What's New, Scooby-Doo?
When the gang arrived at the Los Angeles Conference Center to check out the games there, Ostwald opted to show Velma the game he made. He brought her into the game's booth and briefly demonstrated the game to her and how it worked. He then seemingly turned it off so that Velma could come back and play later, but he had actually sent her to a virtual reality version of the conference center, complete with virtual imitations of the gang, so that he could have her participate in a virtual mystery without her knowing it. As a result, Velma was the only true member of Mystery Inc. that participated in the mystery, since the rest were all virtual reality projections.
The virtual Mystery Inc. accompanied Velma as she attempted to solve the mystery of the Osomons, small furry monsters from a game of Ostwald's that supposedly turned hostile and started running amok. Though the virtual gang behaved similarly to their real counterparts, they had glaring holes in their characterization that ultimately tipped Velma off. First, Fred wanted to stick together and search for clues while the real Fred Jones always wanted to split up. Second, Daphne had no issue wearing shoes that didn't match, something the real Daphne BLake wouldn't do. Third, Shaggy and Scooby actively volunteered to be live bait, but the real Shaggy Rogers and Scooby-Doo would never do this and always need to be bribed with Scooby Snacks. The final clue was when one of the Osomons bit the virtual Shaggy's arm. He screamed "Toinks!", when the actual Shaggy would've said "Zoinks!"
Velma proclaimed that she had solved the mystery, and everything froze in place. She had figured everything out; she was playing Ostwald's game the whole time and never left the booth in the first place. Dr. Ostwald walked in and congratulated her on solving the mystery.