This graphic novel takes place mere minutes after the end of How to Train Your Dragon 2.
"Picking up just after the events in How to Train Your Dragon 2, Hiccup, Astrid, and company are called upon to assist the people of an earthquake-plagued island. But their lives are imperiled by a madman and an incredible new dragon who even Toothless—the alpha dragon—may not be able to control!"
Starting right after the events of How to Train Your Dragon 2, the recovery of Berk has started. Whilst Hiccup is introducing Berk to the reader, Snotlout is just barely clinging onto a runaway armored Thunderclaw. Hiccup eventually swoops in, with Toothless, and fixes the situation immediately. Gobber eventually catches up, to find the armored Thunderclaw. Gobber was getting rid of the dragon armor form Drago's dragon army. However, when he attempted to remove the armor from this specific armored Thunderclaw, it refused to have it be taken off. Snotlout tried to take it off as well, believing that he could tear the armor off any dragon, inciting the Thunderclaw on its brief rampage. It turns out this particular Thunderclaw knocked Drago's prosthetic arm off, and it gets to keep his armor. Whilst this is happening Eret has been knocked down by villagers calling him an outsider, saying they can't trust an outsider when another had destroyed their home. This eventually reaches a revolution, with Hiccup having to draw his Inferno sword out.
Up in the Great Hall, Hiccup, the new Chief of Berk, is finding it difficult getting the people's approval of him. He still feels that he needs to gain their trust as Chief, but before he can dwell on that idea with his mother, Fishlegs comes in with news of a visitor.
People from the island of Nepenthe had come to Berk in search of help from Stoick. Calder had been sent to Berk by King Mikkel the Munificent. However, the Twins are suspicious of him because of his jib. Shortly after a little explaining, Hiccup and Calder are discussing Nepenthe situation. On their Island there has been constant earthquakes growing louder and more noticeable every time. Hiccup agrees to come and help, although Berk is having their own problems. He appoints Astrid as acting Chief, in which the role is quickly given to Gobber as Astrid wants to travel with Hiccup to stop him from getting into any kind of trouble.
Soon afterwards, the riders (including Eret and Valka) head off to Nepenthe. They are followed by Tidal Class dragons such as Thunderdrums, Scauldrons, Seashockers and Submarippers who offer Toothless fish as a tribute to their new alpha. Ruffnut and Tuffnut even travel on Seashockers' backs.
Returning to the dragon riders, Hiccup sees through his spyglass a huge Unknown Dragon but before they could learn more about it they hear a tremor from Nepenthe and decide to leave the dragon for later.
When they finally arrive on the island, Astrid points out that the pools all over the island could be Screaming Death pools, even though they are filled with water.
The riders land just when another tremor happens. After that, Hiccup asks to be taken to their Great Hall so he could speak with their king. Much to his surprise, Mik, son of Mikkel prefers to stay in a more "natural" place from where to rule and welcomes Hiccup in an unusual way (for a king).
Hiccup then asks Mik about the earthquakes. However, Mik says that he's not afraid of them and that Hiccup and the riders would eventually resolve the problem. He also invites them to relax in the hot pools.
Meanwhile, Gobber is having problems with waking up Grump, he then asks Bonesnarl to do his thing. Bonesnarl roars at Grump, waking him up and making him light the forge. Gobber states that he starts to like Bonesnarl.
On Nepenthe, the Twins, Snotlout and Fishlegs are enjoying a bath in a pool, while Toothless, Hookfang, Meatlug, Barf and Belch are enjoying the hot water in another pool. However, Stormfly, Skullcrusher and Cloudjumper don't seem attracted to them. Astrid says that she found some lines through earth but they are too small for a Screaming Death. Valka also notices that there are no dragons beside the ones from Berk.
Later that day, the riders decide to go back to Berk since they didn't find anything. Mik invites them to a last meal before they return and most of the dragons and riders accept. Hiccup asks Mik if he hides anything but before he could answer he passes out and Calder appears behind him. Soon, all the dragons and riders who ate pass out as well. Eret thinks that the food was poisoned but Calder explains that there is something in the pools that has a calming effect on both humans and dragons. Hiccup warns him that three dragons are enough to kill him. Calder says that there is no need for that as he will become a dragon himself. He also invites them to submit to his glory so they could stay alive.
That night, the riders are locked up in a house. Calder pretends to transform into a dragon by putting a costume on and having Inferno in his hand. Hiccup tries to convince Calder that it is impossible that he would ever transform in a dragon, distracting him while Eret frees one of his arms. He punches Calder while Hiccup frees himself with his metal leg. Hiccup escapes during the riders' diversion and he brings the other dragons and frees his friends.
While the riders make their escape, 6 Foreverwings wake up from under the island. They began destroying the village with their lava. Fortunately, Astrid, Fishlegs, Ruffnut and Tuffnut save the people while Valka, Snotlout and Eret make sure that no one hurts the dragons. Hiccup and Toothless take care of the dragons and it isn't until Toothless changes to his alpha mode that the Foreverwings bow to him and then fly or swim away.
Suddenly, Calder shows up thinking that the Foreverwings were a sign and that his evolution will happen soon. Hiccup explains that the Foreverwings were waking up due to the rise of the alpha and had nothing to do with Calder. Furious at Hiccup's statement, Calder attacks him with Inferno. Hiccup throws a canister of Zippleback gas he had attatched to his leg. The canaster explodes on Calder, causing him to fell off a cliff and die. The riders come to see what happened and Hiccup says that he is "better off than The Serpent's Heir".
The next day, Hiccup explains to the reader what he had learned from the experience. He gives Mik a shovel so he could start the reparations of the island. The riders arrive on Berk where they are welcomed by Gobber, who shows them the organised pack of Scuttleclaws. In his house, Hiccup sees the empty space where Stoick's Axe should have been and remembers his father. He gives a speech outside the Great Hall telling that he wants to change Berk in a better, stronger place. During his closing narration, Astrid is seen organizing, Eret is seen making new friends and Hiccup is seen coming up with the plan for Stoick's memorial.
Hiccup is later seen in the Great Hall, thinking about something. Toothless sneeks up and jumps on him. Valka also comes and states that Hiccup is tired and that he should get some sleep. However, Hiccup says that he first has to rule.
- Hiccup Horrendous Haddock III
- Astrid Hofferson
- Gobber the Belch
- Fishlegs Ingerman
- Snotlout Jorgenson
- Tuffnut Thorston
- Ruffnut Thorston
- Eret, Son of Eret
- Drago Bludvist (mentioned only)
- Stoick the Vast (mentioned only)
- Barf and Belch
- Soaky and Wetty
- Scuttleclaw Hatchlings
- Night Fury
- Deadly Nadder
- Monstrous Nightmare
- Hideous Zippleback
- Unknown Tidal Class Dragon
- Whispering Death (mentioned)
- Screaming Death (mentioned)
- Snaptrapper (mentioned)
- The tribe begins construction of Stoick's monument which ties to the end of How to Train Your Dragon 2.
- This is the first appearence of the Foreverwing outside of video games.