In circa-1900 Mexico, T.J. Breckenridge, a beautiful cowgirl, hosts a wild west show that is struggling. Former boyfriend Tuck Kirby, working for Buffalo Bill's wild west show, wants to buy out T.J., but T.J. has an ace she hopes will boost attendence at her show - a tiny horse. The tiny horse, however, comes from The Forbidden Valley and a convoy of gypsies demands the tiny horse be returned to the valley; the horse's genesis is also known to a British paleontologist, Sir Horace Bromley, working in the nearby desert. T.J., her men, and Tuck eventually find The Forbidden Valley with Bromley, and encounter a litany of living dinosaurs. One, a belligerent Allosaurus, is known as Gwangi by the gypsies, and a running pursuit sidetracks into a bloody battle with a styracosaur and eventually to terror in the outside town.