During their summer vacation with their grandfather Mori, 15-year-old Rocky, 14-year-old Colt, and 9-year-old Tum-Tum undertake a test on a pitch-black
obstacle course. Although they complete the course, they fail to master using their other senses to compensate for lack of sight. That night, Mori overhears Rocky and Colt planning not to return the following year due to growing older, which leaves Tum-Tum feeling alone. Mori becomes deeply saddened. Back home, Tum-Tum grows upset upon learning his favorite TV show,
Dave Dragon & the Star Force 5, is being canceled. The next day, the siblings meet their new neighbor Amanda when she accidentally crashes her
remote-controlled helicopter into their window. To make amends, Jessica invites Amanda for breakfast and later to Tum-Tum’s birthday party at Mega Mountain, an amusement park modeled after
Six Flags. At Mega Mountain, Rocky spends time with his girlfriend Jennifer and her friends Veronica, Eric, and Doyle, while Tum-Tum persuades Colt to watch a show with him. Amid the fun, master criminal Mary Ann "Medusa" Rogers and her henchmen stealthily seize control of the park, disabling rides, holding guests hostage, and converting the command center into their base. Medusa demands a $10 million ransom from park owner Harry Jacobson for the patrons’ safety. Tum-Tum and Colt attempt to get an autograph from Dave Dragon but instead witness him being captured by Medusa’s henchmen. Rocky rejoins his brothers and Amanda after spotting suspicious activity on the park’s
surveillance monitors. Together, they inform Dave about the takeover, and he goes off alone to confront the villains. The group retrieves a
radio to contact emergency services, prompting police and fire units to respond. Medusa’s men ambush the police, forcing the
FBI to intervene. In retaliation, Medusa orders her sidekick C.J. to accelerate the Avalanche ride dangerously. Using Amanda’s laptop skills and ninja abilities, the kids rescue trapped riders and override the ride controls. To stop the children’s interference, Medusa dispatches her bumbling nephews—Carl, Buelow, and Zed—to capture them, but they are defeated using carnival game props and park rides. Meanwhile, Dave sneaks into the command center but is captured. The
SWAT team attempts to storm the park but is thwarted by electrical traps set by Medusa’s mercenaries. Amanda hacks into the park’s systems to lock down rides while fending off C.J.’s attempts to regain control. Medusa activates the Zinger roller coaster’s emergency brake, stopping it mid-inversion. Dave attacks Medusa but is subdued and imprisoned with hostages at the coaster’s base. Spotting Rocky and Jennifer on ride monitors, Medusa’s nephews capture and tie Jennifer under the Zinger loop. Rocky ventures alone to rescue her but is confronted by Medusa’s second-in-command, Lothar Zogg. After a fight atop the roller coaster loop, Rocky uses a yo-yo to trip Lothar, sending him crashing into authorities outside. Rocky frees Jennifer just as Medusa releases the coaster to crush them. Park owner Jacobson arrives by helicopter with ransom money, but Amanda destroys the last bag with her remote helicopter, scattering cash over the park. Medusa captures Amanda and escapes underground with remaining funds. Dave fights Medusa’s ninjas but is knocked unconscious. The boys battle remaining henchmen until Medusa plunges the park into darkness, using
night vision goggles to gain advantage. Recalling Mori’s advice to rely on their other senses, the boys overcome the darkness and defeat Medusa’s ninjas, rescuing Amanda, who is handcuffed next to a bomb. Unable to disarm it due to power failure, they attach the bomb—taped to oxygen tanks—and send it as a makeshift torpedo toward Medusa’s escaping ship. The bomb detonates on impact, destroying the ship. Police arrive and arrest Medusa, who accepts defeat. Celebrated as heroes, the boys reunite with family but credit Dave Dragon as the true hero. The media announces plans to revive Dave’s show in honor of his bravery. Rocky and Colt decide to continue their training and invite Amanda to join them next year, which she happily accepts. The film and the series ends with a joyous celebration of Tum-Tum’s birthday. ==Cast==