Steggy comes from a world where all dinosaur species lived together as one. The scientists and researchers establish a project to explore worlds beyond their own. The Earth has the requirements necessary to support life. A squadron of spaceships is sent to the world to investigate, but sometime after landing, all radio contact is lost and all further exploration is called off. But Steggy is intent on finding out what had happened to his ancestors. Bringing a large supply of food packs, he boards the remaining exploring ship and sets a course for Earth.
When he arrives on Earth, his spacecraft is found by Dr. Piltdown and his team of scientists. Believing it to be a dinosaur egg, Dr. Piltdown has it shipped off to the museum. Steggy tries to pursue them, but ends up losing them. Glancing at the window of a toy store, he sees what he thinks is a fellow dinosaur, but when he tries to interact with it, he finds out it is only a toy. A baby then picks him up, but the mother nearby tells the child to get rid of him. Steggy escapes the toy store on a skateboard, and runs into some grass, only to be discovered by a malicious boy who wants to capture him for a science project. Steggy flees for his life, and ends up running into Dale. Naturally, Dale decides he should help the dinosaur, and drags Steggy into an opening at the base of a tree. Dale grabs a beetle from the alcove, and puts it near the opening. The beetle chomps on the boy's finger, making him run off.
Dale considers having Steggy as a pet, but is unsure if the other Rangers would ever agree. A grateful Steggy persistently follows Dale around despite the chipmunk's warnings to back off. Reluctantly, Dale decides to bring Steggy into the Ranger headquarters. But he decides to keep Steggy hidden from the other Rangers. Dale decides to fake ill so he can spend time with Steggy. Once the others are gone, Dale teaches Steggy tricks such as rolling over, playing fetch, and standing still. After that, Dale decides to get Steggy some food, while the dinosaur reads comic books and watches a monster movie. But Dale happens to notice that Steggy has gotten a little bigger after eating the fruits and vegetables.
When Dale hears the Rangers returning, he decides to cover Steggy with a sheet and have him pose as a sofa. When Chip notices that Dale was out of bed, Dale replies by saying he slept on the sofa. Monterey Jack takes a seat on Steggy, but the dinosaur is exposed when Dale mistakenly says "roll over". Gadget knows the Rangers have a mystery on their hands, since there has not been any dinosaurs on Earth for eons. When a news bulletin appears on TV showing Steggy's Spacecraft, the dinosaur panics and runs out the door. Steggy ends up getting stuck in tar, but the Rangers free him by spraying him with water from a nearby hydrant. Then the Rangers are nearly flattened by an oncoming steamroller, but Steggy warns them when he sees it coming. So shocked are the Rangers when they hear Steggy speak. The dinosaur tells them he had learned their language by reading Dale's comics.
Steggy spills the beans about where he was from and why he had arrived on Earth. After telling the Rangers that Dr. Piltdown and his scientists had taken his spaceship (mistaking it for an egg), they know they have to help Steggy out. Steggy has to be carried by the Ranger Plane, since he had been mysteriously growing in size. The Rangers are shocked when Steggy tells them he was already fully grown when he left his home world, and Steggy's weight causes the rickety plane to crash through the museum's ceiling window and land in a cactus. When Steggy lands in the arms of a museum guard, he bolts off towards the dinosaur exhibition. It is then that Steggy finds out what had really happened to the lost colony. Eating the Earth food made then increase in size, but decrease in intelligence. Not soon after, Steggy begins to suffer the very same effects, having eaten those vegetables and fruits.
Steggy's intellect has finally diminished to the point where he can only recall the tricks Dale had taught him earlier. Dale manages to use those tricks to avoid Dr. Piltdown and his scientists. After entering the lab where his spacecraft is held, Steggy gobbles up one of the food packets he had brought along on the journey. Suddenly, Steggy shrinks down to the size he was when he first arrived, and he experiences a significant boost in intelligence along with it. The food from his homeworld is what kept Steggy small and intelligent. After chasing Dr. Piltdown in his spacecraft, Steggy and the Rangers leave the museum. After thanking the Rangers for their help, Steggy returns home in his spacecraft.