If alien invasion films were outgrowths of the paranoia of the Red Scare, then
It Came From Outer Space has a positively leftist theme: the protagonist doesn't want the aliens harmed by the well-meaning sheriff's posse.
The aliens' ship breaks down outside Sand Rock, Arizona, smashing a giant crater into the desert near an old played-out mine, and all they really want to do is steal the parts they need to fix their Dangerous And Very Expensive star-drive and get back on track with their original mission, which is exploring the universe. These aliens insist that they have minds and souls and are good--despite looking like gigantic one-eyed tomato hornworms.
They convince amateur astronomer (and not coincidentally, science-fiction writer) John Putnam that they mean no harm, even though they've kidnapped several of the local townsfolk and assumed their human forms. This, of course, is only so that they can obtain the parts necessary to fix their space ship. As long as nobody molests them, they will release their hostages unharmed.
Of course, nobody believes John, least of all the local sheriff, Matt Warren, especially since Matt's ex-girlfriend Ellen is now keeping late hours with John in his place outside town.
Investigators from the press, the Army, and even an astronomer-friend of John's all come to the conclusion that the crater was made by a giant meteor, nothing more. John is made out to be some kind of crackpot for claiming to have seen a space ship in the crater shortly before a landslide covered it up.
However, the sheriff finds it increasingly difficult to blow off John's story once two telephone linemen go missing. When Frank's doughty wife and George's smokin' hot girlfriend (and I mean it, how would a girl like her wind up dating a telephone lineman in a little hick desert town like Sand Rock, Arizona?!) show up at the police station to report that their men were acting very strange and had left on some unexplained pretext, Matt has to start paying attention.
John and Ellen had earlier encountered George acting very strangely out in the desert, and they suspect that George may have killed Frank. George, incidentally, is played by Russell Johnson, who was making a living playing sacrificial bit parts in B-movies before he hit the big time as The Professor on Gilligan's Island.
But when John, Ellen, and Matt make a very tense ride out to the desert, to look for Frank and George, the linemen and their truck are gone, and the blood John finds on the rocks is explained away by Matt finding a dead coyote under a bush.
Later, in town, John sees George and Frank walking along the street like a pair of zombies, and follows them into a blind alley, where they confront him and warn him in eerily echoing voices not to bother them. They don't want to hurt anybody, and assure John that his friends will be OK, but if they aren't left alone there will be trouble.
John tries again to convince Sheriff Matt about the aliens, and Matt slowly begins to come around. Why would aliens steal a telephone truck? Because it was loaded with electrical equipment. Earlier in the day, a hardware store was broken into, and electrical equipment was stolen. And a prominent astronomer who had come out to study the impact crater disappears, along with three prospectors who had been working the old mine (and who looked like they'd wandered in from the set of The Lone Ranger.)
Meanwhile, Ellen is out driving alone in the desert, when Creepy Frank steps in front of her car. She slams on the brakes and Creepy Frank gets in, and by the terrified look on Ellen's face, reveals himself to be one of the aliens.
The aliens ring up the sheriff's office to tell John that they've got Ellen. This draws both men out to the mine, where John convinces Matt to remain with the vehicle while he goes to talk to the aliens. Matt, you see, would just as soon shoot the visitors, which he proves by smashing a spider that John points out to him. John explains that humans often try to kill what they don't understand, then goes out to find Ellen.
Ellen appears like a will-o-the-wisp, clad in an alluring strapless dress despite the desert cold (we assume it's night) and John pursues her to the mine entrance. The aliens--speaking from the darkness--tell him again that they mean no harm, and will release the hostages