Hiccup and Fishlegs find a coffin out at sea during sword fighting lessons. When theHairy Hooligans open the coffin, they find it contains Alvin the Treacherous, who goes by the name Alvin the Poor but Honest Farmer. We soon realize later in the book Alvin is an outcast, and will become Hiccup's worst enemy. Alvin tells the Hairy Hooligans a riddle that shows where to find the treasure of Grimbeard the Ghastly, the world's greatest pirate and Hiccup's great-great grandfather.
The Hairy Hooligans go to the island mentioned in the riddle, the island of the Skullions, and find a small chest filled with treasure. However, Grimbeard the Ghastly booby-trapped the chest with a smell that will wake up all of the Skullions, huge ear-less, eyeless dragons with an unusually keen sense of smell. Hiccup is nearly killed by a Skullion, but is saved by his father, Stoick the Vast. Hooligans and Alvin the Treacherous run to the ship with the treasure and fight over the loot. As they fight, their ship is boarded by cannibalistic Vikings, and the ship sinks. Everyone jumps ship except for Hiccup, Fishlegs, Alvin the Treacherous and Toothless.
The ship sinks to the bottom of the sea, and they all escape into an underwater cavern filled with air. Toothless finds a door that opens into a cavern full of gold and jewels, and Alvin tries to kill Hiccup to keep him from taking any treasure. Fortunately, in the middle of the battle, Alvin steps on a pile of treasure that rears up and grabs him. Hiccup realizes there is enormous Monstrous Strangulator , an octopus-like dragon creature, in the cave. Alvin is quickly eaten, but Hiccup tricks the Monstrous Strangulator into poisoning itself. They escape the cave, and Hiccup decides the world isn't ready for the treasure. Hiccup and Fishlegs soon surface with Toothless's help, and they all return safely to the village. However, as said at the end of the book, is Alvin really dead?
- This is the only book which title isn't about dragons.