This film from Nathan Juran (working from a script by Christopher Knopf and Robert Creighton Williams, and a story by Charlotte Knight) is basically a space age update of the familiar "King Kong" story, and it works very well.
This time, instead of an ape, the innocent, misunderstood creature is a large sulphur-eating reptile, called the Ymir. The creature unknowingly hitches a ride to Earth on the first spaceship to travel to Venus, then gets loose, grows to immense size and rampages around Rome before finally being brought down.
Ray Harryhausen's special effects work on the monster represents the peak of his career -- the viewer winds up caring more for the ill-fated creature than any of the underwritten human characters. Those human characters come from the '50s sci-fi tropes we all know so well -- square-jawed hero (William Hopper), bland love interest (Joan Taylor) and various concerned scientists and gung ho military commanders (including Frank Puglia, John Zaremba and Thomas Browne Henry). The writing and directing are solid, if unspectacular. All of these elements are merely in service of generating sympathy for the Ymir anyway, and it succeeds admirably.
While computer technology may have made stop-motion animation obsolete, the old style was by no means without charm of it's own. This movie is perhaps the best example of that lost art.
"Why is it always -- always -- so painful for man to move into the future?"
Some versions cut out the scene in which the monster kills an elephant, because it was considered too graphic. However, that battle is probably the highlight of the film.
"King Kong" (1933) and "Mighty Joe Young" (1949).
Take a Look:
Let's all stand around and watch the monster attack a farmer!
Ymir busts out of the makeshift science lab:
The entire film is available in installments beginning here: