It is late Spring when Wally, Jeremy, Shades and Mario are at the La Brea Tar Pits working on a school research project. They discover a mysterious tar egg that cracks open to reveal a friendly dinosaur, whom they name "Denver." Keeping their pet dino a secret is a challenge when rock concert promoter Morton Fizzback makes repeated attempts to capture the young dino and use him as the star in his rock 'n' roll show.