Fact1: Frankenstein sends out messenger bats Having perfected and tested formula, Fact2: Baron inform them of discovery and also to reveal imminent retirement as head of Worldwide Organization of Monsters, Fact3: Monster 's more intelligent mate live in island castle with Boris, Fact4: Flanken is nephew and successor in monster business, Fact5: arrival of It proceeds to go on rampage
Output: Baron Boris von Frankenstein voiced by Boris Karloff achieves his ultimate ambition, the secret of total destruction. Having perfected and tested the formula, he sends out messenger bats to summon all monsters to the Isle of Evil in the Caribbean Sea. The Baron intends to inform them of his discovery and also to reveal his imminent retirement as head of the Worldwide Organization of Monsters. Besides Frankensteins Monster sometimes referred to as Fang and the Monsters more intelligent mate voiced by Phyllis Diller who live in the island castle with Boris, the invites also include Count Dracula, the Mummy, The Hunchback of Notre Dame, the Werewolf, The Invisible Man, Dr. Jekyll and Mr. Hyde, and the Creature from the Black Lagoon referred to as just The Creature. The Barons beautiful assistant Francesca voiced by Gale Garnett enters the lab to confirm that all invitations have been delivered and inquires about one of the addressees named Felix Flanken voiced by Allen Swift impersonating James Stewart. Frankenstein explains that Flanken is his nephew and successor in the monster business. This displeases Francesca, who covets the role for herself. Francesca even asks why there was not an invitation for It. Boris replies that It was not invited since It can be a crushing bore, explaining that It even crushed the islands wild boars in his bare hands the last time It was invited. Frankenstein has his zombie butler, Yetch Swift impersonating Peter Lorre, Chef Mafia Machiavelli, and the zombie bellhops and servants make preparations for the upcoming party while having some zombies patrol the island to make sure that It does not show up uninvited. The monsters begin to arrive on the freighter that Felix is also traveling on. However, when Felix proves to be an incompetent, asthmatic and unsuitably kindhearted human, the monsters plot to eliminate him and gain control of the secret formula. Over time, Francesca develops feelings for Felix after he unknowingly saves her multiple times. As Dracula, Frankensteins Monster, and the Monsters Mate descend upon Francesca, she sends out a letter via messenger bat to an unknown recipient. When the monsters corner Felix upon capturing Francesca, they are frightened at the arrival of It revealed to be a giant gorilla who is a takeoff of King Kong who proceeds to go on a rampage since he was not invited. It snatches up the monsters and Francesca on whom It develops a crush. Felix rushes off to tell his Uncle Boris what happened, and is instructed to head to the boat. Boris leads the zombies in rescuing Francesca from It using biplanes. Boris convinces It to let Francesca go and to take him instead. It complies, releasing Francesca. Felix and Francesca manage to get off the island as Boris and the remainder of the monsters remain in the clutches of It. Displeased that the monsters tried to steal the secret of total destruction for themselves and attempted to kill Felix, as well as having to put up with It, Boris sacrifices his life by dropping the vial containing the formula, destroying the Isle of Evil and everything on it. The destruction is witnessed offshore by Felix and Francesca. Francesca tearfully admits to Felix that she is not human, but is in fact a robot creation of Boris von Frankenstein. Felix answers that none of us are perfect where he then mechanically repeats the words are perfect, indicating that he has also been a robot creation of his uncle all this time.