When Hiccup finds a coffin at sea, he opens it to discover a riddle that will lead to the treasure of Grimbeard the Ghastly, the world's greatest pirate and Hiccup's ancestor.Setting out on a treasure hunt, Hiccup and his friends must master the art of swordfighting! How else will they escape an island of homicidal dragons, defeat a boatload of Viking pirates, and survive a dirty double cross?
The Hairy Hooligans go to the island mentioned in the riddle which is determined to be the dangerous Isle of the Skullions. Snotface Snotlout and his dragon Fireworm triumphantly sniff out a chest full of treasure, as well as Grimbeard's infamous sword, Stormblade. However, Grimbeard the Ghastly booby-trapped the chest with a smell that will wake up all of the resident 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. On the way back to Hooligan Village, the Hooligans fight over the treasure and who the successor to Stoick should be. As they fight, their ship is boarded by the Outcast Tribe and Alvin is revealed to be the Chief of the Outcasts. A battle ensues and the ship sinks. Everyone escapes 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 - Grimbeard's real treasure. 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.