The gang goes to the Old California Festival, where they see a play involving Scooby's favorite hero El Sabueso the hound, who helps the poor. Later that night, while camping out, Scooby dreams he is El Scoobo and Scrappy is Scrappito, and they must save Senorita Daphne and Senor Shaggy from the Commandante. / The kids go back to their high school for the Central High Carnival, and run into their old science teacher Mr. McDabble, who has created a ruby laser that can cut through the hardest metal. But before too long, the ruby ends up missing, and the kids have to find out what happened.