Most of Berk's inhabitants have been infected of an outbreak called 'Eel Pox ' the common symptons are high fever, sneezing, cough, and ill behaviour such as being cold. Most of its infected villagers are quarantined in the Great Hall to avoid anymore infection, as the epidemic grows, Hiccup is called by Fishlegs to go to the academy and meet with Gothi to find the ingredients for a cure. As they arrive, the Riders are ready to go as Tuffnut does not want to be sick and Ruffnut states that if he gets sick so will she, Tuffnut asks if that's true and Ruff responds that it's true because they're twins, stating that whatever happens to him also happens to her. Tuff, foolishly believing this, begins to beat himself, and Ruff pretends to feel the slaps and punches as Tuff repeatedly harms himself.
Hiccup, about to go, hears Stoick who is badly sick and keeps sneezing continuosly. Astrid finally arrives with Gothi who has the very long list of ingredients and the gang prepares to go, but Stoick wants to come with his son, but Hiccup wants his father to stay to look after the village and doesn't want 'a sick dragon rider' to come along with them. Stoick agrees and tells Hiccup to be safe. They rally at a beach and split up to find the ingredients:
- Snotlout goes for the first ingredient: Buckthorn Root. He is slightly roasted after Hookfang burns it in front of his rider.
- Fishlegs and Meatlug go for the second ingredient: A dozen Rock Blossoms. While in the process, Meatlug enjoys eating rocks where the blossoms are found.
- Astrid and Stormfly go for the third ingredient: One handful of Goat Weed. Easily cutting a handful of it, by Stormfly's spine shot.
- The twins go for the fourth ingredient: One wild Dagga Plant. Tuff struggles to climb and reach the top of the cliff, where the plant is located, because Ruff is already at the top and throws rocks at him.
Hiccup thinks that every ingredient is found, but Gothi gestures that's not everything. She says that the last item is not on the island they're at, the last key ingredient is actually a Bloodbane Eel. Fishlegs ask where to find it, she points her staff towards the sea, then Tuff interrupts that they're not going to be able to search the whole ocean. Gothi slams her staff at him and Fishlegs says to not interrupt her, and says that they have to go North to Eel Island. Astrid reminds everyone that their Dragons are terrified of eels, so Hiccup says that he and Toothless will go alone, much to Toothless' dismay, Hiccup tries to reason with Toothless that he's the fastest dragon. The two quickly head out to Eel Island and fails to hear Astrid saying that he has the other ingredients. Fishlegs' assures that they'll be back in no time.
Meanwhile, Hiccup and Toothless locate the island and move deeper into it. The island is shown to be heavily infested with eels that sure lives up to the island's name. Toothless's gets nervous and wants to hurriedly leave and Hiccup quickly tries to calm him down, he unleashes a rope snare from his shield and tries to hook an eel. He grabs one and accidentally pulls it to Toothless' face, blinding him and making him go out of control, throwing Hiccup off his back. Hiccup falls into the river where several eels close in on him. One strangles Hiccup's neck and he begins to drown while struggling to remove the eel. Toothless fires a plasma blast to the river, the eels flee but the other one remains behind and tries to bite Hiccup. Toothless comes in and accidentally ate part of the eel. The two resurface, Toothless swallows the eel and quickly begins to feel wayward, Hiccup is concerned that he has never seen a dragon come close to an eel and eats it. Toothless slowly begins to feel paranoid, and his mouth is bursting with red lights. His vision begins to become very loose and weird as he sees Hiccup's face forming in weird shapes. Toothless flees without reason and Hiccup follows him.
Back at Berk, the riders have returned to the Great Hall. Stoick asks do they have the ingredients, and Fishlegs answers that Hiccup has them. Stoick asks where is he, they don't know and begins to worry, as the infection becomes worse, the symptons have worsen by making them confused and makes them change their personality like Gobber who claims that he is Gobber's Great Auntie Roseworns and wants to give him a kiss, They are disgusted by the personality and sees what Stoick was pointing at, the confused Gobber annoys and harass Stoick and angrily shouts to stop. Meanwhile, Toothless runs deep into the forest unable to see straight, such as being hit by a tree and tripping over a log, the eel worsens Toothless' senses such as his hearing, even small sounds such as the sound a fly makes, irritates him and makes him more paranoid. He is surrounded with noises and gets heavily delusional. Suddenly he coughs a burst of somewhat red fireballs that makes a critical impact when fired. Hiccup is able to locate him, Toothless tries to resist his cough and fires the fireball as far as possible, and a huge explosion erupts. Hiccup is shockingly amazed at the dangerous fireball and figured out why dragons go easy on the eels. Toothless' continuously coughs and shoots red fireballs all around the area without warning. Hiccup backs up and takes cover and waits for Toothless' 6 shot limit, when it finally hits 6. He closes on Toothless, only having to be discovered that the eel has increased its shot limit and is able to fire multiple shots, Hiccup tries to dodge the attacks.
