A year after the previous book's events, Hiccup hasn't found the Dragon's Jewel and is wanted by the entire Barbaric Archipelago, led by Alvin and his mother. The witch sent Hiccup's mother Valhallarama to hunt down the Outcast, not informing her that the target was her own son. After confronting his mother and her dragon, Hiccup sets out to the Amber Slavelands to find the Jewel and Fishlegs.
Meanwhile, Furious sent out a Triple-Header Deadly Shadow to kill Hiccup. Hiccup infiltrates Darkheart Prison with a disguise but finds Alvin enslaved Vikings and sent them out to the Amber Slavelands to find the Jewel. Through his newfound friend Eggingarde, Hiccup discovers Fishlegs went missing while searching for the Jewel with his group, the Amber-Hunters (coincidentally led by Stoick the Vast). Hiccup also learns of the Monster of the Amber Slavelands, a dragon with eyes on its claws.
The next day, the Amber-Hunters set out to find the Jewel but fail like other attempts. That night, Hiccup is kidnapped by Camicazi and her group of escape artists, who rescued inmates from Prison Darkheart. Hiccup opts to stay and find Fishlegs, and orders Camicazi to take Eggingarde back to her mother. The next morning, Hiccup's helmet is found, forcing him to reveal himself to Alvin and the Witch. They force him to look for the Jewel in his Sand-yacht, the Hopeful Puffin 2, which Camicazi hid herself in. To their dismay, Hiccup is kidnapped by the invisible Deadly Shadow and is about to be killed by it.
The middle head of the Deadly Shadow (Patience) notices Fishlegs's lucky lobster necklace around Hiccup's neck. The Deadly Shadow decides to help Hiccup look for Fishlegs as it was his mother's dragon. As they land at the place Eggingarde described as the place where Fishlegs was taken, they find his lobster-pot-cum-rucksack and believe he is dead, Since Fishlegs is dead, the Deadly Shadow decides to kill Hiccup as they made a promise to Furious.
A dragon trap activates, trapping the Deadly Shadow. Hiccup frees the Shadow, but the Monster of Amber Slavelands takes him to its lair. Deducing the monster is related to the Burrowing Slitherfang, Hiccup plays dead in order to strike the Monster at its weak spot in the middle of its head. After killing the Monster, Hiccup discovers the Jewel in its lair, along with a half-dead Fishlegs.
After the Deadly Shadow rescues them, Valhallarama once again attacks Hiccup and the gang and takes both Hiccup and the Jewel to Alvin and the Witch. In the Prison, the Witch tries to convince Valhallarama to surrender the Jewel and Hiccup, but she remains silent. She explains her past Questing career and her Quest to find the King's Things and also gives a speech about Hiccup's cause, ending it by placing the Slavemark upon her head and renaming it the Dragonmark. A fight ensues between the Company of the Dragonmark (the slaves) and Alvin's army. The Dragon Rebellion joins them and chaos erupts. The Company of the Dragonmark retreats to the Bog-Burglar Islands as Hiccup tries to rescue Toothless and the Wodensfang from Alvin. He succeeds, only to have Alvin steal the Dragon Jewel from him. As Hiccup and the gang retreat to their secret hideout, Hiccup discussed his failures with the Wodensfang and about Furious' words he will bring dragons to their final oblivion. The Wodensfang comforts Hiccup and tells him it will be alright, but secretly wonders if Hiccup is right. He looks to the stars and decides he has to trust Hiccup and hope for the best.
- Hiccup Horrendous Haddock III
- Stoick the Vast
- Excellinor the Witch
- Alvin the Treacherous
- Snotface Snotlout
- Gobber the Belch
- Baggybum the Beerbelly
- Hiccup's Windwalker
- Deadly Shadow
- Monster of the Amber Slavelands
- Valhallarama says she never found one of the Lost Things. Unbeknownst to her the ruby heart's stone was actually originally in her possession; she had given half to Humongously Hotshot the Hero before she married Stoick and kept the other half for herself. It is unclear if her half of the stone would be counted as a valid Thing by the Druid-Guardian.