Mad Monster Party?

Baron Boris von Frankenstein perfects a formula for universal destruction and tests it. Satisfied, he packs the knowledge into an invitation-laden plan and sends messenger bats to summon all his monster peers to the Isle of Evil. He also announces his imminent retirement as head of the Worldwide Organization of Monsters. The invitation campaign signals a turning point in monster governance.

Francesca confirms all invitations were delivered and learns Felix Flanken is Frankenstein's nephew and would-be successor. She resents this line of succession and eyes Felix as a rival for power and the role she covets. The scene hints at an upcoming plot to secure Frankenstein's secrets for herself.

A bumbling but good-natured drug store pharmacist from the United States receives his invitation and eagerly accepts. He boards a freighter bound for the Isle as the rest of the monsters travel aboard. The ship's wary crew watch the unusual passengers with a mix of fear and curiosity.

Frankenstein has his zombie butler Yetch, chef Mafia Machiavelli, and the zombie staff organize the party and patrol the island to ensure It does not arrive uninvited. The preparations set a tense tone for the upcoming gathering. The staff also tests the security around the manor as the clock ticks toward the feast.

Monsters begin to arrive on the freighter that Felix is traveling on. During dinner, Frankenstein unveils the destruction formula and explains that he will reveal it the next day, also naming his successor. Francesca eavesdrops and, when caught, forms a rival cat-fight with the Monster's Mate while the others erupt into a chaotic food fight.

Felix arrives on the island and is greeted by Frankenstein and Francesca, who then give him a tour. The Mummy, Dracula, and the Werewolf secretly plot to attack Felix in order to seize Frankenstein's secret formula. Felix's kind-hearted nature repeatedly foils their early attempts.

Frankenstein formally tells Felix about his retirement and names him leader of the monsters. Felix is reluctant to take on the burden, recognizing both the danger and responsibility that come with the role.

Francesca develops feelings for Felix after he saves her on multiple occasions. She secretly dispatches a letter via messenger bat to an unknown recipient, signaling a hidden plan. Dracula, Frankenstein's Monster, and the Monster's Mate also begin to maneuver around Francesca as the plot thickens.

Felix and Francesca are confronted by an uninvited giant gorilla, It, who rampages across the island and crushes the monsters and Francesca. The rampage disrupts the planned gathering and throws the power struggle into chaos.

Frankenstein leads the zombies in a daring rescue of Francesca using biplanes. Boris convinces It to spare Francesca and take him instead, sacrificing himself for the girl, and It obliges. Felix looks on as the plan unfolds.

Felix and Francesca escape the island aboard a boat, leaving the remaining monsters to their fate in Its clutches. The pair share a tense moment of relief as they make their getaway from the erupting island.

Frankenstein sacrifices himself by dropping the vial of the formula, triggering a cataclysmic destruction that obliterates the Isle of Evil. The destruction is witnessed by Felix and Francesca offshore as they flee.

Felix and Francesca watch the wreck from the sea and discuss future plans. Francesca reveals she is not human but a robot creation of Frankenstein, leaving Felix to realize that he, too, may be a kind of machine. Felix ends the moment with the line 'none of us are perfect'.