The film opens with Gobber's house on fire and the Vikings and resident dragons getting it under control. Gobber is convinced that his old nemesis The Boneknapper Dragon, a giant, yet perfectly silent dragon (the only one with no roar) that clothes itself in bones for armor, is responsible. However no one else believes that the Boneknapper even exists, much less that it burned his house, especially when the evidence suggests that the fire was started by his underwear drying at his hearth and catching fire.
After their marriage, the Beast and Belle unite all the famous fairy tale kingdoms into the United States of Auradon, and are elected King and Queen. Villains and their minions are imprisoned on the Isle of the Lost, a slum-like island surrounded by a barrier that prevents the use of magic (deceased villains were resurrected to succumb in this punishment). 20 years later, the Beast and Belle's son Prince Ben is soon to be crown king. As his first proclamation, Ben wishes to give the kids on the Isle of the Lost the chance to live in Auradon. Though skeptical, his parents accede to his idea. The kids chosen are: Carlos, son of Cruella de Vil; Jay, son of Jafar; Evie, daughter of the Evil Queen; and Mal, daughter of Maleficent. The quartet are causing mischief until they are summoned by Maleficent ("Rotten to the Core"), who orders them to use their school trip to steal Fairy Godmother's wand; with it, the villains will break the barrier and control of both sides of magic.
Happening after the events in How to Train Your Dragon, the short shows Hiccup, Astrid, Fishlegs, Toothless and Gobber telling the legend behind the Book of Dragons with the misadventures of its author, Bork the Bold. In addition to Bork's discoveries, the young Vikings reveal their own training methods they developed with new, never before seen dragons. The short shows a total of 14 dragons, each spread into 7 classes: Stoker (Terrible Terror, Monstrous Nightmare), Boulder (Gronckle, Whispering Death), Fear (Hideous Zippleback, Snaptrapper), Sharp (Deadly Nadder, Timberjack), Tidal (Scauldron, Thunderdrum), Mystery (Changewing, Boneknapper) and Strike (Skrill, Night Fury). But there are new dragons that are not in the book that are shown in the cartoon series.
Le Roi Arthur et ses sujets ont un problème : le chevalier noir (Sam) leur a volé leur « épée-qui-chante ». Bugs le bouffon est envoyé afin de la récupérer. Pendant que Bugs récupère l'épée, Sam est réveillé par Gerry, son dragon, qui n'arrête pas d'éternuer ; Bugs réussit cependant à s'échapper en volant l'épée tandis que Sam se fait rôtir par Gerry. Bugs se réfugie dans le château et écrase Sam en baissant le pont-levis. Ce dernier s'écrase à nouveau sur le mur du château en voulant se catapulter. Sam tente d'accéder à nouveau au château mais il se fait assommer par le lapin. Après une brève course-poursuite, Sam et Gerry font exploser le château en se réfugiant dans la poudrière et Bugs en sort vainqueur.
Scooby-Doo, Shaggy, and Scrappy-Doo are on their way to a Miss Grimwood's Finishing School for Girls, where they have been hired as gym teachers. Once there, however, they find that it is actually a school for girl ghouls. The pupils include Sibella, the daughter of Count Dracula; Elsa Frankenteen, the daughter of Frankenstein's monster, Winnie, the daughter of The Wolfman; Phantasma (usually called Phanty for short), the daughter of a phantom, possibly The Phantom of the Opera; and Tanis, the daughter of The Mummy - all parodies/tributes to the Universal Monsters of the 1930s-40s. Other residents of the school are a floating white hand; an octopus, who is the school's butler; Legs, a spider that helps with the upcoming volleyball match; Miss Grimwood, the headmistress; and her pet dragon Matches (who dislikes Scooby at first, but later becomes friends, and has a strong friendship with Scrappy). Shaggy and Scooby want no part of this at first, but eventually they agree to stay as teachers.