An American bomber pilot in World War II, Steve Holloway, Mikel Conrad is rescued by a U.S. Navy flying boat after drifting at sea in a liferaft. When he awakens from a coma at a military hospital, he tells doctors how his copilot died when he and three of his crew members, Benny, Richard Monahan, Andy, Morgan Jones, and Ed, Mark Lowell, crashed following a bombing mission. The survivors use a liferaft to finally reach an unknown island after being adrift for eight days.
The island is inhabited by prehistoric dinosaurs and reptiles and a tribe of Amazons, who are cavewomen descended from Druids. The Amazons are led by Sandra, Doris Merrick, and they are being plagued by Neanderthal men intent on kidnapping and mating with them. The Neanderthals had previously killed all of the Amazons’ men, and they fear the aircrew to be in league with “the hairy men.” Steve tries to explain the aircrew’s predicament, but hearing that they were fighting a war further convinces the Amazon's leader, Sandra, Doris Merrick, of the aircrew's ill intent. However, Sandra later frees the men from their cave cell without telling her followers.
The men follow Sandra's recommended escape route but encounter giant bison, prehistory’s flesh-eating plants, and other formidable dangers. Realizing their chances for survival are slim, they return to the Amazons. During their journey back, they are attacked by a Glyptodont, a giant armadillo, but the Amazons come to their aid just in time. Myra, Midge Ware, Valdra, Judy Brubaker, Tennus, Carol Brewster, and Cleo, Autumn Russell reveal that the aircrew’s release was not a friendly act but Sandra's attempt for the indigenous animals and plants to dispose of the four men.
The Neanderthals attack, killing Ed, and the remainder of the aircrew use their pistols to kill them or drive them away. Steve recommends Benny and Andy leave the island in the liferaft. He takes the Amazons to their mountain temple from which they can better defend themselves. Benny and Andy refuse to leave Steve, knowing that the Neanderthals are preparing for another attack, while little of their ammunition remains. The mountain is actually a volcano which suddenly and violently erupts, killing Benny, Andy, and the Amazons. Steve barely makes it back to the liferaft before escaping as the eruption consumes the island.
Despite giving Holloway a serum to restore his speech, the doctors at the hospital find his adventure incredible. When they closely examine the medallion that Sandra gave Holloway, its design proves its Druids origin, verifying his story.
