In Mexico at the turn of the 20th century, a beautiful cowgirl named T.J. Breckenridge (Gila Golan) hosts a struggling rodeo. Her former lover Tuck Kirby (James Franciscus), a heroic former stuntman working for Buffalo Bill's Wild West Show, wants to buy her out. Along the way, he is followed by a Mexican boy named Lope (Curtis Arden), who intends to join the rodeo on a quest for fame and fortune. T.J. is not interested in Tuck because of this, but Tuck is still attracted to T.J., especially when T.J. jumps off a diving board on her horse. T.J. finally accepts Tuck when he saves a matador from a bull and the two kiss on the lips.
T.J. has an ace she hopes will boost attendance at her show - a tiny horse, El Diablo. Tuck meets a British paleontologist named Horace Bromley (Laurence Naismith), who is working in a nearby Mexican desert. Bromley shows Tuck fossilized horse tracks, and Tuck notes their similarity to El Diablo's feet. Tuck sneaks Bromley into the circus for a look at El Diablo, and Bromley declares the horse to be an Eohippus.
The tiny horse came from a place known as the Forbidden Valley. A Gypsy known as Tia Zorina (Freda Jackson) claims that the horse is cursed, and demands that it must be returned. Later, she and other gypsies collaborate with Bromley to steal El Diablo and release him in the valley. Bromley hopes to follow the horse to its home in search of other prehistoric specimens. Carlos (Gustavo Rojo), an ex-member of the Gypsy tribe now working for T.J.'s circus, walks in on the theft and tries to stop it, but is knocked out.
Tuck arrives just as the Gypsy posse leaves. Carlos sees him as he is regaining consciousness. Tuck notices that the horse is missing, and sets off after Bromley. When T.J. and her crew discover Carlos, Carlos claims that Tuck has stolen El Diablo for himself. Carlos, T.J., and the others decide to follow Tuck and Bromley into the valley.
Making their way into the Forbidden Valley, Tuck, T.J., and the rest of the group meet up and soon discover why the valley is said to be cursed when a Pteranodon swoops down and snatches Lope but due to the weight it falls back to the ground. After Carlos kills the Pteranodon by twisting its neck, they spot an Ornithomimus, a small dinosaur which they chase after in the hopes of capturing it. Just as the Ornithomimus is about to escape, it is killed by Gwangi, a vicious Allosaurus which chases Bromley and the rest of the group. However, a Styracosaurus appears and drives Gwangi away. As Gwangi leaves, he takes the dead Pteranodon with him.
Later, Gwangi pursues the people to their base camp and the cowboys try to rope him down, but he breaks free when the Styracosaurusreappears. Gwangi battles and kills the Styracosaurus and later manages to catch and kill Carlos, but is knocked out while trying to exit the valley in pursuit of the rest of the group.
Securing the creature, Tuck and other men in the group take Gwangi back to town to be put on display in T.J.'s show. On the opening day of the show, a Gypsy dwarf sneaks in and begins to unlock Gwangi's cage in an effort to free him, only to be killed and eaten when Gwangi breaks free. The crowd begins to flee as Gwangi attacks, and Tia Zorina is trampled to death in the chaos. Bromley is crushed by a broken piece of the cage, and Gwangi attacks and kills a circus elephant before rampaging through the town. Tuck, accompanied by T.J. and Lope, tries to hide the crowd in a cathedral, but Gwangi finds them and breaks in. Tuck urges the crowd out through a back exit, leaving Tuck inside with Gwangi, T.J., and Lope.
Gwangi tries to eat Lope, but Tuck manages to distract him by stabbing him with a flag. Tuck is eventually able to throw a torch onto the floor near Gwangi, setting the building on fire. Tuck and the others manage to escape and lock the door behind them, trapping Gwangi in the burning building. Roaring in agony, Gwangi dies in the fire as Tuck, T.J., the crowd, Lope (with tears in his eyes) and the townspeople look on.
- James Franciscus as Tuck
- Gila Golan as T.J.
