An Americal rocketship returning from a stay on the planet Venus crashes in Sicily, killing all but the commander Colonel Robert Calder (William Hopper). He is nursed to health by Marisa Leonardo (Joan Taylor), daughter of zoologist Dr. Leonardo (Frank Puglia). A glob of what looks like gelatin brought back on the ship expands into a dragon-like creature 15 inches high. It continues to grow, defying attempts to imprison it. It goes on a death-dealing rampage. Calder and the military surround it in the Coloseum in Rome.