The movie opens with title and credits amid a scene of two mountain peaks and a cloud filled valley. At the U.S. Government Proving Grounds in White Sands, New Mexico a V-2 missile is launched. In the control room its progress is monitored by civilian and military personnel. The rocket scientist, Dr. Michael Rostov (John Hoyt) asks Dr. Robert Phillips (Hugh Beaumont) for altitude and distance. They are testing a new type of propulsion, an atomic powered engine. It was tracked, but it failed to respond to commands to turn. Rostov asks Dr. Stanley Briggs (Whit Bissell) to calculate its location traveling for three more hours at its present rate of speed--where would it crash? The project director, Simmons of the AEC, tells the assembled military representatives that he needs their help to locate and recover the rocket.Sitting in front of a fireplace having coffee are Major Joe Nolan (Cesar Romero) and his date for the evening, Marla Stevens (Hillary Brooke). They engage in small talk and it is becoming clear to both that he doesn't remember their last date. They dance in her living room, and then kiss. The doorbell rings, and Marla goes to answer it. An Air Police Sergeant (Murray Alper) is there looking for Major Nolan. He is to report back to the base immediately. Sgt. Willie Tatlow (Sid Melton) is a mechanic and replacing parts in an airplane when the Air Police Sergeant comes to get him to report to the base Commanding Officer. Lt. Danny Wilson (Chick Chandler) is next on the list for the Air Police. He is just about ready to go on leave when he is summoned by the C.O. They depart in a DC-3, with Nolan as Pilot, and Wilson as Co-Pilot. Tatlow is the mechanic on this flight. They have been tasked with locating Rostov's missing rocket. On board are: Rostov, Phillips and Briggs. Tatlow, who hates heights, gives the science team coffee. He brings coffee to the flight crew. Nolan goes back to talk to the scientists and tells them they will be landing for fuel in about three hours. They land, refuel and resume their flight plan. They reach the point where the rocket was calculated to run out of fuel. They spot an island and change course. Their radiation detectors respond, as the Geiger counter shows a very high level of radiation. It seriously affects the airplane and it crashes on the island. Fortunately trees and underbrush cushioned the impact and no one is seriously injured, although Briggs did cut his leg. Briggs tells Nolan that when the airplane went out of control the Geiger counter detected intense radioactivity. They exit the airplane.Now armed and supplied with food and water, they trek through the jungle. They encounter what appears to be a deserted village. They discover a boy and young woman (Acquanetta) are the only inhabitants of this island. She describes the rocket crashing on the island and the natives leaving the island because it was considered a bad omen. The brother and sister stayed to help their injured father. She escorts the party to the sacred mountain, but refuses to accompany them any further. She claims no one who goes ever returns. They begin the long climb to the top.Nolan leads the climbing party. At one point he encounters volcanic gas that nearly kills him. They now know why the mountain was classified as taboo and the reason no one returned. Rostov seems to be overly concerned with Briggs' ability to make the journey. Nolan takes note of their discussion. They double back then find another route up the mountain. They bivouac in a sheltered section of a ledge for the evening. An approaching storm provides both thunder and lightning. They engage in small talk while they wait out the storm. Rostov is off by himself and one bolt of lightning reveals a large lizard above their encampment. Rostov calls out, but no one else sees the creature. It starts to rain and they huddle inside the shallow cave.The next morning they continue their climb. The storm has passed but they are reaching an altitude where they are in the clouds. Rostov is having to help Briggs with the ascent. At one point Rostov is helping Briggs, but he loses his grip and Briggs falls off the mountain to his death. Nolan does not believe Rostov's story. They reach the plateau on top of the mountain. (Note: The black and white film is now tinted green and white.) The landscape is now like a prehistoric jungle. Phillips explains that, "You are looking at a kind of world that hasn't existed for millions of years." They notice the air is thin but there's plenty of oxygen.Nolan and Wilson rest, smoke and talk about Rostov. He is concerned that only three people are familiar with the rocket: Rostov, Phillips and Briggs, and now Briggs is gone. He voices his concern for the safety of Phillips. Rostov, Briggs and Sgt. Tatlow pick up radiation and a general direction. The party continues their trek in search of the lost rocket. Wilson is taking a break and eating when he notices some tracks. Phillips notes that he recognizes the tracks from a trip to a museum. They are Brontosaurus tracks.In a clearing they see the Brontosaurus eating. It charges them. Phillips stumbles and climbs a tree to escape the beast. It bumps the tree to dislodge the scientist while the party on the ground fires at it with their rifles. The tree snaps, but Briggs is relatively unscathed. They camp out for the evening around the fire.The next morning Nolan wakes first and notices that Rostov and Phillips are gone. They search for them. They spot Rostov tending to Phillips. Nolan immediately assumes he is trying to kill Phillips. Rostov explains that Briggs wedged his foot in the rocks and he was trying to assist. Rostov then alerts the party to an approaching Triceratops. It confronts a second member of its own species and they begin to fight. That distraction allows the humans to free Phillips. Phillips explains that he woke up early and decided to do some photography. He assures Nolan that Rostov saved him. Nolan takes the opportunity to apologize. Rostov tells Nolan he is Russian and is used to suspicion.While resting, Phillips discovers they are right in the middle of the Uranium field. It is stronger than the radiation from the rocket so there is nothing more to guide them. Nolan asks, "But how do we find the rocket?" "I wish I knew," is Phillips reply.They are low on food, water and ammunition, but they continue the search. They shoot and kill a Pterodactyl as a potential meal and as they search for where it fell, they discover the rocket. As they walk to it the Brontosaurus and Triceratops surround the rocket. They scare them away with gunfire. This gives Rostov and Phillips a chance to enter the rocket and retrieve their equipment with the rocket's telemetry. Unfortunately Willie Tatlow is mortally wounded by one of the dinosaurs. They bury him then begin their descent down the mountain. They manage to avoid injury during a rock fall. The volcano is becoming active. They hasten their descent. The island is starting to come apart. They enter the village, and then find their way to the ocean and a few abandoned canoes. They paddle away and are witness to the island exploding and sinking into the ocean. We close with the four survivors on the canoe looking back.
Output: Question: Why does Tatlow die?, Answer: He is mortally wounded by a dinosaur.