- Richard Carlson as Champ
- Laurence Naismith as Professor Bromley
- Freda Jackson as Tia Zorina
- Gustavo Rojo as Carlos
- Dennis Kilbane as Rowdy
- Mario De Barros as Bean (as Mario de Barros also)
- Curtis Arden as Lope
Gwangi was originally conceived by Willis O'Brien (1886-1962), the man who created the special effects for the original King Kong (1933). The plot was inspired by Sir Arthur Conan Doyle's book The Lost World (1912), with added elements from King Kong (capturing a monster and bringing it to civilisation where it runs amok). In O'Brien's scenario, then called Valley of the Mists, cowboys discover an Allosaurus in the Grand Canyon. After finally roping the dinosaur, they put it in a Wild West show, but the creature, now called Gwangi, breaks free and fights lions in the show that have also escaped. After killing the lions, Gwangi goes on a rampage around the town and is run off a cliff by a man in a truck. O'Brien died before The Valley of Gwangi was filmed. A similar film, The Beast of Hollow Mountain, appeared in 1956, produced by O'Brien and a Mexican film company with the same story. Harryhausen was not involved with the visual effects for that film.
Gwangi was described in O'Brien's original script as an 'Allosaurus', although O'Brien apparently didn't draw much distinction between Allosaurus and T. rex, as he also referred to the T. rex in the original King Kong (modeled by Marcel Delgado) as an allosaur. According to Ray Harryhausen, his own version of Gwangi (and O'Brien's Gwangi too, as well as Delgado's Kong T. rex) was based on a Charles Knight painting of a T. rex - one of the two most famous paintings by Knight, and one that is instantly recognizable by the eye being placed too far forward on the skull (this was based on concurrently incomplete skeletal remains and the eye was mistakenly placed in one of the nasal sockets), as well as incorrectly portraying T. rex with a three-fingered hand. This famous T. rex image is also reflected in Harryhausen's Rhedosaurus in 'The Beast From 20,000 Fathoms', and even in the 2005 King Kong's 'Vastatosaurus'. In a DVD interview Harryhausen said "We sometimes called it an 'Allosaurus'... They're both meat eaters, they're both Tyrants... one was just a bit larger than the other." Gwangi was scaled to be 14 feet tall, within the size range of an average adult T. rex (although not the largest), and at the upper limit of the largest Allosaurus specimens.
The Valley of Gwangi was the last dinosaur-themed film that Harryhausen animated, and he made much use of his experience in depicting extinct animals from his earlier films. Close to a year was spent on the special effects (there were over 300 'Dynamation' cuts in the film, a record number for Harryhausen), with the roping of Gwangi being the most labour-intensive animated sequence. It was achieved by having the actors hold on to ropes tied to a "monster stick" that was in the back of a Jeep. The jeep and stick when filmed with Gwangi are on a back rear projection plate and hidden by his body, and the portions of rope attached to his body are painted wires that are matched with the real ropes. The coordination of Gwangi's animation with live actors on horseback (and the horses appearing to react to Gwangi) was particularly difficult to film, and the source of an editorial lapse in a following scene. Gwangi bites through the ropes around his neck when first lassoed and later has his jaws roped together when unconscious. However, he is then shown being transported in a cart again held only by ropes around his neck but with jaws now un-bound.
The first animated sequence in the film is a diving act done by T.J. and her horse. Because it was decided that it was too risky to have a rider and horse jump off a 40-foot high platform into a tank of water, a model horse and rider were used. After tempting Gila Golan's horse to jump from a mock-up platform onto a trampoline, the film cut to an animated model suspended on wires (actually it was just a tiny toy horse and rider bought in a toyshop). The splash was real, triggered by an electric charge inside the tank.
After local Gypsies steal the Eohippus, it is released into the Forbidden Valley. For all the scenes where the cowboys are chasing the creature, the animated model was used. However, in one long shot, a baby goat was used instead because the model would have been too small.
The pterosaurs were mistakenly given bat's wings (with elongate fingers supporting the membrane; pterosaurs had one finger forming the wing's leading edge but none on the membrane). The wings appear to mimic those of a pterosaur from an earlier Harryhausen film, One Million Years BC (1966). Close-up sequences of the pterosaurs in Gwangi were provided by life-size models. For the scene when Lope is snatched from his horse by the Pteranodon, the boy was raised by wires painted out in the studio and Harryhausen animated the eight-inch high model pterodactyl to correspond with his movements. However, once the creature gets up to a certain altitude the real boy was replaced with a model which was used until he crawls away from the creature which is being killed by Carlos on the ground. Bromley the paleontologist mistakenly calls it a Pterodactyl (a common error) while he is inspecting it on the ground.
