Maj. Joe Nolan leads a South Pacific expedition to recover an atomic-powered rocket that vanished without a trace. His fellow serviceman and pilot, Lt. Danny Wilson, joins him, along with Sgt. Willie Tatlow. The mission includes three scientists who helped build the rocket, among them Michael Rostov, a Russian-born physicist, and Robert Phillips, another scientist on board. Their transport aircraft crash-lands on a remote, unknown tropical island in the area where the rocket was lost on radar. Only two occupants survive the crash: a native woman and her young brother, the latter of whom is cared for by the Native Girl, whom explorers later refer to as such. The woman explains that something fell from the sky onto the cloud-shrouded plateau that dominates part of the island, and the rocket’s fiery arrival drove the rest of the native population to abandon the island.
The expedition presses on, seeking clues amid a harsh trek that ends on the escarpment. Stanley Briggs dies accidentally during the steep ascent, a loss that marks the team as they fight through rock faces and treacherous ground. After exhausting climbs, they emerge from a toxic gas cloud cover into a lush, prehistoric jungle that hides more than just ancient flora. A vast field of uranium lies beneath the surface, a discovery that explains why their electronic tracking equipment failed long before. Early encounters with prehistoric life unfold: a Brontosaurus charges as Robert Phillips retreats up a tree, bullets bouncing off the creature’s thick hide while the others try to keep him safe. That night, the group camps, their nerves frayed but their resolve intact, while the island’s ominous stillness intensifies the danger around them.
Morning brings tension as Michael Rostov and Phillips vanish from sight, and Nolan is quick to suspect Rostov of foul play, pointing to the earlier near-miss that could have allowed Briggs to fall. Rostov protests, insisting he was simply trying to help Phillips. The tension between science and survival deepens as a Triceratops nearly attacks, only to be rivaled by another Triceratops that challenges and fights the first to the death, leaving the group momentarily shaken but safer for the moment. Rostov opens up about a painful past, revealing that he is a Holocaust survivor who lost his wife and unborn child, a confession that adds a heavier weight to his presence on the expedition.
As food becomes scarce, Wilson shoots a Pterosaur for sustenance, and the team discovers the rocket’s surroundings are ringed by more dinosaurs—Brontosaurus and a pair of Triceratops. Nolan devises a careful strategy using their weapons to deter the prehistoric guards and allow Rostov and Phillips to retrieve the data they came for from the rocket. The mission’s fragile balance shatters when Tatlow is fatally gored by a wrathful Triceratops, a tragedy that unfolds just as Nolan and Wilson deliver the final blow to the animal. They bury their comrade and press on, but the planet beneath them trembles with violent earthquakes.
With time running out, the four remaining survivors—Nolan, Wilson, Rostov, and Phillips—make a desperate retreat down the plateau toward the island’s flatlands, choosing an outrigger canoe as their escape route. From a safe distance, they watch as the island endures further seismic upheavals, culminating in a catastrophic eruption of the formerly dormant volcano that consumes the island and everything on it. In the end, the quartet remains afloat on the calm sea, their hope pinned to the horizon as the island’s fate closes behind them.
