Scientists at an observatory discover that a new planet has moved into Earth's solar system and is orbiting the sun. An emergency session of Congress is called. It is decided that as the new planet is so close to earth, the United States should strive to be the first to explore it. Over the next few months, teams of scientists work to build a stable spaceship and devise a satisfactory method for the astronauts to obtain power on the planet. An astronomer is able to photograph the new planet, which he names Nova, and prove that it has plant life, indicating that its atmosphere is capable of sustaining humans. The aerospace engineers succeed in building a spaceship and a device that can supply electrical power to the explorers after they land. The device is activated by atomic power and is capable of providing an atomic explosion if needed. In order to explore Nova fully, it is decided to assemble a group of scientists representing a broad range of disciplines. So zoa-geographer Dr. Ralph Martin, minerologist Dr. Nora Pierce, physician Richard Gordon and chemist Dr. Patricia Bennett are assigned.
Soon after, the ship takes off and begins its months-long journey without any mishaps. Upon landing on Nova, Ralph and Pat, wearing cumbersome space suits, test the atmosphere and soil, determining that it is safe for them to explore. Shedding their gear, Ralph and Pat are joined by Nora and Dick. During a brief exploration, they find a freshwater lake. Nora is intrigued by an island situated in the middle of the vast lake, as the vegetation on the island is clearly different than that of the forested area in which they have landed. The others insist on conducting their experiments before exploring any further, and so the group hikes to a rock formation several miles away.
After Nora gathers her samples, the scientists attempt to find their way back but get lost as darkness falls. When an exhausted Pat is frightened by a snake, Ralph insists on camping for the night and resuming their search for the ship in the morning. The group builds a serviceable shelter and the two men begin alternating watches while the women sleep. During Ralph's watch, Pat awakens, and the couple, who have fallen in love, slip away from camp in order to kiss privately. Ralph falls down a hillside, and is attacked by a fierce beast resembling an alligator. Pat's screams alert Dick and Nora. After Ralph vanquishes the animal, the others carry him back to camp.
In the morning, Ralph is still unconscious from his injuries. Dick and Nora decide to return to the ship for supplies while Pat watches over Ralph. While Nora and Dick are gone, Ralph regains consciousness, although Pat's happiness over his recovery dissipates when they are approached by an enormous insect. Ralph shoots and kills the insect. Pat's spirits revive when the others return, for they are accompanied by a kinkajou, a friendly monkey-like mammal. Named Joe, the little animal quickly becomes the pet of the group, and the scientists decide to continue their experiments at the camp until Ralph is well enough to travel.
The next day, Nora and Dick use an inflatable raft to travel to the island to conduct their studies, while Ralph and Pat stay behind. Nora and Dick are horrified to discover that giant, dinosaur-like beasts inhabit the island and are chased by one into a cave. Dick is injured when he saves Joe from being eaten by the beast but is able to return to the cave, where he is tended to by the frightened Nora. After watching the enormous lizard best an opponent in a bloody fight, Dick compares it to a Tyrannosaurus Rex, the "king dinosaur". Dick is able to send up a distress flare, and Pat and Ralph respond by taking another raft over to the island. Unable to reach the cave, however, Ralph and Pat watch helplessly as their friends must run for the beach while the king dinosaur battles another foe. Dick, Nora and Joe reach the beach, where Ralph sets the timer on the atomic power source to ignite in half an hour. Despite being chased by the beast, the four friends reach their raft and frantically paddle to the other side of the lake. Having reached safety just in time, they watch as the power source explodes, killing all of the prehistoric beasts in an atomic explosion. Wryly commenting that they have brought "civilization" to Nova, Dick and the others prepare to return home.
- Directorial debut of Bert I. Gordon.
- Several portions of the soundtrack were re-used in The Violent Years.