Back on Berk, its been hours and Hiccup hasn't returned yet, the Eel Pox infects Tuffnut, Ruffnut, and Snotlout. Astrid, Gothi, and Fishlegs remain immune for now, Astrid assigns Fishlegs to search Hiccup to Eel Island and reluctantly agrees, while Tuffnut becomes delusional and jokes himself that he saw for the first time a person on the back of a dragon, after a quick laugh he passes out and asks questions who is he and who are the people near him.
Back on the island, Hiccup is in trouble as Toothless' cough and sneezes worsens, initially making him unable to resist all of it, and devastates the entire area, one red fire ball explodes near Hiccup's good leg, another one shoots him and crashes into a tree. Toothless sees this and feels sorry for him and decides to flee far away from Hiccup so he can be safe. Hiccup regains consciousness for a moment and sees Toothless fleeing but loses consciousness again. A while later, Hiccup awakens and hears russling in the bushes and trees, Hiccup defends himself but turns out to be Fishlegs and Meatlug, the two are glad for a brief moment, Fishlegs ask what cause all this devastation Hiccup answers that it was Toothless. He also explains that Toothless ate an eel, and the eel is not agreeing with him. Hiccup grabs a net from Meatlug and builds a trap for Toothless, Fishlegs ask what if the trap doesn't hold him, Hiccup responds that Meatlug could always sit on him. Fishlegs says that Meatlug can hear him but she's actually asleep.
Back on Berk again, Snotlout has taken effect of the worst symptons of Eel pox making him delusional, by sermonizing dragons and a Terrible Terror which he throws away, and the Terrible Terror brutally attacks Snotlout. Hiccup and Fishlegs head out to find Toothless. On the way, they see a Typhoomerang burn mark, discovering that Typhoomerangs inhabits the island which makes the place a perfect habitat due to the fact that they eat eels. On a brief moment on Berk, Snotlout continues to go talking crazy with the dragons, but the Terrible Terror attacks Snotlout again making him fall down the stairs of the Great Hall. Meanwhile, Hiccup and Fishlegs discuss Toothless' behavior and figures out that Toothless is experiencing a different kind of Eel pox, and wonder if the cure for it is also the ingredients Toothless is carrying, if they give them the medicine maybe it will counteract the eel making Toothless normal again. Fishlegs briefly disagrees but he doesn't have any other ideas and goes with Hiccup's discovery. They find Toothless who continues to become wayward, they try to figure out a plan to get to Toothless carefully, but they are spotted by a Typhoomerang. Toothless attacks the dragon but is unable to shoot straight, and Toothless runs away while being in pursuit by the Typhoomerang. Toothless hits a dead end, and is in trouble. Meatlug attacks the Typhoomerang giving Toothless a chance to get away. Hiccup tells Fishlegs to keep the dragon busy while he aims for Toothless, he jumps off Meatlug and perfectly falls and ride on Toothless. Toothless shakes violently to get Hiccup off but Hiccup remains stubborn and tries to put his leg on Toothless, Toothless acts as a wild bull and desperately tries to get Hiccup off. Without reason, Toothless runs straight towards the end of the cliff Hiccup tries to put his leg on Toothless or both of them will drown to death. They almost fall but makes it, Hiccup is able to control Toothless's tail fin making him able to fly, but Toothless again remains stubborn and tries to shake off Hiccup. Meanwhile Fishlegs is having trouble to make the Typhoomerang out of the way tries to call help for Thor that his desperate to even take the twins' help.
Back on Berk, The twins, having taken heavier effect of the Eel Pox, have become bright and intelligent by doing science experiments by studying air and its resistence to motion. Ruff proposes to increase their sample size, Tuff agrees, their next sample is by dropping a large yak alongside with a normal size dragon rider, which Tuff agrees. Ruff goes to find a suitable yak, while in a sick feeling.
