes every time they crash, one of which knocks the Twins and their dragons at a large distance. Fishlegs observes that the workers have trained the Quakens to obey via vibrations from their hammers. Hiccup tries to devise a plan to free the dragons and reveals he's brought yet another type of tail fin to use on Toothless.
Meanwhile, Astrid, Heather, and Snotlout spot the other convoy as the ladies are annoyed by Snotlout's vanity.
Back at the facility, Hiccup, and the Twins successfully distract the workers allowing Fishlegs and Meatlug to free the Quakens. Hiccup orders the workers to leave, but the workers laugh in response. One of them knocks a hammer down to the ground, and the Quakens turn to face Fishlegs and try to attack him. The three escape on their dragons as the worker with the hammer pounds on the ground more. Hiccup and Toothless lead the Quakens to them to save the Twins, but struggle in the process because the tail fin Hiccup put on Toothless improved tight turning at the cost of altitude control. Fishlegs flies in to try and calm the dragons down and train them, but it proves useless. Fishlegs is appalled that the Quakens respond better to cruelty than violence.
While the Twins try to tend to their continually injured dragons, the trio tries to plan an attack to injure Viggo's marble operations while saving the Quakens. They notice a fissure, and the island shakes again, but this time not from the Quakens. They realize that the mining operation is causing the island to become unstable, which is why the group of dragons left in the first place. This gives Hiccup a plan to stop the operation and save the Quakens.
Meanwhile, Astrid, Heather, and Snotlout follow the convoys to Viggo's base, and find out that the marble being shipped to Viggo is being used to construct a highly durable fortress. Astrid notices that it's still vulnerable as it isn't finished. They attack the area, but realize that the marble's durable enough to withstand dragon fire and Windshear's strikes, meaning that dragons won't be able to infiltrate or escape.
Back at the facility, Hiccup warns the workers that the island's unstable and orders them to shut it down and free the Quakens. The workers follow Hiccup's orders reasonably, with the exception of the one controlling the Quakens, who orders them to attack the Riders. The Quakens chase the Riders and cause more damage to the island. One of the quakes traps Hiccup and Toothless in a crevice, and one of the Quakens falls in with them and gets trapped. Fishlegs noticed the other Quaken hesitates in order not to hurt the other one. Fishlegs takes advantage by grabbing the discarded hammer controlling them. Tapping it gently, he shows the other Quaken his compassion towards Meatlug and starts to get the Quaken to follow him, instructing it to free the other trapped Quaken in the crevice. Hiccup and Toothless escape the crevice as Fishlegs instructs the other Quakens to fly with him. They all escape the island via flight (with the exception of the Twins, who have to sail with Barf and Belch as they're too injured at this point to fly). Astrid, Snotlout, and Heather return to relay the news of the marble stronghold back to Hiccup. Though the marble makes it impervious to dragon fire, Fishlegs has an idea on how to handle it.
The Riders arrive at the marble stronghold, and Fishlegs uses the Quakens to defeat the Hunter troops and destroy the stronghold. Having successfully bonded with the beasts, Fishlegs drops the hammer and frees them of their control. Hiccup congratulates Fishlegs on his accomplishment, but Fishlegs worries about how many more dragons Viggo has out there. Hiccup assures him that they will stop Viggo and free his captive dragons, and instructs Fishlegs to take the Quakens to Dark Deep to enjoy their "retirement."
Later, Viggo and Ryker arrive at the ruined stronghold and are angered knowing it was the Riders who did this. Viggo assures his brother that the Riders will pay.
- The Catastrophic Quaken makes its second appearance.
- Hiccup begins experimenting with alternative tail fins for Toothless.
- A yellow tail that allows for extended stable gliding without flapping, but hampers maneuverability.
- A blue tail that increases maneuverability, but can't be used to gain altitude.
- Tuffnut alludes to the concept of 'Stockholm Syndrome' wherein a kidnapping victim is more likely to relate to their kidnapper and remain compliant with them, rather than fighting back for themselves. After Fishlegs makes this connection about the dragon slaves and the dragon hunters, Tuffnut states "I had a cousin in Stockholm that happened to once."
- The line, "Release the Quakens!" is a reference to Clash of the Titans.
- The Quakens blindly following the orders of the Hunters might be an allusion to Drago's Bewilderbeast.
- The way Fishlegs uses the hammer to communicate with the Quakens is similar to how Valka uses her staff to communicate with dragons.
- A running gag in this episode is that Barf and Belch constantly get injured.
- This is also one of the first times where the two heads are referred to as individuals, like when the front leg on Belch's side is hurt and Hiccup asks what happened to Belch.
- Both times Hiccup replaces Toothless' specialized tail fins, the main one has a dark red lightning bolt pattern on it.
- The fortress that Viggo was building is similar to Tomorrow Kingdom from the books. Both were constructed by enslaving dragons to build it, though humans were enslaved as well for Tomorrow.
- Jay Baruchel as Hiccup Horrendous Haddock III
- America Ferrera as Astrid Hofferson
- Christopher Mintz-Plasse as Fishlegs Ingerman
- T.J. Miller as Tuffnut Thorston
- Andree Vermeulen as Ruffnut Thorston
- Zack Pearlman as Snotlout Jorgenson
- Mae Whitman as Heather
- Night Fury
- Deadly Nadder
- Monstrous Nightmare
- Hideous Zippleback
- Catastrophic Quaken
- Hiccup's Prosthetic Leg
- Toothless' Saddle and Tail Fin
- Astrid's Kransen
- Berk Satchel
- Dragon Hunter Weapons
- Dragon Hunter Ships