The Terrors were giant, slug-like aliens which were covered in a thick coat of shaggy hair and had a large, gaping maw with which to consume their prey. Although described as omnivorous, the Terrors were only ever seen consuming humans.
After a spaceship crash-lands in Angel County, California, a single Creeping Terror exits the craft and shambles off into the countryside, leaving another still chained inside. Once a forest ranger and the local sheriff stumble across the ship and enter to investigate, they are both eaten by the beast.
Meanwhile, the first monster has gone on a cross-country rampage, killing and eating multiple victims including several picnickers, a pair of fishermen, and some young couples who had been making out in the local Lover's Lane.
The government, now aware of the situation, has sent in a group of soldiers and a famous scientist to deal with the problem. Joining forces with the new sheriff, the group determine that the Creeping Terrors are actually mindless creatures designed to consume biological samples and send any information gleaned back to their home planet via the spaceship, for reasons unknown.Eventually confronting the loose Terror, the soldiers attempt to kill it with gunfire, but this proves ineffective, and the beast eats them all. Fortunately, the commander throws a grenade at the monster, and this proves enough to destroy it.
The remainder of the group finally manage to destroy the other Terror as well, but not before it had sent it's data back to it's extragalactic masters. Whether they chose to do anything with this information is unknown, as the movie ends on that note, and no sequel was ever produced.