The Riders praticipate in a daily training exercise on how to handle an ambush. Gobber, Bucket, and Mulch play along as the enemy by firing multiple catapults at them. After a while, Hiccup orders the Riders to fall back but Snotlout disobeys and decides to really finish off the enemy. Snotlout lets Hookfang fire a fireball at a catapult. Gobber and the others dodge the attack but the catapult is activated and throws a stowaway boulder. Snotlout relaxes and doesn't pay attention of what great damage he unleashed. The boulder hits Astrid causing her to fall off Stormfly. Astrid gets hit by a few branches and almost hits the ground, but Hiccup and Toothless save her. Astrid regains consciousness but breathes heavily. Snotlout arrives still cheering for what he did earlier on and the Riders all look at him angrily. Hiccup tells Snotlout he almost killed Astrid but Snotlout tells him she's fine. Astrid goes angry at Snotlout literally wanting to kill him. Hiccup is sickened by Snotlout always following his own ways and disobeying others, and in punishment he temporarily suspends Snotlout from the Academy until further notice. Snotlout responds by running off, and the Twins' misunderstood the word 'grounding'. Astrid tells Hiccup he didn't have to do that, but Hiccup proves a point to Astrid by asking how can they count on Snotlout the next time they encounter Dagur.
Meanwhile, on Outcast Island, Dagur talks crazily to the sketched drawing of the Night Fury. The Berserkers and Outcasts look on as if they're scared of him. Savage informs Dagur the testing of the Dragon Root resulted perfectly as Dagur thought it would. Keep in mind Dragon Root makes dragons go wild and crazy and want to tear each other apart. Savage suggests they plant the Dragon Root on Berk but points out they have to wait for the right time, so they'll keep on eye at Berk first. Dagur agrees with Savage's plan.
At the Academy, the Twins plan on renovating their pen with Snotlout's after they misheard Hiccup's suspension of Snotlout as kicking him out of the Academy. Hiccup tells them he didn't kick Snotlout out but the Twins make a point they have to merge their pens, or Hiccup has only one remaining choice: total expulsion or he runs the risk of loosing the other Riders' respect. Hiccup is left speechless, and tells them no one is merging their pens and volunteers to talk with Snotlout when there's a possibility Snotlout will come back.
Meanwhile, Snotlout and Hookfang find a small island that's infested with only large rocks and no vegetation. Snotlout is satisfied and names it Snotland. As he claims every growing thing in the island and Hookfang marks it by burning it, he spots a mysterious wrecked Outcast Ship on shore and go's to checks it out.
At Berk, Stoick had a tough day of leading and Hiccup also had one too with Snotlout, and he informs his father Snotlout disobeys Hiccup's every order and asks him if he should expel Snotlout for good. Stoick asks if that's what he really wants. Hiccup says no but he doesn't want to lose the respect of the other Riders and asks how can he be someone's leader and still be their friend. Stoick responds he can't always be like that, and tells Hiccup someday he'll become Berk's future chief and when that day comes he has to make choices not for his friends or himself but for Berk.
On 'Snotland', Snotlout jumps in the wrecked ship and is about to attack, but discovers no one is in it. An earthquake occurs and it's revealed the island is inhabited also by the Screaming Death. Hookfang quickly grabs Snotlout and flies off but, as he tries to shake off the Screaming Death, he loses his grip on Snotlout and falls. Snotlout is knocked him unconscious. The Screaming Death spots Snotlout and is about to finish him off, but a stranger rescues him in time. Hookfang thinks he died, but spots the person who rescues him. Hookfang safely heads back to Berk along with the mysterious person who rescued him. The villagers all think Snotlout is dead, and they are all excited for a viking funeral to be held, but get their plans spoiled when it's revealed Snotlout is actually alive. Stoick and Hiccup arrive to see what's all the commotion and are shocked to see Snotlout is with Alvin, whom everyone thought was dead.
At the Great Hall, Gobber presses charges on Aliv the Treacherous, who recalls his terrible crimes he caused on Berk, all of which had something to do with treachery. Despite saving Snotlout, which Alvin calls a valuable life, Stoick sends Alvin to be locked up until a proper sentence can be held for him and isn't interested with his pleas. Meanwhile, Astrid and Fishlegs inform Hiccup about what Snotlout told them, that the Screaming Death is closing in on Berk.
The Riders head to the Academy and find the Twins are busy decorating Snotlout's pen. Hiccup, Astrid and Fishlegs train hard to figure out a way to stop the Screaming Death. Snotlout barges in and asks if everybody missed him and is sure he's ready to be accepted back into the academy. Despite the fact he discovered the Screaming Death's location and claims to have captured Alvin unconsciously like a fox (which Tuffnut points out it doesn't even make sense to him) Hiccup only informs him he is still suspended after breaking the rules again and almost getting killed in the process. In punishment he locks away his dragon in his pen (along with Tuffnut in it, accidentally), and Snotlout's protests fall's on deaf ears. Meanwhile, Dagur and Savage see the perfect advantage to put their plan into motion by using the dragon root.
