Scooby and the gang are visiting the location set of the film The Ape-Man of Forbidden Mountain. Daphne's uncle, John Maxwell, is glad to have the gang as extras, as no other actors will work on the film. Apparently, there is a real Ape-Man scaring everyone off.
There is a twist at the end because John Maxwell decides to rewrite the film so the gang can play leading roles, with Scooby-Doo as the Ape Man.
Several things from the original TV version were removed, such as the Ape Man sending a stuffed ape dummy hurling towards Shaggy, the parrot that says "Stick 'em up, what's the password?", the Ape Man dressing up as Scooby and performing the mirror routine, and Shaggy taking a picture of the Ape Man unmasked before he is captured (instead, the Ape Man is unmasked at the end of the mystery.)