Cowboys discover a lost world populated by dinosaurs.

Promotor Tuck Kirby arrives in a small Mexican town to visit T. J. Breckenridge, his ex-partner and girl friend who now owns a financially troubled Wild West show. Tuck suggests that she sell the show and marry him, but T. J. boasts of a new attaction that promises to make her wealthy. She shows Tuck a miniature horse which she obtained from Carlos, a gypsy who defied native superstition by removing the creature from the nearby Forbidden Valley. Baffled, Tuck takes the tiny animal to British paleontologist Prof. Horace Bromley who claims that it belongs to a species believed to have been extinct for over 50 million years. Before the horse can be put on display, however, it is stolen by gypsies and returned to the Forbidden Valley. Despite warnings about Gwangi, the monstrous 14-foot reptile that rules the valley, Tuck, T. J., and several cowboys enter the area and find themselves in a prehistoric world. After one of the cowboys is carried off by a giant prehistoric creature, the men manage to capture Gwangi, and T. J. makes plans to put the monster on exhibition in her show. The gypsies intervene again, this time by setting the monster free. Gwangi kills the professor and rampages through the town until it traps Tuck and T. J. inside a cathedral. Tuck pierces the monster's skull with a lance, and Gwangi, after knocking over a burning lamp, dies in the fire as the two lovers escape.