The first American spaceship to Venus, the XY-21, crashes into the Mediterranean off the coast of Sicily. Fishermen row to the slowly sinking spaceship, crawl through a hole, and pull two still-living spacemen out before it sinks, though one dies in the hospital. In Washington, Major General McIntosh (Thomas Browne Henry) learns that the ship, piloted by Colonel Bob Calder (William Hopper) has been located. A little boy named Pepe (Bart Bradley) finds and opens a cylinder marked USAF on the beach, then sells the gelatinous mass inside to a zoologist named Doctor Leonardo (Frank Puglia). His granddaughter Marisa (Joan Taylor), a medical student, is taking care of the injured spacemen. Calder awakes to find his crewmate Doctor Sharman (Arthur Space) dying of the disease that killed the others.
After Marisa returns to the trailer where she lives with her grandfather, a small creature hatches and Leonardo locks it in a cage. In the morning it has tripled in size. General McIntosh arrives with Doctor Justin Uhl (John Zaremba) and meets with the Italian government. Marisa and Leonardo hitch the trailer to the truck and drive to Rome, where he wants to confer with other scientists. Police begin to search for the Venusian species known to have been on the ship. For the reward, Pepe leads them to the empty container, and they follow Doctor Leonardo.
By nightfall, the creature is human-sized. It breaks out of the cage and heads for a nearby farm. It terrorizes the animals, breaks open some bags of sulphur to eat, and kills a dog that attacks it. Calder arrives at the barn, trying to trap the creature and force it into a cage, but the farmer attacks it with a pitchfork, and it injures the farmer. It breaks out and disappears into the countryside. The police commissioner wants to destroy it.
The Italian Government gives Calder permission to capture it if he can, so he plans to drop a giant electric net over it from a helicopter, but the police attack it with flamethrowers. As it has no heart or lungs, bullets do not harm it. Calder uses sulphur as bait and jolts it with the net. Later, reporters observe it sedated in the Rome Zoo. Marisa has eyes for Colonel Calder. The equipment shorts out, and the creature revives and escapes.
It battles a zoo elephant and sends the crowds running. The two beasts rampage through the streets, damaging cars and buildings. It nearly kills the elephant. Calder rams it with his car and it dives into the Tiber. Soldiers toss grenades into the river after it, and it emerges, grown to monstrous size. It hides in the Colosseum and bazookas drive it to the top. The ledge on which it stands is destroyed with tank-and bazooka-fire and it falls to its death. Marisa and Calder embrace.
The creature is called an Ymir, though this is not mentioned in the movie. Its voice was the trumpet of an elephant recorded at high speed. The movie was placed in Rome instead of the original Chicago because Harryhausen wanted to vacation there. The film was colorized by Harryhausen and Legend films five years after the death of the director. Critics loved the fact that the puppet had a well-defined personality and evoked sympathy for its plight.
Ray Harryhausen appeared as a man feeding the elephant. The elephant wasn’t tall enough, so they hired a short man to play the zookeeper. It was Harryhausen’s last film in black and white. He began to turn away from SF toward Fantasy at this point. The distance from Venus to Earth is 23.6 million miles, pretty close to reality for a movie. The Ymir liked sulphur because Venus is covered in volcanos, they thought at the time, and that’s what his species lived on. Gene Simmons of Kiss patterned his stage movements on the Ymir.