Adventure-seeker Ted Osborne (Phillip Reed) and his fiancee Carole (Virginia Grey) are at a cafe in Singapore, looking for a charter to an island supposedly inhabited by dinosaurs. They come across the ruthless, two fisted, alcohol-suffering Captain Tarnowski (Barton MacLane). They decide to talk, and Osborne asks is Tarnowski is willing to give them a charter in his ship to the unknown island. Initially Tarnowski refuses, but then Osborne tells that during World War II he was a pilot in the US Navy. He had flown over many remote islands, and on one he reported seeing large, moving things that looked like Brontosaurus dinosaurs. He shows Tarnowski a photo, and the captain finally agrees to take them there. Before departing, Tarnowski introduces them to John Fairbanks (Richard Denning), an old friend of his, who agrees to help them with their quest. Fairbanks and a group of friends had been shipwrecked on the island with Fairbanks being the lone survivor of attack by the dinosaurs. As Fairbanks had been drinking incessantly to forget the events of the past since his rescue, his account of the dinosaur island was believed to be the result of alcoholism and insanity. Since Fairbanks and Osborne's stories collaborated, Tarnowski agrees to allow his ship to be hired, but specifies that no one in the crew be told of their destination.
They sail aboard Tarnowski's ship, heading for weeks in the direction of French Polynesia, where the island is supposed to be located. During the voyage, it becomes clear that Fairbanks is obviously becoming attracted to Carole, even though she is the fiancee of Osborne. Tarnowski acts mad at Fairbanks because he is afraid of the dinosaurs, and that the island might not be a safe place for Carole.
However, unknown to almost everyone, a group of unfriendly natives have been hiding in the hold of the ship. Before the crew reaches the island, they mutiny and attack the crew. Fortunately, they lose the fight and afterwards agree to help them on the island.
The next morning, the ship's first mate spots the island in the distance. As they approach, he notices feeding Brontosauruses. With the conformation of living dinosaurs inhabiting the island, they become cautious. The natives help them unload supplies, and a camp is set up in a forest clearing not far from shore. Whilst exploring the island, they find more Brontosaurus and later, a dimetrodon (a sail-backed synapsid). Then, as they venture out onto the flats of the island, they find Ceratosauruses. Knowing these creatures can be a threat, the natives are asked to guard the camp at all times.
After a near fight between Fairbanks and Osborne because of Carole's safety, a giant sloth appears and prowls around the camp. Carole screams and the men are alerted, but they soon fend it off with gunfire. The next morning they go back to the flats where they find even more Ceratosaurus. The natives try to persuade them to leave and when the first mate agrees with them, he gets into a fight with Tarnowski and is killed when one of the natives accidentally throws a knife into his back. The native is shot and killed by Tarnowski while the crew focus on driving away the dinosaurs with grenades and gunfire. Fairbanks still tries to convince Osborne to leave the island but still he refuses. Eventually though, he feels he has observed the dinosaurs long enough and decides to leave shortly. Tarnowski, however, will not leave the island until he brings back one of the dinosaurs alive, which none of the other men think is right. He starts saying bad things about the men due to the overdose of whiskey he has been drinking.
The natives, knowing that the crew are not treating them well, try to steal the lifeboat and get off the island. The men are alerted, but the natives are able to dodge their gunfire and escape. However, they are all killed when a giant wave topples and smashes the boat. They come back to the camp and find it destroyed, because Tarnowski carelessly threw away his match. Knowing that they must soon leave the island, they gather wood and begin building a raft. While Fairbanks and Osborne are working on the raft, Carole is obtained by Tarnowski, who plays a cruel trick on her. A Ceratosaurus attacks the pair but Tarnowski kills it with a grenade. Tarnowski carries her off, while Fairbanks and Osborne argue about her safety again. Tarnowski tries to convince Carole to leave the island secretly with him but she refuses, and eventually, after he says he will protect her, she doses off with him. Fairbanks meanwhile sets off in search of Carole.
While Carole and Tarnowski are waking up, the dimetrodon attacks again. Carole is able to fend it off with Tarnowski's gun, and as he tries to kiss her Fairbanks arrives. The two men fight and Tarnowski is knocked out, giving Fairbanks and Carole time to escape. Then, the giant sloth reappears and devours Tarnowski. As Fairbanks and Carole reach the shores of the island, they are trapped behind a rock during a fight between the giant sloth and a Ceratosaurus. The dinosaur loses and the sloth walks away. The two are shortly reunited with Osborne and another passenger, and the four break camp and leave the island on Tarnowski's boat, looking back as the island slowly disappears from sight.Virginia Grey as Carole Lane
Phillip Reed as Ted Osborne
Richard Denning as John Fairbanks
Barton MacLane as Captain Tarnowski
Dick Wessel as ship's first mate
Dan White as Edwards
Phil Nazir as Chief Lascar
Ray "Crash" Corrigan as the giant sloth
Snub Pollard as a Singapore Barfly
Unknown Island is not in the public domain. The copyright (Certificate # R 630 246) was renewed on April 22, 1976 with rights assigned to Wade Williams.