The movie opens with a long shot of Griffith Observatory and a narrator (Marvin Miller) telling us, "Earth had a new neighbor; a new planet had moved into our solar system." Title and credits follow over a rocky vista in the background.
The building of a passenger carrying rocket ship is started. We are treated to a lot of stock footage of an engine test. The world's largest telescope is able to see the new planet, which is given the name of Nova. There appears to be life on that planet. There follows more aerospace related stock footage. They will be getting an atomic powered battery that can also be used as a bomb. The space ship is now ready for its passengers. We are introduced to the crew. A biologist, Dr. Richard Gordon (Douglas Henderson) is a zoo-geographer. He is holding an animal skull. Dr. Nora Pierce (Patti Gallagher) is the geologist/mineralogist. A medical doctor, Dr. Ralph Martin (William Bryant) is the third member of the team. Dr. Patricia Bennett (Wanda Curtis) is the chemist; her specialty is nuclear or radio-chemistry. On October first it was noted that the position of Nova was in the best location for a space voyage.
A rocket gantry moves towards a V-2 rocket; then moves away. The rocket launches and leaves the atmosphere. Earth-based observers follow the progress of the ship with telescopes and radar. The ship approaches and lands on Nova in a wooded area.
Two scientists exit the ship for some preliminary tests. Drs. Patricia Bennett and Ralph Martin begin their exploration of planet Nova. They take note of the volcano and proclaim the planet is quite young. Their objective is to see if the planet is habitable for humans. The temperature, air pressure, radiation levels, and oxygen content conclude, "Human and animal life, as we know it on earth, can exist in the atmosphere and environment of planet Nova." Drs. Richard Gordon and Nora Pierce join their colleagues on the surface and without protective space suits. They walk a short distance to a lake. They see an island and Nora wants visit it and explore. Nora and Pat decide a bath and clean clothes are in order. The men arm themselves with rifles and carry backpacks, the women carry small sample bags. They walk through a pine forest. Nora checks a few rocks and concurs with an earlier observation that Nova is a fairly young planet. Nora reiterates her desire to visit the island.
They walk back to the ship. Near a fallen tree they stop to rest. Pat leans against it and encounters a snake. Ralph shoots and kills it. They decide to spend the night. They build a shelter in their improvised camp. Ralph and Dick take turns keeping watch. Pat wakes up early and she and Ralph embrace and kiss. They are engaged to be married. They go for a walk, leaving the camp unguarded. An alligator appears in the underbrush. Ralph trips while walking and rolls down a hill right into the alligator. They wrestle. The commotion wakes Nora, and she in turn wakes and alerts Dick to the situation. Ralph manages to kill the gator, and then staggers away where he collapses. Dick carries him back to camp and tends to his wounds.
The next day Dick tells Pat to stay with Ralph, as he is in no shape to travel. He and Nora will return to the ship for supplies. Ralph wakes and spots a giant insect. He kills it with his rifle. Dick and Nora return to camp with supplies and their portable nuclear battery/bomb. They acquire the company of a kinkajou, and they name him Joe. They eat and agree to visit the island. Dick tells the group he and Nora will take the raft over to the island to explore. A snake enters the camp. Dick sees it as it approaches a sleeping Ralph. It is too close to use his pistol. On its own it moves off. The next morning Dick and Nora depart. They stop at the ship to get the raft. Pat and Ralph set up a portable lab and begin tests. Dick and Nora take the raft over to the island. They land and note that it is colder. They wander into a canyon and discover a cave. They hear a roar and see a giant lizard. Dick shoots it with his rifle with very little effect. His gun jams and he pushes Nora back towards the safety of a cave. The lizard chases them into the cave. The opening is too small for it to get them. Dick leaves the cave to get Joe and is attacked and injured. The lizard stays by the opening trying to get Dick and Nora when a giant alligator comes along. The two reptiles fight, watched by a third. Dick uses the flare gun to signal they are in trouble. Ralph and Pat see the signal flare and grab some gear and the atomic battery/bomb. They stop at the ship to pick up another raft and paddle over to the island. The lizard defeats the alligator and returns to the cave to continue his quest for a meal. When Ralph and Pat arrive on the island they spot the other raft and search for their trapped colleagues.
Another lizard fight breaks out. Dick and Nora take the opportunity to escape the cave and climb towards Ralph and Pat. The first lizard defeats the third and starts toward the four scientists. Ralph tells the others, "I brought the atom bomb. I think its a good time to use it." He sets it for thirty minutes. They hastily depart. On their way back to the raft they encounter a giant armadillo and a wooly mammoth. The lizard gives chase as they continue their run for the shoreline. They all board one raft and paddle furiously for the mainland. They reach the safety of the other shore and a shallow depression in the ground. The bomb explodes. They peek over the embankment to watch the rising mushroom cloud. Ralph says, "Well, we've done it." Dick adds, "Yeah, we sure have done it, we've brought civilization to planet Nova." They walk back to the ship. We close with a shot of an expanding mushroom cloud.