T.J. Breckenridge is a woman who grew up in the circus, surrounded by all the magic and glamor of the world of greasepaint and big-tops. Her father died when she was very young, leaving her in the care of his partner, "Champ Connors," who came to think of her as his own daughter. Champ trained her to stunt ride, so that while she was still a kid, she became a starring act in the traveling rodeo that Champ and her dad had established. When she came of age, she inherited dad's half of the show, and quickly proved herself to be a shrewd businesswoman, with a genuine concern for the people who worked for her.
One day a smooth-talking cowboy trick-shooter named Tuck Kirby walked into the business office and into T.J.'s life. Young and inexperienced, she fell for him right away, and he seemed to return the affection, until one day, he walked out just as suddenly as he had arrived.
Some years later, when the show was beginning to fail, old Tuck Kirby showed up again, wanting to make an offer on her prize horse for a competing show. It would have meant the end of the show, but it was a good offer, that might have saved her from poverty. There was no way T.J. would take it, though, first because it would have left the other employees without jobs, and second because of her lingering bitterness over Kirby's sudden departure. She soon realized, however, that she still had feelings for Kirby, after he explained that he ran off out of terror at the prospect of being "caught," but that now he was looking for a chance to settle down and buy a ranch.
Meanwhile, fate dropped a bizarre fortune on T.J.'s door, in the form of a tiny horse, identified by a local paleontologist as an Eohippus. This horse would be enough of a draw to rescue the show, maybe even make it possible for her to pay off her workers and get that ranch with Tuck. But then it was stolen by gypsies, who hoped to return it to the mysterious Valley of Gwangi.
Tuck and T.J. pursued the tiny horse into the valley, where prehistoric beasts had survived to the present day, and soon met Gwangi - a gigantic Allosaurus who dominated his valley with an iron fang. The assembled cowboys quickly lassoed Gwangi, but he broke free and pursued them out of the valley, being hit in the head with a rock and knocked unconscious in the process. Now Tuck, Champ, and T.J. had the makings of a star attraction that would really bring in the crowds!
But, as anyone who has seen "King Kong" knows, monsters make very unstable attractions...