- On the Isle of Evil, Baron Boris von Frankenstein reveals his imminent retirement and that he has refined the secret of total destruction, only to be upset when the other monsters try to steal the secret and kill his nephew, Felix Flanken.
- When Baron Boris von Frankenstein decides to retire from the monster-making business, he calls an international roster of monsters to a creepy convention to elect his successor. Everyone is there including Dracula, the Werewolf, the Creature, Dr. Jekyll and Mr. Hyde and many more. But von Frankenstein's title is not all that is at stake. The famous doctor has also discovered the secret of total destruction that must not fall into the wrong hands.—Anonymous
- NOTE: All of the voices in this film are by Allen Swift, except where noted.
Baron Boris von Frankenstein [the voice of Boris Karloff], the scientist who once discovered the secret to creation, has just perfected the secret to destruction. In order to celebrate his discovery, the Baron invites all his monster friends - Dracula, the Werewolf, the Mummy, the Invisible Man, Dr. Jekyll and Mr. Hyde, the Creature from the Black Lagoon, and the Hunchback of Notre Dame - to his Caribbean home on the Isle of Evil for a big party. The Baron also has another reason for hosting such a gala. He has decided to retire as head of the World Organization of Monsters and to pass on his secrets to his only "living" heir - geeky, allergy-ridden pharmacist Felix Flanken, son of the Baron's sister, the white sheep of the family. As the S.S. Herring bearing all the guests nears the island, the Baron briefs his zombies about the necessity of remaining calm, cool and collected no matter what happens. Only the Baron's servant, Yetch, is having trouble keeping his head (and hands) attached. This is partly because it has been so long since Yetch has seen all of their friends, but also because he is besotted with the Baron's secretary, the lovely, well-stacked, red-headed and cool-as-a-cucumber Francesca [the voice of Gale Garnett].
As the Baron and Francesca prepare to greet their guests, they are joined by the Frankenstein Monster and his Mate [the voice of Phyllis Diller]. First to arrive is Dracula, who is enchanted to see Francesca ("You have always been my type...O-negative, isn't it?"). Next is the Werewolf ("Wolfie, you old dog! Ha-ha! This convention is going to be a howling success!"). The Invisible Man arrives ("You're looking well...I think."). The Hunchback arrives, carrying in the Mummy in his mummy case, then comes Dr. Jekyll and Mr. Hyde, followed by the Creature. Meanwhile, in the kitchen, Chef Mafia Machiavelli is preparing the dinner menu for the party. There is octopus soup and a poison ivy salad with toadstools, poison berries, and three kinds of dressings - arsenic, cyanide, and nitroglycerin. The antipasto consists of black widows marinated in their own poison, smoked lizards and snakes, and marinated mice. For the main course, there is boar's head, roast vulture, and minced hyena casserole. In addition, the entertainment is to be provided by the living skeleton rock and roll band Little Tibia and the Fibulas. As the dinner commences, the guests are all trying to figure out just who will be the Baron's successor. Dracula is assured that there is no one more evil, treacherous, and sneaky than he. Francesca thinks that she might be able to use Dracula in order to attain the secrets for herself. The Monster and his Mate feel that there can be no one closer to the Baron than themselves ("He CREATED us!"). Ultimately, a big fight breaks out, and the guests spend the rest of the night sleeping it off.
The next morning, the Baron's nephew Felix arrives. The Baron's plan is to have Francesca show Felix around the island. Francesca's plan is to set Felix up for an ambush along the way. Fortunately, Felix bumbles his way through Dracula, the Mummy, and the Werewolf and all of their attempts to kill him. Feeling betrayed by Dracula, Francesca concocts a new plan which involves inviting the uninvited IT to the island. In an attempt to escape from the wrath of the other monsters, however, Francesca finds herself falling in love with Felix. As the two lovers make their way to the opposite side of the island where Francesca has a boat hidden, they are followed by the monsters who have decided unanimously to kill Felix. Just as the monsters capture Francesca and catch up with Felix, IT (a.k.a. King Kong) arrives. IT storms the castle, but when IT finds a photo of Francesca, IT falls madly in love with her. Grabbing Francesca and all of the monsters in one oversized hand, IT climbs to the highest mountain where IT is swarmed by airplanes flown by Baron von Frankenstein and his zombies. The Baron orders IT to release Francesca and take him instead, which IT does, allowing Felix and Francesca to get in the boat and escape. In his final act, the Baron drops the vial containing the secret of destruction and the Isle of Evil, along with everyone and everything on it, is destroyed in a gigantic explosion. Safe and out at sea, Felix makes plans to marry Francesca, but Francesca sadly reveals that she cannot marry him because she is really only a robot created by the Baron. Felix consoles her by saying, "Well, Francesca, none of us are perfect...are perfect...are perfect...are perfect...are perfect...", revealing that he is also a robot created by the Baron. [original synopsis by bj_kuehl]
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