Off the coast of Sperlonga, Lazio, Italy, fishermen watch as a spaceship crashes into the sea. They row out to the site and pull two spaceman from the nose-down craft before it completely sinks into the sea.
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 metal capsule 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 crew mate, Dr. Sharman, in the last throes of the fatal disease that killed his other eight crew.
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 Venusian species. As police divers begin to search for it, McIntosh offers a 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 flees. Confused, the beast blunders onto a nearby farm, terrorizing the animals. The creature eats sulfur and rips open several bags it discovers. While feeding, the creature encounters the farm dog and kills it, alerting the farmer. Calder and the others reach the barn, trapping the beast inside. Calder explains that the creature is not dangerous unless provoked. However, he immediately provokes it by trying to prod the creature into a cage, and it injures the farmer when he stabs it with a pitchfork. After the creature breaks out of the barn and disappears into the countryside, the police commissario insists that it be destroyed.
After the Italian government grants Calder permission to capture the creature, 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. Aware that sulfur is the creature's food of choice, Calder uses it 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.
Now huge, the creature battles an elephant and sends the panicked patrons scurrying. Now on the streets of Rome, the two destroy cars and damage buildings. The creature finally defeats the elephant and continues its rampage. Calder tracks the beast to the River Tiber, where it submerges. Grenades are lobbed into the river, and it surfaces. It heads for the Colosseum and destroys an ancient temple, killing many 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, then fires a second time and scores another critical hit, and the creature plunges to its death. A relieved Marisa runs into Calder's arms.
20 Million Miles to Earth was in production in Rome, Italy in September 1956, using only William Hopper of the main cast, and in the U.S. from October 30 to November 9 of that year. Rome was chosen as the location for filming because Harryhausen wanted to vacation there. The working title of the film was The Giant Ymir, and it has also been released as The Beast from Space. All references to the name Ymir were removed from the released version of the film, as Harryhausen was concerned that audiences might confuse it with the Arabic title "Emir".
Ray Harryhausen wanted the film to be shot in color, but the filmmakers were not given a budget large enough to accommodate color film. In 2007, five years after the death of the film's director, Harryhausen worked with restoration and colorization company Legend Films to create a colorized version of the film. That version, along with the original theatrical black-and-white version, was released as part of a 50th Anniversary Edition of the film on July 31, 2007.
There are minor differences between the two versions. One example: the black-and-white version begins in "A Fishing Village in Sicily", not Sperlonga, Italy.
20 Million Miles to Earth has an 80% rating on Rotten Tomatoes, based on ten reviews. Marco Lanzagorta of PopMatters gave the film nine out of ten stars, calling it "required viewing for those interested in modern cinema."