Maj. Joe Nolan (Cesar Romero) is the head of an expedition to the South Pacific to retrieve an atomic-powered rocket that vanished without a trace. He had previously lived in a South American jungle, as has fellow serviceman and pilot Lt. Danny Wilson (Chick Chandler), who is also on the expedition. Aircraft mechanic Sgt. William Tatlow (Sid Melton) is also recruited for the expedition, which includes the three scientists who helped build the rocket.
Their transport aircraft mysteriously crash-lands on a remote, unknown tropical island in the area where the rocket was lost on radar. They find only two occupants left on the island, a native woman (Acquanetta) and her young brother. The woman indicates something fell from the sky atop a forbidding, cloud-shrouded plateau that dominates part of the island. The rocket's fiery arrival caused the rest of the native population to abandon the island.
Expedition member Stanley Briggs (Whit Bissell) is accidentally killed on the steep ascent of the escarpment. After long stretches of tedious rock climbing, the expedition finally closes in on the top. Emerging from what turns out to be a toxic gas cloud cover, they discover a lush, prehistoric jungle inhabited by various dinosaurs and a large field of uranium, which is what has disabled their electronic tracking equipment.
The group comes upon a Brontosaurus, which then attacks Robert Phillips (Hugh Beaumont) as he quickly retreats up a tree. This results in Nolan and Wilson shooting at it, but they quickly discover that the dinosaur's thick hide absorbs bullets with little effect. Later that night they set up camp. When Nolan wakes up, he finds Phillips and Russian scientist Michael Rostov (John Hoyt) gone. He then discovers that Rostov got himself stuck in a large rock near a Triceratops; he accuses Phillips of arranging the accident on purpose, but Rostov insists that he tried to help Phillips escape. The Triceratops nearly attacks the group, but another makes a challenge and the two dinosaurs fight to the death.
Nolan suspects that Rostov, the scientist who helped make the rocket, is up to no good because he also appeared to be able to save Stanley Briggs on their climb up but didn't. Eventually Rostov reveals himself to be a victim of the Holocaust in which he lost his wife and unborn child.
A Pterosaur is later shot by Wilson for food near the rocket's landing site. They soon discover the rocket is surrounded by a Brontosaurus and a pair of Triceratops, but Nolan devises a strategy that results in their successfully scaring off the dinosaurs using their weapons. Rostov and Phillips retrieve the needed data from the rocket. With his back turned, Tatlow is then gored to death by an angry Triceratops, just as it is being shot by Nolan and Wilson. The team digs a grave and makes a cross marker for Tatlow. When violent earthquake tremors begin, the team makes a hasty retreat down the side of the plateau.
The four surviving members manage to successfully return to the island's flatland with the rocket's critical component, just in time to escape the island using a native outrigger canoe. The survivors watch from a distance as the island is first rocked by more violent earthquakes and then the catastrophic eruption of the island's formerly dormant volcano, which finally destroys everything.
Where is a Pterosaur shot?
Near the rocket's landing site