Back on the Island, Toothless flies and shakes violently hoping to get Hiccup off his back, Hiccup struggling to control him. Meanwhile Fishlegs sees that the Typhoomerang is all out of shots, but the Typhoomerang dives down and grabs several eels with its mouth and eats them, proving that eels only increase their firepower. Fishlegs and Meatlug are in deep trouble and almost lose control, Fishlegs and Meatlug have had enough and uses their new ability called the Gale Force Gronckle that involves Meatlug spinning rapidly and wacking the Typhoomerang with its tail, the Typhoomerang is badly damaged and flees, while Fishlegs is glad but disoriented for a while after the attack, they head for Hiccup. Hiccup still struggling finally sees the spot where the trap is located. Hiccup, with no other choice, pulls Toothless to aim for the spot the two fall violently, Toothless closes on Hiccup in anger. Toothless gets caught in the trap and tries to gets free while Hiccup apologizes, Toothless fires a red blast and burns the net unable to hold him much longer, Toothless gets free and is about to attack Hiccup, but luckily Fishlegs and Meatlug pummels him down by sitting on top of him. Toothless is finally under control, but Fishlegs tells Hiccup to be careful for he might shoot, Hiccup is sure that he won't shoot and tells Toothless to calm down that he is just sick. Fishlegs mixes and prepares the cure for Toothless, he mixes it in a bowl and gives it to Hiccup. Hiccup offers it to Toothless, he smells it and doesn't like it and is about to fire another red fireball, Hiccup quickly pours it into his mouth and Toothless swallows it. Hiccup and Fishlegs aren't sure if it worked, suddenly Toothless breaks free and faces Hiccup, thinking that his about to shoot, he suddenly acts weird and spits the eel he ate on Hiccup. Toothless feels all better now and recognizes his rider, the two are glad to finally bond with each other. But Fishlegs interrupts that they should celebrate back on the island due to the fact that the island is inhabited with Typhoomerangs. The two head home to Berk.
On Berk, Astrid still waits for Hiccup and Fishlegs return, but she is getting the ill feeling of the Eel Pox. Hiccup and Fishlegs return and Astrid is glad to see them safe and sound. Later at night, the cure is rassioned and given to the infected villagers, Gobber, who's not crazy anymore, serves it to the villagers. Snotlout, already having the cure, is still acting delusional. Gobber suggest giving Snotlout a double dose of that cure, the twins already having the cure, still acts as intelligent people and having invented ice cream but Ruff calls it 'Ruff cream' but Tuff argues that it was suppose to be named 'Tuff cream'. Gobber suggests to give them a triple dose of the cure, Gobber is disgusted of how they freeze perfectly good milk.
Hours later in the Great Hall, Fishlegs and Hiccup are doing some updates in the Book of Dragons by putting the symptons of a dragon eating an eel and its reason why dragons don't eat eels with the exception of Typhoomerangs. The two happily agree that they're never going back to that island. While heading out, Hiccup greatly thanks Fishlegs help and support thinking that they would never finish that problem. Fishlegs feeling happy, offers Hiccup a group hug, Hiccup runs but gets pinned by Fishlegs' and Meatlug's weight, and the three happily bond with each other, while Hiccup is getting his bones crushed.
- Jay Baruchel as Hiccup
- America Ferrera as Astrid Hofferson
- Christopher Mintz-Plasse as Fishlegs Ingerman
- T.J. Miller as Tuffnut
- Chris Edgerly as Gobber the Belch
- Nolan North as Stoick the Vast
- Zack Pearlman as Snotlout Jorgenson
- Andree Vermeulen as Ruffnut
- Toothless - Night Fury (Hiccup's Dragon)
- Stormfly - Deadly Nadder (Astrid's Dragon)
- Meatlug- Gronckle (Fishlegs's Dragon)
- Hookfang - Monstrous Nightmare (Snotlout's Dragon)
- Barf and Belch - Hideous Zippleback (Ruffnut and Tuffnut's Dragon)
- It's shown that when eels are eaten by most dragons, it can cause sickness, delusions, and enchance their fire attacks and the number of shots they have which is why they fear them.
- Typhoomerangs regain their fire when they eat eels.
- In Frozen, Toothless' shot limit was counted to be about 14-15 shots, but in this episode, his shot limit is only 6.
- Toothless' shot limit is 6 but it was shown that since eels increase shot limits, Toothless' shots are shown to become bigger and more powerful.
- The twins became quite intelligent when they were sick. They have re-thought the science of gravity and invented ice cream.
- Like Gobber, Fishlegs can read Gothi's writing.
- The Terrible Terror that kept attacking Snotlout wasn't the same one he trained in Worst in Show, but does have a similar personality.
- Despite the fact that Eel Pox normally causes people to go crazy due to the fever and delusions induced, Stoick managed to stay sane despite catching the Eel Pox while Gobber, Snotlout, and the twins quickly succumbed to the illness and became feverish and frenzied.
- This may show that Stoick is stronger and has a better immune system that allows him to fight off the Eel Pox to a better degree than the other citizens of Berk, especially since Snotlout and the twins contracted the illness much later than Stoick and quickly succumbed to the delusions that the illness induces while Stoick didn't.
- Fishlegs has taught Meatlug a new skill that's very strong to fight off a fully grown Typhoomerang, called the Gale Force Gronckle. However, the likeliness of whether they will use it again is questionable, as Fishlegs very soon became very dizzy from the spinning.
- It's shown that Hiccup, Fishlegs, the dragons (with the exception of Toothless after he ate a Bloodbane Eel), and possibly Gothi weren't sick and didn't contract Eel Pox.
- Ruffnut's proven to be slightly smarter than Tuffnut, because she got him to hurt himself by saying it would hurt her too since they're twins.
- Eels do not make noise, nor are they usually very aggressive unless provoked.