The first U.S. spaceship to Venus, the XY-21, crashes into the Mediterranean sea off the coast of Sicily, Italy as fishermen watch. They row to the spacecraft, enter through a hole in the spacecraft and pull two spacemen from the nose-down craft before it completely sinks.
In Washington, D.C., Major General A.D. McIntosh discovers that the missing spaceship, piloted by Colonel Bob Calder, has been located. As McIntosh flies to the site, Pepe, a little boy, finds and opens a translucent cylinder marked “USAF” on the beach. It contains a gelatinous mass, which he sells to Dr. Leonardo, a zoologist studying sea creatures. Meanwhile, Leonardo's granddaughter Marisa, a third-year medical student, is summoned to take care of the injured spacemen. When Calder regains consciousness, he finds his crewmate, Dr. Sharman, in the last throes of the fatal disease that killed his other eight crewmen.
After Marisa returns to the trailer shared with her grandfather, a small creature hatches from the mass, and Leonardo locks it in a cage. By morning, the creature has tripled in size. McIntosh arrives, accompanied by scientist Dr. Justin Uhl, and meets with two representatives of the Italian government, informing them the spaceship has returned from Venus. Leonardo and Marisa hitch the trailer to their truck and head for Rome. Calder's spacecraft carried a sealed metal container bearing an unborn Venusians species. As police divers begin to search for it, McIntosh offers a half-million lira reward for the capsule's recovery, prompting Pepe to lead them to the empty container. When Pepe tells them that he sold the mass to Dr. Leonardo, McIntosh and Calder pursue him.
That night, Leonardo discovers that the creature has grown to human size. Soon after, it breaks out of the cage and heads to a nearby farm, terrorizing the animals. The creature eats sulfur and rips open several bags it discovers in a barn. While feeding, the creature is attacked by the farmer's dog, and the creature kills the dog, alerting the farmer. Calder and the others reach the barn, trapping the creature inside. Calder explains that the creature is not dangerous unless provoked. However, he provokes it by trying to prod the creature with a pole into a cage-like cart, and the creature injures the farmer after he stabs the creature with a pitchfork. After the creature breaks out of the barn and disappears into the countryside, the police commissioner insists that it must be destroyed.
After the Italian government grants Calder permission to capture the creature alive, he devises a plan to ensnare it in a giant electric net dropped from a helicopter. The Italian police conduct their own pursuit, shooting at it with flamethrowers. Gunfire has little effect on the creature, as it has no heart or lungs. Aware that sulfur is the creature's food of choice, Calder uses bags of sulfur as bait, luring the creature to a secluded site and subduing it with an electric jolt from the net. Later, at the American Embassy in Rome, McIntosh briefs the press corps and allows three reporters to view the creature, which has been placed in the Rome zoo. There, Calder explains that the creature is being sedated with a continuous electric shock so it can be studied. Marisa, who is aiding her uncle, begins flirting with Calder. Suddenly, electrical equipment shorts out and the creature awakens.
The fully grown ymir from 20 Million Miles to Earth.
The creature battles a zoo elephant and sends the panicked patrons scurrying. Taking their fight to the streets of Rome, the two beasts destroy cars and damage buildings. The creature finally kills the elephant and continues its rampage. Calder rams the creature with his car and tracks the creature to the River Tiber, where it submerges. Soldiers lob grenades into the river, and the creature surfaces, now gargantuan in size. It heads for the Colosseum and destroys an ancient temple, killing a few soldiers. As the creature disappears into the ruins, Calder charges after it with a group of bazooka-carrying soldiers, driving it to the top of the structure. Calder scores a direct bazooka hit. Direct fire from a tank then destroys the ledge the creature is clinging to, and it plunges to its death. A relieved Marisa runs into Calder's arms.
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