Scooby and Shaggy head off to Dr. Phibes' lair on information that Phibes has called a meeting of the world’s most fiendish fiends.  Before arriving, Scooby and Shaggy intercept the most fiendish of all the fiends (a muscle-bound mutation named "Menace"). Before sending Menace to the moon in the Mystery-Space Shuttle, Scooby uses a Super Scooby Snack with "Morphing Power" to take on Menace's form. Scooby then enters Phibes' big meeting disguised as Menace, only to discover that Phibes has called all the superfiends together for the soul purpose of destroying Scooby and Shaggy! Soon after learning that he has a million dollar bounty on his head, Scooby's snack wears off, and he changes back into himself. Scooby flees with all the fiends chasing and is rescued briefly by Shaggy outside the lair. Scooby and Shaggy lead the fiends back to the mansion, where a combination of Robi's "abusive butlering" and their caginess bring down the fiends one by one. In the end, Menace returns from his trip to the moon to make one final assault on Scooby and Shaggy. Our heroes defeat Menace, but he manages to escape with a Scooby Snack. Menace delivers the snack to Phibes who finally has what he's been after all along: Uncle Albert's top secret formula!