On a peaceful evening in the woods, a couple of bird watchers encounter a mysterious gryphon, setting off a chain of events that pull the Mystery Gang into an intriguing adventure. Velma Dinkley, Mindy Cohn, receives a call from her mother, Mrs. Alma Rumblebuns, Diane Delano, urging her to check on her younger sister Madelyn, who is attending the esteemed Whirlen Merlin Magic Academy, situated within an ancient Irish castle.
As the gang arrives at the castle, they are greeted by its charismatic owner, Whirlen Merlin, and his brother, Marlon Whirlen, who manages the castle's operations, as well as the enchanting Crystal, the former stage assistant of Whirlen. The atmosphere is thick with mystery, especially when it’s revealed that the gryphon has been frightening students and staff alike, threatening the security of the academy.
Madelyn, who has harbored a long-standing crush on Shaggy Rogers, invites him into the lush gardens for a romantic outing, where she shows him an ancient sundial. She shares her belief that the gryphon was originally intended as a protector for the school, only for the creature to turn aggressive and chase them away. As many students depart, the gang stays on, eager to investigate the unusual sightings and disturbances.
Meanwhile, the sinister figure of Calvin Curdles, Jeffrey Tambor, a powerful ice-cream guru, makes continuous offers to buy Whirlen’s castle. The gang promises to look into whether Curdles is involved in the gryphon’s behavior, suspecting foul play. In their exploration, Velma and Daphne stumble upon a secret passage leading to the attic, where they uncover Alma Rumblebuns’ past relationship with Calvin Curdles, hinting at deeper motives.
Shaggy and Scooby, Matthew Lillard, explore the castle's effects room and accidentally create a fog, revealing the castle's hidden secrets. Meanwhile, Madelyn uncovers a book detailing the ancient staff of Lord O'Flannery—the castle’s original owner—believed to control the gryphon. Despite warnings from groundskeeper Amos, John DiMaggio, that the staff’s resting place on a haunted island guarded by a banshee, the gang ventures into O'Flannery’s crypt. They retrieve the staff, but are chased away by the ghostly banshee and narrowly escape with their lives.
The gryphon soon appears again, forcing the gang into the castle’s depths in a frantic chase. During this chaos, Madelyn is kidnapped by the creature. The gang uncovers that Amos has been secretly working with Calvin Curdles to drive Whirlen to sell the castle. As they attempt a rescue, Curdles aggressively pushes Whirlen to surrender the property.
Inside the castle tower, the gang discovers that the supposed gryphon is actually a giant puppet, manipulated by a blimp concealed behind a fog machine—an elaborate scheme devised by Marlon Whirlen. His goal was to make everyone believe in a real gryphon and create a magical spectacle. Marlon’s frustration with doing all the work without recognition led him to stage the illusion with his brother's help. His plan was also to boost his own fame as a magician, which involved scaring away trespassers to build suspense.
The revelation doesn't stop there: the "banshee" was merely a hologram, and the entire puppet show was part of Marlon’s plan to become a celebrated magician. But the scheme falls apart when the gang intervenes, and they manage to rescue Madelyn. During the climax, the gang’s efforts cause the puppet gryphon to crash down, exposing its true mechanical nature. Marlon admits his motives, expressing regret and explaining he only wanted recognition, not to destroy the school forever.
In the end, harmony is restored—Marlon apologizes, and Whirlen forgives him. Velma reveals that Amos had seen Marlon repairing the gryphon and alerted Curdles about it. It’s revealed that Curdles’ real motive was to buy the castle to win back Alma Rumblebuns’ affection, which she graciously accepts. The academy’s future looks bright as Madelyn is invited to become their apprentice, with Daphne assisting her. Curdles sponsors a grand reopening of the Merlin Brothers’ Magic Academy, promising a new chapter filled with magic, friendship, and new beginnings.
