In a fishing village in Sicily, men are hauling in their nets. One of them hears something passing over head. A spaceship breaks through the cloud cover and crashes into the sea nearby. At first the fisherman leave the area, but one of the crew worries there might be people trapped inside and insists on going back to investigate
Getting close to the wreck, they find a gash in the hull and decide enter and search for survivors. Working their way to the control room they find two men alive but badly injured. Both men are evacuated before a series of explosions rock the vessel and it sinks.
Later in Washington in the office of Major General McIntosh (Thomas Henry) discusses the crash. They are unaware of the survivors and assume the craft has been lost with all hands. A report comes in identifying the crash location in Sicily. McIntosh and Doctor Uhl (John Zaremba) make arrangements to travel to Sicily and examine the the crash site.
Back at the village Commissar Charra (Tito Vuolo) collects information from the fishermen about what they saw inside the ship. Although the men only saw three crewmen, everyone is convinced the crew had to be bigger given the huge size of the ship.
One of the children, Pepe (Bart Braverman) finds a canister on the beach and is in the middle of burying it when he is summoned to answer questions about a visiting American doctor visiting the area. The local village doctor is away delivering twins and no one knows when he will be back. The boy is instructed to fetch the American doctor in hope he can care for the injured astronauts.
Before fetching the doctor Pepe returns to the canister and removes a gelatinous mass, which he wraps in a blanket. Pepe finds Doctor Leonardo (Frank Puglia) but is disappointed to learn he is not a real doctor but a zoologist. Leonardo suggests his granddaughter Marisa (Joan Taylor) who is medical student traveling with him may be able to help.
Doctor Leonardo has been paying Pepe to recover shellfish for his studies. Pepe shows him the contents of the canister after bartering how much he will pay for it. Intrigued Leonardo begins examining the mass, he is convinced he can see a shape buried inside.
At the village hospital Marisa makes the men as comfortable as possible. Col Robert Calder (William Hopper) the commander of the flight regains consciousness and immediately shows grave concerns for the other survivor Dr Sharman (Arthur Space) The Captain presses Sharman for information about how long the creature in the canister could survive, before he can say too much he dies, becoming the 9th victim of the flight.
Back at Leonardos caravan, a creature only a few inches tall hatch from the gelatinous mass. Dr Leonardo is fascinated at the anatomy of the beast and moves it to a cage for further examination. The doctor and his daughter, returned from the hospital, decide to leave the creature in the cage overnight
Next morning Leonardo examines the cage and finds the creature is well over 2 feet tall. He decides he needs to talk to the Pepe and find out exactly where the creature was found.
Later that day a US seaplane arrives with McIntosh and Uhl. They go to meet Col Calder and have a conference. A representative of the Italian government Commissario Charra (Tito Vuolo) arrives and is briefed on events. McIntosh explains a biological specimen is missing and it is an unborn animal from Venus.
Charra offers a diving team to go down and inspect the wreck and locate the specimen. McIntosh goes on to interview the fishermen. The General describes the missing canister,although afraid, Pepe admits he found the cylinder and that he sold the specimen to Doctor Leonardo. The General sends Calder after Leonardo who is now on route to Rome.
Leonardo stops on the side of the road to refasten one of the ropes on the cage. The creature sees its opportunity and breaks free escapes. Leonardo is checking on Maria who was injured by the creature when Calder arrives. After having the situation explained to him he sets of to track the creature through the surrounding farmlands
The creature stumbles into barn where it becomes confused and agitated by the various animals it encountersfused and tentative about the various animals it encounters. Moving to a storage area it finds a bag of sulfur and begins to consume the contents till attacked by a dog. Drawn by the noise of the two animals fighting the farm owner arrives and tries to shoot the creature
Calder also drawn by the noise stops the farmer, telling him the priority is to capture the creature alive. The attempt at capture fails and the farmer is attacked. Desperate Calder opens fire with his own gun. Although struck several times the creature seems to retreat back into the barn uninjured. Calder breaks off his own attack and locks the creature in the barn
During the night the creature escapes. From reviewing Dr Sharmans notes it is understood the animal lives on sulfur and is probably heading for deposits of the mineral around Mount Etna. Charra then informs Calder that the Italian government will allow no further attempts to capture the creature. After the attack on the farmer the animal is far too dangerous.
Calder strikes a deal that if he tracks and locates the creature first he can try and capture it before Charras forces arrive to destroy it. Calders plan is to use two helicopters, one loaded with Sulfur to tempt the animal and the craft has a net to be dropped once the creature becomes distracted.
Charras forces find the animal first, and although they drive it back towards a waterfall they dont have the firepower to defeat it. Unaware of Charras efforts, Calders group approaches, drops the sulfur then the net. As the animal struggles to release itself a mild electric shock is run through the net paralyzing the creature instantly.
Later at the US embassy in Rome, the General briefs the press on events, and allows a limited number of press representatives to visit the zoo where the creature is being held. The creature is chained down and kept unconscious with a continuous electrical current while various tests are performed.
Calder introduces the press to the scientists working on the creature. So far little beyond the creatures amazing sense of smell is known. Calder sees Marisa working with her father, after finding a quiet corner it is obvious romantic attraction is building, with even an agreement for a date after the research on the monster is complete.
During an experiment a chain snaps and the equipment producing the paralyzing electrical current is destroyed. The monster quickly revives and breaks free of its bonds, and the building housing it. Still disorientated the creature encounters an elephant that immediately reacts by charging.
Patrons at the zoo panic as the battle between the elephant and the creature continues. Calder contacts McIntosh and explains the situation; he immediately authorizes the use of tanks and artillery to destroy it. Meanwhile the creature fatally wounds the elephant and flees into the city.
Calder briefly spots the creature before it disappears into the Tiber River, Calder transmits this information to McIntosh who orders the river shelled in hopes of bringing the beast to the surface. The tactic works and the creature emerges destroying a bridge before heading towards the coliseum for a final confrontation with troops patrolling there.
The creature arrives and is attacked by infantry supported by a flame throwing tank. Calder and the general arrive to take charge of the situation. More troops pour into the area but creature has disappeared among the ruins.
The soldiers re-establish contact with the creature. Trying to escape it climbs higher till finally cornered in the upper reaches of the ruins. A barrage of bazooker and tank fire seem to wound the creature before a miss directed shell hits the ruins causing the creature to fall to its death