The scene where Gwangi pounces on the Ornithomimus has been copied many times in dinosaur movies, primarily in Jurassic Park, when the Tyrannosaurus Rex pounces on the raptors. The smaller dinosaur had never appeared on the movie screen before and although its movements were unlikely it was one of Harryhausen's favorite sequences in the film. The battle between Gwangi and the Styracosaurus features battle moves such as biting, stabbing, butting, and pinning. After Gwangi is captured, he is wheeled back to town in a cart and for several of the long shots of this scene the crew built and used a full-sized mock-up of Gwangi, again because an actual model would have been far too small.
Harryhausen originally planned to have used a real elephant in some of the scenes for the fight with Gwangi. This did not work out because he wanted to have used a 15-foot tall elephant (the world's biggest elephant was two feet shorter than this). So the live 8-foot elephant was only used in the beginning when T.J. is seen briefly riding on its back. For some of the elephant fight scenes Harryhausen used the animation table as the bullring floor.
For Gwangi's death scene a number of special effects were used. When the torch hits the ground near Gwangi, the flames are seen quickly developing and surrounding Gwangi. The flames were added in by double printing the camera. The outside of the burning church was a mixture of composites: the lower half was the real church photographed on location in Spain, and the upper, burning half was a miniature, again added in by double printing the camera.
The model of the Eohippus was supposed to have had toes but appears to have had regular hooves with 'toes' painted on (the sound effects of the animal moving also resemble hooves). The model of the Styracosaurus featured an inflatable air 'bladder' to simulate the animal breathing heavily after its combat with Gwangi (a feature first used in models made for much earlier films by Marcel Delgado).
Some of the models used in the film featured were reused model armatures from earlier films. Gwangi, the Ornithomimus, and the Styracosaurus were all made from the Ceratosaurus, the phororhacos, and the Triceratops, who were stripped down and had their armatures modified for further use. The actual model of Gwangi was about 12 inches high and the Ornithomimus was about 8 inches high. A solid-latex, non-armatured model of Gwangi was also used for the scenes when he knocks himself out while trying to exit the valley in pursuit of the cowboys (Harryhausen was never pleased with this, as the solid model didn't look right).
Actress Gila Golan's Israeli accent was so strong that all of her lines were redubbed on the film by a voice actress. Actor Laurence Naismith, who plays Professor Bromley, had earlier appeared in Harryhausen's Jason and the Argonauts as the shipbuilder Argos.
The movie was filmed in Almería and Cuenca, Spain. The bullring scenes were shot in Almería's plaza de toros and the finale at Cuenca's cathedral. The unusual rock formations of Ciudad Encantada near Cuenca were used for the forbidden valley.
By the time of the film's release, interest in 'monster' films of this type was waning. Management at Warner Brothers and Seven Arts also changed and the film was released with little promotional effort on a double-bill with a biker film; it thus missed its target audience and was not as successful as earlier Harryhausen efforts.
The scene where Gwangi suddenly appears from behind a hill and snatches a fleeing ornithomimus in his jaws was later copied in the big-budget dinosaur movie, Jurassic Park.
During the 1980s hit TV series Scarecrow and Mrs. King, anytime a television was shown on in the series, The Valley of Gwangi was on the screen.
Justin Parpan's 2006 children's read-aloud book, Gwango's Lonesome Trail (Red Cygnet Press, Inc., ISBN 1-60108-004-2) features a pre-historic dinosaur named "Gwango" roaming the contemporary American Southwest.
In an episode of the situation comedy Friends, Ross watches the movie while in a hospital.
In the 2011 animated movie, Scooby-Doo! Legend of the Phantosaur, during a night time chase scene through the town a movie theater can be seen in the background playing two dinosaur-themed monster movies,The Valley of Gwangi and The Beast from 20,000 Fathoms (another Harryhausen film from Warner Bros).