|“||Okay guys! This is it! It's us three against a huge mucus-shooting, nasty killer dragon!||”|
|— Snotlout to Hookfang and the Fireworm Queen|
|“||A wounded Fireworm seeks the Dragon Riders' help in protecting a precious treasure trove against an unfamiliar creature with strange and slimy powers.||”|
While gathering ammunition for their catapult (with the exception of the Twins, who carve their faces into the mountain like Mount Rushmore), the Dragon Riders notice rustling in the nearby forest. They try to escape, but Snotlout and Hookfang crash during their flight. They are being chased by the Fireworm Queen, who seems particularly interested in Hookfang. Hiccup notices three slash marks on her neck, leading the group to guess she needs help defending her home from the Dragon Hunters and trusts Hookfang due to their history. Snotlout and Hookfang follow the queen as the rest of the riders join him for backup.
Hiccup thinks it's been uncomfortably quiet from the Dragon Hunters recently and warns the Riders to proceed with caution as the Queen leads them to the caves. She eventually leads them to a mysterious dragon digging in the depths, and Fishlegs uses his new Dragon cards to identify it as a Cavern Crasher, a scavenging dragon known for pushing other dragons out of their homes. The Riders try to fight the beast, but the dragon's speed and durability prove too much for them. It leaves a trail of flammable slime that Snoutlout and Hookfang ignite that leaves the group stuck in the burning cave. The Queen helps them escape the flames and goes after the Crasher. Hiccup tries to come up with a plan, but Snotlout and Hookfang go off on their own to help the Queen as Snotlout is determined to repay her for saving Hookfang's life as the rocks fall down and trap the riders.
Going deeper into the caverns, Snotlout starts having second thoughts about his idea, before being rebuffed by Hookfang as they catch up to the Queen. The Riders try to escape the caverns and come across the Cavern Crasher. They fail to catch the creature with bolas, and it retaliates by spraying flammable mucus all over them. The Twins try to go for the beast, but the flammable mucus they're covered with explodes when they try to blow up the dragon, causing the Twins and their dragon to crash. They try to avoid the beast's fire blasts as they're even more vulnerable with the mucus covering them.
The queen leads Snotlout and Hookfang to her hive, which contains dozens of baby Fireworm eggs. Snotlout realizes that's why she wanted their help, to defend her children from the Crasher intending to raid her nest for her eggs. Hookfang helps her organize the eggs and the Queen gives one to Snotlout to hold. The egg hatches and Snotlout starts caring for the baby Fireworm. He nearly hurts it when the infant burns his hand, but Hookfang helps cover it up from the Queen.
The Riders continue fighting the Cavern Crasher, but it stops fighting them when it hears the Queen, causing the Riders to realize what it's after. They try to stop it by bringing down rocks, but the Crasher flattens itself and squeezes through the cracks, (as Tuffnut had predicted earlier). Hiccup suggests to the Twins to use their "brain power" and skills to help them get through the rubble.
Back at the hive, Snotlout, the Queen, and Hookfang are caring for the multiple baby Fireworms when they are alerted of the Crasher's arrival. When the creature arrives, the Queen tries breathing fire on it, but it proves resistant to her attacks. Hookfang tries trapping it in rubble by bringing down rocks from the ceiling, but it squeezes through. The beast uses its mucus to attack the Queen and launches a fire blast backward that nearly fries the other Riders. Hookfang picks up the Queen and combines their firepower to try and incinerate the Crasher. While the Crasher survives, the attack cooked the mucus right off its body and its fighting with the Riders earlier weakened his fire blasts. The Riders arrive to back-up Snotlout and the Queen, and the force of all its opponents proves too much for the Crasher and it leaves the Queen to care for her young. Hiccup is impressed with Snotlout's performance and credits him with the overall rescue.
The Riders return to ready their catapult and the Twins have carved Snotlout's likeness into their monument.
- This episode is the first canon appearance of Fishlegs' Dragon Cards, which were previously seen in How to Train Your Dragon 2.
- The Fireworm Queen featured in this episode is the same one that appeared in "Race to Fireworm Island".
- The Cavern Crasher makes its debut.
- The lullaby that Snotlout sang to the baby Fireworm is similar to "Ba Ba Black Sheep" with Viking lyrics.
- Fishlegs stated "We've been down these tunnels, remember?" despite the fact that Hiccup and Snotlout were the only two Dragon Riders to go into the tunnels in "Race to Fireworm Island".
- During this episode, it becomes apparent that Ruffnut and Tuffnut are master sculptors.
- Or maybe Barf and Belch are, considering they were doing the chiseling.
- This is yet another example of the Twins' hidden intelligence.
- Jay Baruchel as Hiccup Horrendous Haddock III
- America Ferrera as Astrid Hofferson
- Christopher Mintz-Plasse as Fishlegs Ingerman
- T.J. Miller as Tuffnut Thorston
- Andrée Vermeulen as Ruffnut Thorston
- Zack Pearlman as Snotlout Jorgenson
- Hiccup's Prosthetic Leg
- Toothless' Saddle and Tail Fin
- Astrid's Kransen
- Fishlegs' Dragon Cards
- Dragon Hunter Weapons (mentioned)
- Dragon Hunter War (mentioned)