Scooby-Doo and the gang head to Atlantic City for a vacation that quickly spirals into a mystery when a werewolf breaches a jewelry shop, setting off an alarm and stealing a ruby necklace before slipping away into the hills that overlook a nearby circus. The chilling incident casts a shadow over the Brancusi Circus, which Marius Brancusi owns and has been trying to modernize, and the locals whisper that the creature has been stalking the troupe for months, scaring away performers and snatching jewelry wherever they go.
On arrival, Fred’s curiosity about the circus leads the friends to mingle with the performers, while Velma notes a chilling legend from Ingolstadt about a werewolf named Hans who gathered jewels to fuel his transformations. The clues begin to weave together as Velma and the others realize they’re dealing with a werewolf threat that could be connected to the circus itself. To get to the bottom of it without tipping off the scheming culprit, the gang agrees to go undercover as circus performers, putting their distinct talents to work behind the scenes.
Undercover roles are filled with a mix of danger and whimsy: Whitney Doubleday becomes the animal trainer for Shaggy and Scooby, while Velma takes on the Human Cannonball act, teamed with the strongman Archambault. [Fred] dives into trapeze duties alongside the new performer Oliverio, and his partner Lena adds a flirtatious spark that complicates Fred’s focus. Meanwhile, Daphne steps in as a clown to fill in for the reluctant Schmatko, who is haunted by his past as a Soviet-era actor. But trouble strikes when Schmatko is abducted by the werewolf during a break in the show, throwing the troupe into a fresh panic.
As performances continue, the werewolves strike again, wearing the stolen outfits of missing artists and daring the gang to stop them. The troupe narrowly escapes with the audience believing the danger is part of the act, and the reviews begin to skew toward Shaggy as the star of the act, much to Scooby’s exasperation. The plot thickens when Archambault, Lena, and Marius vanish, and a private show for a celebrity singer Wulfric Von Rydingsvard promises a pivotal moment in the night’s schedule. Velma connects a new thread: a carbonado, also known as a black diamond, belongs to Wulfric, hinting at a possible motive rooted in jewel theft.
The show culminates in a dramatic reveal during the performance: the “werewolves” are not cursed beasts but Doubleday and his trained baboons in disguise. A clever use of Doubleday’s own commands helps to subdue them, while Archambault is cornered after a failed bid to escape with the box-office takings. The gang learns that Archambault had long conspired with Doubleday, using the circus as cover to plunder jewelry and money, and that Archambault had even planted a clue about the Ingolstadt werewolves to misdirect the investigation.
With Doubleday captured and the jewels recovered—or left momentarily missing as the dust settles—the Brancusi Circus packs up to leave Atlantic City. In a final gesture of thanks, Wulfric Von Rydingsvard grants the gang a private performance, sealing a night of spectacle with a satisfying end to the mystery.
