- Stoick releases Thornado so he can care for three baby Thunderdrums; Bing, Bam, and Boom.
- We see Hiccup's Viking helmet for the last time.
Hiccup finishes his latest contraption, The Thunder Ear notifies the Vikings when the Screaming Death is coming. Hiccup demonstrates by letting his father be the first to test it, Toothless signals their Thunder Ear subject, Fishlegs and Meatlug. They began singing a song where they are located several miles away from Berk, and still can be heard thanks to the Thunder Ear. Stoick also suggests that they can also use it to hear enemy ships near Berk's territory. Suddenly Stoick rotates the Thunder Ear and hears Gobber bragging to a Viking, that Stoick will be lost without him and he even calls him his 'right-hook man'. Stoick again rotates it to its former spot, suddenly Stoick and Hiccup both hear a weird and deafening somewhat giggling sound. They both decide to investigate the noise.
As they head to where it was heard, They discover three baby Thunderdrums on the sea stack, as soon as they get close to it the baby Thunderdrums soon start to harass them, and Thornado furiously quiets them down and finally behaves. Hiccup recommends that they bring them to the care of Berk, but Stoick knowingly declines as the baby Thunderdrums can make quite a racket. Stoick also reassures Hiccup that they'll be fine by bragging Thunderdrums are the toughest species in the entire Archipelago, much to Toothless' dismay. They head back to Berk and Hiccup gets one last look at the baby Thunderdrums.
They arrive later at night, Stoick orders Gobber to be on night watch patrol with the Thunder Ear, but Gobber tells Stoick that he has to play his panpipes to the village later, but Stoick denies his excuse as he mentions, Gobber's 'right-hook man' will be doing his chief duties the 'one who will be nothing without Gobber'. Gobber is surprised to hear those words, luckily Hiccup tells him that Stoick heard his conversation with the Thunder Ear.
The next morning, a loud and violent noise shakes and rattles Hiccup and Stoick's house, they both hastily investigate it by going outside and is surprised to discover that the baby Thunderdrums is seen rampaging through Berk such as making inappropriate and loud noises all over the village. The village is getting leveled, as the baby Thunderdrums keep bumping any obstacles in their way and destroy anything that looks interesting, including Hiccup's contraption the Thunder Ear. The Riders fails to keep the baby Thunderdrums under control, as they prove to be more slick and slippery. With no one able to keep them in control, the baby Thunderdrums continue to ravage across the town and plaza, but finally, Thornado keeps them in discipline and the baby Thunderdrums finally behave. As soon as some of the Riders arrive, The Twins are interested in keeping one, but Stoick orders Hiccup to put the Thunderdrums off the island (who the Twins briefly think the chief was referring to 'em due to the fact they are also troublemakers) but Hiccup suggest instead that they put them in training as a battle squadron for Dagur's armada, Stoick agrees with Hiccup's suggestion but warns them that the next time they cause trouble they'll be quickly sent off, Hiccup promises his father that he won't regret this decision. However, as soon as Stoick and Thornado disappear the baby Thunderdrums quickly start off to be destructive and loud again. They manage to get them to the Academy but have a hard time to think and concentrate clearly with all the noise the Thunderdrums are producing.
Hiccup reveals that he actually has no plan at all to control 'em and they'd better hurry before, as mentioned by Snotlout, their 'ears explode' but the Twins are so curious about that they'd actually want to test it by encouraging the Thunderdrums to scream as loud as they can for their ears to actually explode. Thornado is seen watching on and seems worried about the baby Thunderdrums, he leaves after hearing Stoick is calling him. Luckily, Fishlegs gives them dragon nip to behave temporarily, and the Twins have already named the Thunderdrums proving that they did something useful for the Academy, they name the first two Thunderdrums 'Bing' and 'Bam', but the third name for the third dragon doesn't make sense as they name him 'Lloyd' an odd name for the dragon. Fishlegs suggests that they name the third one 'Boom' to be more legit and logical to the other two despite the Twins still thinking it should be Lloyd. Hiccup moves on to the next phase of actual training, first, he has to earn their trust which quickly goes well for a brief moment, until one of them blast Hiccup with a sonic burst, luckily Toothless catches him but Fishlegs gets pinned down after getting hit by Hiccup. Next, Astrid claims that the Thunderdrums don't understand the word 'stay' and they continue to harass Stormfly, as they land on her tail, Stormfly whips her tail and sends the Thunderdrums away, but continues to harass Stormfly and she runs for her life.
Next up, Snotlout trains them on how to shoot in great accuracy, but the Thunderdrums does not understand him of hitting the targets on the barrel, so Snotlout tells them to destroy the barrels but as soon as he tells them to do it, Snotlout is in front of the Thunderdrums and gets blasted altogether by the Thunderdrums. All of the Riders' training to the Thunderdrum fails even the Twins' training to them about how to burp, as they get blasted towards Snotlout who just finishes assembling all of the barrels. Hiccup and Toothless get more irritated as the Thunderdrums keeps messing around, but suddenly they all behave and Hiccup sees Thornado has shown up to help. Incredibly, the baby dragons calm down and listen to the older Thunderdrum. The Riders are pleased, except the Twins.
Stoick arrives at the academy to get Thornado, and sees the improvement in the dragons. But when the two leave, Bing, Bam, and Boom immediately take off, running wild around the village. Thornado again calms them down, but Stoick decides it's too dangerous to have the triplets on Berk.
The Dragon Riders take the trio to Dragon Island, but they keep following them back to Berk. Stoick finally joins them, and Thornado's command keeps the baby Thunderdrums from taking off. But just when Hiccup, Fishlegs, and Stoick leave, they notice wild dragons attacking babies. The Hiccup and Fishlegs realize the reason the triplets were out at the sea stacks was most likely because they were chased there.
The Riders then fight off the wild dragons, finishing them off with a combined Sonic Blast from Thornado and the triplets. The victory is short-lived, as another sweep of dragons begins to show up. Hiccup and Fishlegs are concerned about what to do. They cannot take the Thunderdums back to Berk, but they cannot just leave them alone. Then Stoick, seeing how Bing, Bam, and Boom look up to his dragon, decides to set Thornado free so he can take care of them. While the choice is hard for him, he says it's what any chief, or any father, would do.
Hiccup, Fishlegs, and Stoick take off for Berk and Stoick waves goodbye to Thornado.
- This is the first and only Defenders of Berk episode that focused on Thornado.
- Stoick calling the baby Thunderdrums cute and cuddly in the beginning might be a shoutout to another DreamWorks TV show, Penguins of Madagascar. One of the most popular lines in that show is, "Cute and cuddly boys. Cute and cuddly."
- Gobber calls himself Stoick's 'right-hook man', even though he has a hook on his left hand.
- Gobber's pan pipes return.
- He says he is going to finish his 'One Viking Spectacular' with a performance of the "Reindeer Waltz" on it.
- Ruffnut and Tuffnut had a pet yak named Sam, which they unknowingly ate and wore the hide as jackets.
- Fishlegs comes up with a new verse for the "I'm a Viking" song-
- Well, I've got my club, and I've got my bludgeon
- And 16 Berserkers locked in my dungeon
- I'm a Viking through and through!
- 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
- Brook Chalmers as Silent Sven ♣
- ♣ designates a character that appears but does not speak
- Barf and Belch
- Screaming Death (mentioned)
- Night Fury
- Deadly Nadder
- Monstrous Nightmare
- Hideous Zippleback
- Terrible Terror
- Screaming Death (mentioned)
- Astrid's kransen
- Hiccup's prosthetic leg
- Hiccup's Viking helmet
- Thunder Ear