The first U.S. spaceship to Venus, the XY-21, crashes into the Mediterranean off the Italian coast, and local fishermen race toward the wreck. Entering through a gaping hole in the nose-down craft, they manage to pull two spacemen from the sinking vessel before it disappears beneath the waves. In Washington, Major General A.D. McIntosh [Thomas Browne Henry] discovers that the missing ship, piloted by Colonel Bob Calder [William Hopper], has been located and he heads to the site with urgency. On the beach, a curious boy named Pepe [Bart Braverman] finds a translucent cylinder marked “USAF” containing a gelatinous mass egg; he sells this strange find to Dr. Leonardo [Frank Puglia], a zoologist studying sea creatures. Leonardo’s granddaughter Marisa [Joan Taylor], a third-year medical student, is summoned to help care for the injured spacemen, while Calder regains consciousness to find his crewmate Dr. Sharman [Arthur Space] dying, just as eight other crewmen have perished in the ordeal.
As the drama unfolds, a small creature hatches from the egg and Leonardo locks it in a cage. By morning, the entity has grown several times over, prompting McIntosh to arrive with Dr. Justin Uhl and meet with Italian officials to announce that the spaceship has returned from Venus. Calder’s spacecraft carried a sealed metal container housing an unborn Venusian species, and as divers begin to scour the wreck site, Pepe leads authorities to the container’s location after revealing that he sold the mass to Leonardo. The pursuit intensifies as Calder and McIntosh race to find the mass, with Leonardo and Marisa aboard a trailer bound for Rome.
That night, Leonardo realizes the creature has reached human size. It escapes its cage and heads toward a nearby farm, where it terrorizes the livestock, devours sulfur, and rips open sacks found in a barn. The farmer fights to protect his animals, and the creature maims the dog that tries to stop it. Calder and the police close in, ultimately trapping the creature inside the barn. Although Calder explains that the being is not dangerous unless provoked, the creature is provoked when the farmer is stabbed by a pitchfork, and it retaliates by breaking free and vanishing into the countryside. With growing concern from the authorities, the request is made to capture the creature alive.
A plan is hatched to snare the creature with an electric net dropped from a helicopter, and both the Italian police and Calder’s team pursue the beast through countryside and streets. The creature proves resistant to flames and bullets, seemingly lacking a heart or lungs, and sulfur becomes its preferred food. Using sulfur as bait, Calder lures the creature to a secluded area and finally subdues it with a jolt from the electric net. Back in Rome, at the American Embassy, McIntosh briefs the press and allows three reporters to view the subdued creature, which is placed in the Rome zoo. Calder explains that it will be sedated by continuous electric shocks so scientists can study it, while Marisa, helping her uncle, begins to flirt with Calder as a new emotional tension grows between them. But a sudden electrical short cuts the power, and the creature awakens with renewed vigor.
The creature rapidly grows to a gargantuan size and clashes with a zoo elephant, sparking a chaotic confrontation that spills into the streets of Rome. Cars are destroyed, and the monstrous pair rampages through the city, maiming the elephant before the chase spills into the open countryside. Calder drives a path through the chaos, driving the creature toward the River Tiber where it submerges. Soldiers lob grenades into the water, and the creature resurfaces to lay waste to the Ponte Sant’Angelo, damaging the Temple of Saturn, and assaulting the Colosseum itself. As the battle rages, Calder leads a squad equipped with bazookas to force the creature to the edge of a high ledge. A direct bazooka hit collapses the structure as the beast clings on, and a final tank blast collapses the ledge, sending the creature to its death. In the aftermath, a relieved Marisa runs into Calder’s arms, closure arriving as the danger has passed and life begins to return to normal.
Throughout the ordeal, characters such as Fisherman Mondello, Felix Roi, Signore Contino, Dr. Koruku, and others from the extended cast weave in and out of the narrative, underscoring the tension between science, military interest, and the humane impulse to study and protect despite fear. The film combines awe at a space-faring discovery with the moral and practical questions of how humanity should respond to a creature born of another world, how authority balances risk, and how love and desire can emerge even in the shadow of catastrophe. By the end, what began as a perilous rescue in the Mediterranean becomes a tense, spectacular showdown on one of history’s oldest monuments, culminating in a triumph of leadership and resolve as Calder and Marisa confront an extraordinary extraterrestrial event that tests every facet of their limits.
