The episode starts with Hiccup running out of ink while trying to draw Toothless and Trader Johann's ship arriving in Berk. The villagers eagerly board the ship and trade, among them Fishlegs, Gobber, and Stoick, who picks out a sword as a gift and leaves on his Thunderdrum, of which he finally picked out the name Thornado, to meet with the Chief of the Shivering Shores. Even Mildew recieves a mysterious item in exchange for some cabbage. Toothless sniffs it and he sneezes at the Berk Dragon Academy, the riders have target practice with their Dragons, every dragon takes a shot of shooting the statue but when toothless tries his turn he sneezes and misses his sneezing escalates bursting runaway plasma blasts
. While flying with Astrid and Stormfly, she points out a bush of flowers she finds beautiful, but Toothless becomes disoriented in flight and crashes, with a Terrible Terror sharing the same symptoms. Gobber tries to cure Toothless when Astrid points out the other Dragons are becoming sick as well. Hiccup, Astrid, Fishlegs, and Gobber go to Gothi, who concludes after giving Toothless a thorough examination, the Dragons are allergic to something new that just came to the island and advises to get rid of everything, or the Dragons' illness will worsen.Outside the village, the villagers throw everything new in a pit to be buried, with Gobber and Fishlegs being more reluctant about giving up their respective silk underwear and botany book. Hiccup and the others try to comfort their Dragons, but the illness gets worse. Stoick returns on Thornado and sees what's happening. That night while pondering with Hiccup, Thornado falls ill as well. Hiccup investigates where Stoick landed with the others while Ruffnut and Tuffnut have a headbutting contest. Hiccup notices a Terrible Terror sniffing a bush of flowers near the twins - flowers that weren't there before. Recalling from his new botany book, Fishlegs identifies them as the Blue Oleander, a flower that is poisonous to reptiles, including Dragons. Hiccup orders Fishlegs to retrieve the buried botany book from the pit to find an antidote. Tuffnut reports he saw Mildew plant the flowers earlier. Hiccup, Astrid, and Stoick visit Mildew, who swears he wasn't trying to harm the Dragons, but the look on his face when they leave says otherwise as does the fact that he had planted Oleander flowers. Fishlegs returns with the book and says no antidote is in it, but the Book of Dragons mentions a Dragon that eats the the Blue Oleander: the Scauldron. Fishlegs points out the Book of Dragons says the Scauldron has no venom (it being an antidote since it feeds on the flower). Gobber appears and says the book is wrong and he faced a Scauldron before, stating they're 60 feet long, shoot boiling water, and have lots of venom. Hiccup recruits Stoick and Gobber to help find a Scauldron.
Hiccup and Stoick return to their house to comfort their respective Dragons, then set off on their expedition. The scene shifts to a Viking longboat with Hiccup, Stoick, Gobber, two other Vikings, one of whom is named Sven, and Mildew onboard, with the very latter complaining he did his part by digging up the flowers. Stoick sternly tells him they've been forced in to this situation because of him: if they have to put their lives on the line, so does he. Hiccup dumps some of the flowers into the ocean in order to lure a Scauldron to the boat. A Scauldron appears and Hiccup scatters more flowers on the deck to lure the Dragon onto the ship. After a few suspenseful moments, the Scauldron attacks the ship, first snatching Sven's oar away, and ramming the hull from underneath. It crawls on deck to eat the remaining flowers. Stoick and Gobber secure the Dragon's head by Stoick lassoing the beast's horn and Gobber throwing an anchor-line around the Dragon's neck, the two large Vikings using brute strength to pull the head back out of the ocean and onto the ship. Stoick gives his son a tense reminder about the venom which Hiccup finishes: get it inside you and you'll be dead within in 24 hours. Hiccup holds a bucket that has a cloth lashed tight over the top so he can milk the venom from it's fangs, and Sven wedges the Scauldron's jaws open with a thrown barrel, only for it to be crushed. The Scauldron knocks Sven off the ship with its powerful tail and is about to do the same to Hiccup. Stoick releases his line to push Hiccup out of the way and catching the tail as it slams into him, is battered about by it, eventually pinning it to top of the ship's mast with his body. Hiccup grabs Stoick's line and throwing it over a cleat to give him the extra power he needs, he and Gobber pull the head in and on Stoick's instructions, use the mast to wedge the Dragon's jaw open instead. Stoick orders Mildew, who hid behind the mast and is now in between the Dragon's jaws, to grab the bucket; he protests but Stoick yells for him to do it or he'll personally throw Mildew overboard. Mildew grabs the bucket, then hesitates out of fear. The Scauldron unleashes a boiling stream of water that breaks the mast and frees it's head (Stoick ordered everyone to take cover). The Dragon chases after Mildew for a flower stuck to his rear and eats it, biting him in the process, and disappears back into the ocean. Knowing the unpleasant experience it had will keep it from approaching the ship again no matter what they do, Hiccup, Stoick, and Gobber are disappointed they didn't get the venom, then hear Mildew screaming in pain, realizing they got the venom after all.
Back on Berk, Gobber painfully extracts the venom from Mildew's butt and gives it to Hiccup to administer to the Dragons, claiming 'never [had he] seen anything so disturbing as that old man's bare behind' he goes into the forest to scream in horror. Later on, the Dragons come around their illness, with Toothless shooting a fireball and Thornado unleashing a sonic roar to show their good health. The episode ends with Hiccup drawing Toothless and Hiccup's friends riding their Dragons with a Terrible Terror chasing after them as Hiccup does the closing narration. The episode ends with toothless having pepper into his nose and he sneezes
"A deadly flower, a venomous sea Dragon, and Mildew's butt: three things I never really thought I'd have to deal with in the same day. But I'll do anything for my Dragon."
- Jay Baruchel as Hiccup Horrendous Haddock III
- America Ferrera as Astrid Hofferson
- Julie Marcus and T.J. Miller as Ruffnut and Tuffnut Thorston
- Brook Chalmers as Sven
- Tim Conway as Mulch
- Chris Edgerly as Gobber the Belch
- Michael Goldstrom as Trader Johann
- Nolan North as Stoick the Vast
- Zack Pearlman as Snotlout Jorgenson
- Thomas F. Wilson as Bucket