Scooby-Doo, Don Messick and his friends are drawn into a peculiar assignment as they head to Miss Grimwood's Finishing School for Girls, stepping into a place that appears welcoming at first but quickly reveals a more fantastical core. The headmistress, Miss Grimwood, Glynis Johns, runs the school with a firm yet fair demeanor, and the campus hides a quirky menagerie of students who are daughters of famous monsters: Sibella Dracula, Susan Blu the daughter of Count Dracula; Elsa Frankensteen, Pat Musick the daughter of Frankenstein Senior; Winnie Werewolf, Marilyn Schreffler the daughter of the Wolfman; Phantasma, Russi Taylor the ghostly daughter of a phantom; and Tanis the Mummy, Patty Maloney. The school grounds buzz with other peculiar residents—a disembodied floating white hand, an octopus butler, a two-headed shark in the moat, and Legs the spider that assists with an upcoming volleyball match. The little dragon named Matches hops into action under the watchful eye of the friendly dragon keeper, voiced by Frank Welker, while the entire place carries a sense of adventurous mischief.
At the center of the school’s daily life is a volleyball game that pits the Grimwood girls against the neighboring Calloway Military Academy. The boys, led by Cadet Tug Roper, Scott Menville, rig the ball with a remote control, hoping to win. Scooby-Doo, voiced by Don Messick, accidentally swallows the remote, and the control slips into the girls’ hands—allowing them to clinch a victory they didn’t see coming. The moment underscores a playful tension between the cadets and the Grimwood girls, who soon find themselves drawn into the school’s odd, magical rhythm. The atmosphere remains light-hearted, but the stakes rise as the families of the students begin to show up for a Halloween open house, testing the nerves of Scooby and Shaggy, who are there to teach gym class with their usual mix of bravery and bumbling charm.
The Halloween open house turns out to be a warm, if chaotic, affair. The girls’ fathers—including Colonel Calloway, Ronnie Schell—arrive in good spirits, trading stories and sharing laughs with the unusual residents of the school. The mood is festive, and the fathers’ protective presence is clear: they warn Scooby and Shaggy not to let any harm come to their daughters, a stern reminder of the responsibility that comes with caring for these magical youngsters. Amid the laughter and music, new dangers begin to stir as a power-hungry witch named Revolta, Ruta Lee, and her sinister ally, the Grim Creeper, Andre Stojka, gather their forces. Revolta hatches a plan to kidnap the Grimwood girls and bend them to her will, setting her sights on turning the school’s bright futures into something far darker. Shaggy, temporarily hypnotized, is drawn into a field trip to the Barren Bog with the group, and Matches, the dragon, is kept back, even as the siege of danger begins to unfold.
As Revolta begins her work, her spider-bats quietly begin to close in on the girls, while Elsa (Frankensteen) and Tanis manage to rescue Cadet Jamaal, Bumper Robinson and the others from a life-threatening quicksand trap. The team—Scooby-Doo, [Don Messick], Shaggy, Casey Kasem, and Scrappy-Doo—joins Elsa and Tanis on a desperate dash toward Revolta’s castle, where the danger crescendos into a trio of challenges: a shape-shifting Mirror Monster that can mimic evil versions of those who gaze into its reflective surface, a towering Well Dweller, and Revolta herself. The quest is fueled by a desperate clock: at midnight, Revolta’s potion is poised to render the Grimwood girls evil forever, siphoning their strength away.
When midnight nears, Revolta’s witches’ magic is activated through hairdryers, but Scooby and Shaggy intervene, unintentionally reversing the potion’s effects. In a key turn, Elsa Frankensteen throws Revolta’s wand into the potion, triggering a spectacular explosion that breaks Revolta’s dark influence and sends the school’s heroes racing toward safety. The Cadets, moved by the bravery of the Grimwood girls and the courage of Scooby-Doo’s unlikely crew, arrive in a pedal-powered helicopter to help evacuate everyone, and Revolta swears revenge as she retreats to regroup.
In the aftermath, a joyful celebration erupts—the Grimwood Girls and their cadet protectors host a party that honors courage, solidarity, and the power of unlikely friendships. Yet the closing note hints at future challenges: as new monsters—an alien, the Gill-man, and even Godzilla—seem poised to enroll their own offspring at the school, Scooby, Shaggy, and Scrappy-Doo flee in a mix of relief and exhilaration. They cast one last werewolf howl toward the departing sea of monsters and their daughters, a farewell that echoes into the night as the school recovers from defeat and looks toward a future filled with both danger and wonder.
