The party that is referred to in the movie's title is held by the mad scientist Baron Boris von Frankenstein at his home on a tropical island. The Baron throws the party in celebration his discovery of "the secret of ultimate destruction", a formula which gives him the power to destroy anything and everything. The guests at the party include many of the world's most famous monsters, as well as the Baron's unassuming nephew Flankin.
The film features the voice of Boris Karloff as Baron Boris von Frankenstein, Phyllis Diller as the monster's wife, Gale Garnett as Baron Frankenstein's beautiful assistant Francesca and Allen Swift as all other characters. Many of the voices that Swift provided were based on those of famous Hollywood actors.
The movie was originally released in theaters in March but has become a Halloween favorite on television.
Mad Monster Party? was unrated by the Motion Picture Association of America on its first release. It received a G rating in Australia and a U certificate (short for "universal" and meaning "suitable for all", roughly equivalent to an American G rating) in the United Kingdom.
The film opens on a tropical island where Baron Boris von Frankenstein lives with his laboratory assistant Francesca, the first monster that he created, the monster's wife and various servants, most of whom are zombies and other monstrous creations of the Baron too.
The Baron's latest creation is "the secret of ultimate destruction", a formula that can instantly destroy anything and everything. To celebrate his discovery the Baron decides to throw a party to which he invites several of the world's most famous monsters, namely Count Dracula, Dr. Jekyll and Mr. Hyde, Quasimodo, the Invisible Man, the Mummy, the Swamp Creature and the Werewolf. He also invites his mild-mannered human nephew Felix Flankin.
Baron Frankenstein plans to announce that he is retiring and will be entrusting "the secret of ultimate destruction" to a trusted successor. The monsters are horrified when Francesca informs them that the trusted successor is the human Felix Flankin. Francesca initially helps the monsters in their attempts to do away with Flankin. However, she becomes fond of Flankin and eventually helps him to escape.
The final scene features Francesca telling Felix Flankin that there is no possibility of a relationship between the two of them because she is not really human, she is just one of the Baron's creations, before ending with the suggestion that Flankin was created by the Baron too.
A sequel of sorts to Mad Monster Party? followed in In 1972 with the Rankin Bass Halloween TV special Mad, Mad, Mad Monsters. The special, created using cel animation instead of stop motion, features many of the same monster characters as Mad Monster Party? and has a Baron Frankenstein whose voice is an imitation of Karloff's (he died in 1969). Mad, Mad, Mad Monsters is presumably not intended as a direct sequel since many of the characters who appear in it perish at the end of Mad Monster Party?. It could be considered a prequel, since it deals with the creation of a bride for Frnakenstein's monster. Although Mad Monster Party? still has a strong cult following, Mad, Mad, Mad Monsters has faded into relative obscurity.