Scientist Baron Boris von Frankenstein achieves his ultimate ambition, the secret of total destruction.

Having perfected and tested the formula, he sends out messenger bats carrying invitations 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.

Frankenstein's beautiful assistant Francesca confirms that all invitations have been delivered and inquires about one of the addressees, a Felix Flanken.

When the monsters corner Felix upon capturing Francesca, they are frightened at the arrival of "It" (a giant gorilla and knock-off of King Kong) who proceeds to rampage since he was not invited.

Felix and Francesca manage to escape the island in the boat as Frankenstein and the remainder of the monsters remain in "It"'s clutches.

Displeased that the monsters tried to steal the secret for themselves and attempted to kill Felix as well as having to put up with "It", Frankenstein sacrifices himself by dropping the vial of the formula, destroying the Isle of Evil and everything on it.

Felix expresses a desire to begin a family with Francesca, who tearfully admits that she is not human, but in fact a robot creation of Frankenstein's.

The film was created using Rankin/Bass's "Animagic" stop-motion animation process, supervised by Tadahito Mochinaga at MOM Productions in Tokyo, Japan.

[7] Known as stop-motion animation, it was the same approach used in RKO's King Kong, Art Clokey's Gumby and Davey and Goliath, and many other films, commercials and TV specials.

Classic monster films were enjoying a resurgence in popularity in the late 1960s, along with more comedy-centered examples, The Addams Family and The Munsters.

Rumors that Forrest J. Ackerman had a hand in the script have never been confirmed and his name never appeared in the on-screen credits or in original promotion for the film at the time of its release.

Karloff and Diller's characters are both designed to look like the actors portraying them, while Baron Frankenstein's lackey, Yetch, is a physical and vocal caricature of Peter Lorre.

was one of several family-friendly projects Karloff lent his voice to in his final years (including the 1966 television adaptation of How the Grinch Stole Christmas!).

The animation for the special is provided by Osamu Tezuka's Mushi Production with supervision by Steve Nakagawa, who was also known for his work with Iwao Takamoto at Hanna-Barbera Studios.