As it turns out, Julie was not the owners' daughter; Velma sensed this when Julie's supposed pet cat backed away from her when she got closer. Julie had heard about the gold stolen from the Klondike mine three weeks prior, and planned to collect it for herself. She and her henchmen colluded to smuggle the gold to the U.S., and to scare off townspeople, Julie disguised herself as the Beast from Bottomless Lake, a giant fish-like creature.
Julie's defeat and reveal came when she, Shaggy, and Scooby were caught in a net during their chase, and following this, Velma revealed the Beast as Julie. After the reveal, Julie and her henchmen were arrested.