20 Million Miles to Earth

The first US spaceship returning from Venus crash lands off the coast of Sicily. A container with a small, lizard like alien is recovered, but the creature escapes. Exposed to Earth's atmosphere, its metabolism accelerates, causing it to grow into a gigantic monster. The creature, known as the Ymir, makes its way to Rome. The military is called in to stop its destructive rampage through the ancient city, leading to a final, epic battle in the Roman Colosseum.