Adventure-seeker Ted Osborne has convinced his finacee Carole to finance his expedition to an uncharted South Pacific island supposedly populated with dinosaurs. Piloting their ship is Captain Tarnowski, a ruthless alcoholic suffering from malaria- induced bouts of insanity. When they arrive at the island, they discover that the stories they have heard are all true. Will they survive to tell anyone what they've found?

In Singapore, Ted Osborne and his fiancée, Carol Lane, charter Captain Tarnowski's boat for an expedition to find and photograph the prehistoric creatures that Ted discovered on a remote island while he was a navy pilot in the South Pacific. Tarnowski suggests that they take along a local drunk, John Fairbanks, who claims to have been on the island. Fairbanks, however, does not want to return there as several of his shipmates were killed by the "monsters," but Tarnowski shanghais him. The steamer leaves and soon they are well beyond regular shipping lanes, a fact which causes dissension among the lascar crew. The expedition reaches the island and, through binoculars from the ship, sees a prehistoric creature. Ted, Carol, John, the Captain and a few others go ashore in a small boat, and as they move inland, they encounter dinosaurs. After they have made camp, one of the seamen is attacked by two of the creatures, and the captain is forced to shoot the man to spare him further agony. This incident provokes more unrest among the lascars, who want to leave and return to the ship. John also advises Ted to get everyone off the island before they are killed, but Ted refuses. A giant sloth-like creature then threatens the camp but is distracted by another beast. Life is further complicated when the lecherous Tarnowski grabs and kisses Carol and is threatened by Ted. Carol insists upon leaving, and although Ted wants more pictures of the giant sloth, he agrees to go the next day. However, Tarnowski refuses to leave until he captures one of the creatures alive. John leads the party farther inland, where they are confronted by several flesh-eating tyrannosauri. When first mate Sanderson tries to turn back, he and the captain begin to fight. One of the crew throws a knife, killing Sanderson, and is shot by the captain. The creatures move toward the party but are scared off by grenades the captain has brought. Later, Tarnowski suffers a recurrence of jungle fever. Although he allows Carol to return to the ship, he orders Ted and John to remain to help with the intended capture. Carol cannot leave, however, as the lascars have taken the small boat, leaving the others marooned on the island. The stranded crew attempt to build a raft, unaware that Tarnowski has found and concealed another small boat. The captain attacks Carol again, but is interrupted by a dinosaur, which he then kills with grenades. In the meantime, Carol has fainted and Tarnowski carries her off, planning to take her back to the ship and abandoning the others. When Ted refuses to look for Carol, John tells him he is a coward and sets off to find her himself. While Tarnowski is sleeping, Carol takes his gun, but is forced to use it on one of the creatures. John arrives in time to save Carol from Tarnowski, and they both return to the beach in time to see Tarnowski fall victim to the giant sloth. Later, as John and Carol look on, the sloth vanquishes a tyrannosaurus. Back on board the ship, Ted realizes that he has lost Carol to John.