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.

Following Jeremy to his job at the supermarket, Denver hides in the storeroom and stuffs himself full of potato chips. Meanwhile, Casey heads from the La Brea Tar Pits with Wally's dog, Rocky, to look for another dinosaur. Sitting on the remains of Denver's tar shell, she and Rocky drift back in time to a prehistoric jungle.

Wally borrows his sister Heather's video camera and the starry-eyed kids set out to make a video--starring themselves. As they squabble over leading roles, Denver wanders off with the camera and inadvertently tapes an auto theft ring stealing a truck. While looking for Denver, Wally is captured by the unscrupulous thieves. But Denver alerts the boys, who rescue Wally and catch the thieves.

While camping at a mountain lake with the boys, Denver is mistaken for a horrible lake monster. Hordes of thrill-seeking tourists descend upon the lake, and the boys must use their wits to protect the environment and save their dino buddy.

The boys decide to do a science project on dinosaurs for the school science fair. When their big dino model is sabotaged by a competing group of kids, the boys use Denver as a live stand-in. Is winning worth cheating?

Denver and the kids make friends with a circus owner's son and help him liven up a lackluster show with bicycle and skateboard stunts. The updated circus is a tremendous success and Baxter's Big Top is saved from financial ruin by Denver and his friends.

Wally's first day on the job at the zoo takes a turn for the worse when the self-serving Dr. Funt captures Denver for his new high-tech exhibit. With the help of Wally's pals and a mischievous orangutan, Denver is rescued and Funt's inhumane approach to zookeeping is discredited.

The kindhearted Denver rescues Scott, one of the boys' arch rivals. Denver wins his heart and thwarts Scott's plans to expose Denver to the media in order to gain publicity and votes for the upcoming student election. Instead, Jeremy wins the election, and Scott vows to keep Denver a secret.

While visiting Jeremy's Uncle Eli on his Nevada ranch, Denver and the boys attend a local rodeo. Uncle Eli hopes to win the rodeo prize money in order to finance a preserve for a herd of wild mustangs.

Casey's movie-star cousin, Lana Lake, invites Casey, the boys and Denver to watch the filming of her latest sci-fi movie, "Leapin' Lizards on the Loose." But it really becomes lights!...camera!...action! when Denver is accidently cast as the lead Space Lizard!

When Heather gets a job at a dinosaur theme amusement park, Denver roams freely around the park and fits right in with all the dino-costumed employees. But when the roller coaster gets stuck with the kids on board, Denver risks exposing his true identity to perform a daring and heroic rescue.