Shaggy Rogers Matthew Lillard discovers he suffers from a disorder that makes him irrationally overreact to frightening stimuli, and he fears he can no longer stay involved with Mystery Inc. To help him cope, his friends escort him to a spa in the town of La Serena, hoping the tranquil setting will ease his anxiety. There, they meet the spa’s owner and hypnotherapist, Mr. Hubley Fred Willard, along with paleontologist Professor Svankmajer Finola Hughes and her assistant Windsor Matthew Gray Gubler, who are overseeing a paleontological dig. The moment the group settles in, a startling event unfolds: a large, looming “ghost dinosaur” attacks the dig site, and Shaggy is paralyzed by fear, though Scooby-Doo Frank Welker heroically rescues him. Fred Jones Frank Welker then uses Mystery Inc.’s trusty van, the Mystery Machine, to knock the phantom creature off a cliff, offering a temporary sense of relief.
Hubley explains that the ghostly creature is the Phantosaur, a figure rooted in legend that Native Americans were said to summon in order to reclaim land from European colonists. The hypnotherapist reveals a startling twist: he has hypnotized Shaggy into shifting between his ordinary self and a fearless persona whenever a specific keyword is spoken. The catch, however, is that no one remembers what that keyword is. This revelation leaves the group with more questions than answers as they try to understand what they saw and how it was done.
As Fred, Daphne Blake Grey DeLisle and Velma Dinkley Mindy Cohn pursue the clues at the dig site—looking for dinosaur tracks and what may appear to be snake-like trails—Shaggy finds himself entangled in trouble again. He fights a biker gang at a buffet and is invited to join a motorcycle race after hearing the keyword, “bad.” The clash draws him deeper into a potentially dangerous social circle, even as the others keep investigating the suspicious activity around Svankmajer’s team. Later that day, Daphne teaches Shaggy how to ride a motorcycle, broadening his skills just as chaos threatens to erupt.
When Shaggy hears the keyword a second time, he participates in the race and miraculously rescues the bikers’ leader, Tex Kevin Michael Richardson, from certain danger, earning Tex’s respect. But the excitement is cut short as the Phantosaur returns, this time accompanied by a pack of Velociraptors that interrupt the race. The group observes a van belonging to the mining company, its cables and tires matching the earlier snake-trail clues, and they, along with the bikers, flood the van to reveal a deeper plot. It appears the Phantosaur was an animatronic creation operated by someone intent on scaring off Svankmajer’s team so they could seize the mine’s silver resources.
Hubley arrives again, claiming his hypnotherapy equipment has been stolen, and Mystery Inc. continues their investigation—only to encounter another Phantosaur before it vanishes again. The next day, the team returns to the mine to present their findings to Svankmajer, who claims her camp was attacked and that she had sent Windsor home for safety. Yet the group soon discovers a far more intricate plan: Svankmajer had hired graduate students to pose as Velociraptors, and the second Phantosaur was a projection created by her and Windsor using Hubley’s stolen equipment. They used super-heated air and paint strippers to emulate the Phantosaur’s fearsome fire breath. Svankmajer also reveals that her team had uncovered a dinosaur skeleton preserved in quartz crystal and intended to scare La Serena’s residents in order to claim it for themselves.
Despite the mounting evidence against them, the tension peaks as the paint strippers trigger explosives meant to seal the mine. Shaggy overcomes his fear at last and manages to rally help from Tex’s gang to rescue the others from danger. In the aftermath, Svankmajer and her team are arrested, while Shaggy is unhypnotized. Unfortunately, Hubley’s misstep leads to an unexpected twist: he accidentally hypnotizes the rest of Mystery Inc. into believing they are Shaggy, setting up a new, comic complication for the team to untangle.