Hours later, Hiccup arrives at the town's dungeon where villagers protest against Alvin. Hiccup goes inside to meet Alvin and wonders what he wants to talk about, Alvin tells Hiccup he also wants Dagur out of his island and claims he changed his ways or is at least trying to, and wants Hiccup to help him. Hiccup asks Alvin what's in it for Berk. He responds if the Berserkers are gone the Outcasts will be Berk's allies. Hiccup walks away to think about this decision.
At Stoick's house, while eating their meals Stoick asks Hiccup what he is thinking and Hiccup asks his father of what Alvin really did to Berk to get cast out. Stoick ignores Hiccup's questions of the origins of Alvin being cast out of Berk and only tells Hiccup Alvin is their main enemy and it will never change. Stoick walks away.
At the Academy, the dragons sleep soundly in their pens. The Berserkers open up the Academy's gates allowing Dagur to enter the arena. He orders Savage to bring in the Dragon Root into the center of the arena as the other Beserkers and Outcasts open up the dragon's pens. Dagur and Savage walk away from the arena and locks up the Academy's gates. Within seconds, Hookfang, Stormfly, and Barf and Belch gets caught in the Dragon Root's scent and set a rampage on each other.
Meanwhile, Snotlout is with Alvin claiming he could've gone away alive on the Screaming Death attack on the island. Alvin tells Snotlout they're both a lot alike, because nobody appreciates the things they do. He informs Snotlout him being locked up as return for the favor of saving his life is wrong and to make Snotlout quickly make the decision of opening up his cage, he informs him more he could've left him there and he'll be a free man again. All Alvin wants is to get his island back and if he lets him go he'll never be seen again. A warning signal is heard informing everyone Dagur has been spoted in the arena and the dragons are in trouble. Alvin informs Snotlout if he let's him go now that he can help him fight off Dagur.
Hiccup and Toothless spot the Dragon Root in the academy and Hiccup orders Fishlegs to get inside and take away the root. He ignores Snotlout trying to tell him something. As the Riders try to calm their dragons, Fishlegs grabs the root and is about to escape the arena when the door closes, locking their only way out. Hiccup and Toothless are cornered by Beserkers and Outcasts and Dagur orders him to give up his dragon or let his friends get slaughtered by their own dragons. Hiccup has to make a choice quickly as the Riders struggle to dodge their own dragons' attacks. Stoick, Gobber and other vikings ambush them demanding they open the gates. Stoick comes close to the lever that opens the exit but Savage is about to stop him. Alvin (who Snotlout freed) tells Stoick to duck and kicks Savage. Dagur is shocked to see Alvin alive. Stoick gives Alvin a weapon to fend off the Beserkers and Outcasts, as Stoick opens the exit. Fishlegs exits but along the way, he hits Stoick knocking him down. Savage informs Dagur their plan is failing and they must retreat, but Dagur relents and spots Stoick still knocked out and captures him.
Gobber spots Dagur and Savage and calls Hiccup as they see Stoick is being taken back to Outcast Island as Dagur's hostage, and warns Hiccup if anyone interferes he will kill Stoick, and they walk away. Hiccup is desperate to rescue his father by quickly planning an attack on Dagur before he escapes. Alvin warns Hiccup if he makes this move Dagur will continue his plan to kill his father and tells him to calm down and he only wants Hiccup's dragon as Alvin is good at one thing...Treachery. Hiccup trusts Alvin's opinion.
Meanwhile Stoick is now being sent to Outcast Island as their hostage, and Alvin is caged in the Riders' academy to keep in touch, and Hiccup promises he'll rescue his father as soon as possible no matter at what cost. However though, the Screaming Death closes in on Berk leaving the fate of the entire archipelago in a terrifying crosshair.
- Jay Baruchel as Hiccup
- America Ferrera as Astrid Hofferson
- Christopher Mintz-Plasse as Fishlegs Ingerman
- T.J. Miller as Tuffnut
- Brook Chalmers as Sven
- Chris Edgerly as Gobber the Belch
- David Faustino as Dagur the Deranged
- Mark Hamill as Alvin the Treacherous
- Nolan North as Stoick the Vast
- Zack Pearlman as Snotlout Jorgenson
- Paul Rugg as Savage
- 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)
- Screaming Death
- Stoick directly addresses for the first time the fact that Hiccup will become chief in the future.
- Alvin said he changed by saving Snotlout from the Screaming Death and Stoick from Savage.
- Dagur kidnaps Stoick in this episode as a hostage to trade for Toothless.
- This is the first time Alvin has ridden a dragon, though Hookfang most likely let him because he saved Snotlout.
- Alvin was banished from Berk 20 years ago, though the reason remains unknown until the next episode. It did involve several forms of treachery (hence his name).
- Stoick captured at the end is similar to the ninth book, when Stoick was captured by Excellinor.
- Though Alvin has appeared in the first half of two-part episodes before, this is the first time he had a major role.
- Alvin and Snotlout talking to each other might be loosely based on the books when Snotlout was working for Alvin.
- The Dragon Root that Dagur planted must have had a much stronger effect on the Dragons than it did in A Tale of Two Dragons, because they were willing to attack their respective riders.
- It's shown that not only the riders but the villagers also dislike Snotlout as they started celebrating when they thought he was dead and were thoroughly disappointed when he revealed himself to be alive.