A group of paleontologists discover what they believe is a dinosaur egg. It's actually a spaceship in which Steggy, a mouse-sized stegasaurus from another planet, has come to Earth to find out what happens to his ancestors. Steggy becomes separated from his spaceship and in trying to find it, runs into Dale. Dale, who's been desperately wanting a pet, brings Steggy home, not realizing he's intelligent. When the Rangers learn Steggy's true nature, they help him find his spaceship at the museum. But Steggy starts growing to full dinosaur size and gets dumber as he grows. The scientists find Steggy and trap him, but by doing the pet tricks Dale taught him, he escapes.