A fireball streaks across the desert sky and crashes. We see an aerial view of Sand Rock, Arizona in the late evening in the early spring. Inside an isolated ranch house outside of town we meet John Putnam (Richard Carlson) and his girlfriend Ellen Fields (Barbara Rush). He is an amateur astronomer, she the local school teacher. They sit in front of a fire in his partially completed home enjoying a cup of coffee. They walk outside to take in the night air when they spot the "meteor" from the opening shot.
From inside the crater, by the old Excelsior Mine, we see a sphere partially exposed that looks like a soccer ball. A hexagonal shaped "door" opens and we see some weird machinery and in shadow one of the occupants--a single eyed blob of a creature. It exits the ship and explores the desert leaving in its wake a shimmery trail.
John and Ellen charter Pete Davis (Dave Willock) to take them to the crater in his helicopter. They circle the crater and land near the rim. The three climb to the top and peer inside. John walks down to the floor of the newly formed crater insisting that Ellen and Pete stay up on the rim. John sees the ship partially buried in the side of the mine, and also notices the shimmery substance near the exit of the ship. He stretches his hand out to touch the glowing material, but hesitates when he notices something inside the ship and stands up to face it. The door closes and slams shut triggering an avalanche of loose debris, burying the ship and almost killing John. John climbs back out of the crater and explains what he saw, but Ellen and Pete do not believe him.
Two cars approach the crater. Pete and Ellen beg John not to discuss what he saw--they think he was imagining it. Sheriff Matt Warren (Charles Drake) and Dave Loring (Alan Dexter), who runs the local newspaper, arrive on the scene. John starts to relay his story and the disbelief and ridicule starts. Three more cars arrive at the crater. John, Ellen, and Pete fly back to the airport. John and Ellen drive back to his house. Along the way they are being observed. They encounter the creature on the highway as it appears to block their path on the road. John pulls to the side of the road thinking they hit it. Finding nothing they continue down the road.
The next morning the crater is a buzz of activity. A KLTV television news van pulls up. The Sheriff's office has set up a cordon around the crater admitting only authorized personnel. The military has sent a small group out to explore. John and Ellen arrive only to be hounded by the press. John is there to meet with Dr. Snell. Dugan (Robert Carson) from American Press, and Lober (Dick Pinner) from Press Dispatch want to question John about the Sand Rock Star newspaper headline, "Star Gazer Sees Martians". They get no answers and turn their attention to Ellen, who is equally reluctant to talk to the press. John walks down into the crater to talk to Dr. Snell (George Eldredge). His assistant, Bob (Bradford Jackson) is taking radioactivity measurements, but finds none. Snell is convinced it was just a meteor, and refuses Johns request to dig it out. As John heads back to his car, where Ellen was patiently waiting, he is confronted by Matt. He has known Ellen a long time and is concerned about her involvement with John. He is jealous. John and Ellen drive off.
As they are driving the see the county telephone linemen, George (Russell Johnson), and his boss, Frank (Joe Sawyer). John asks them if they've seen anything. Frank tells them he's hearing something very strange on the wires. Frank and George head down the road and encounter the creature. As he exits the truck, George is attacked and "globbed" or absorbed. John and Ellen suspect something is wrong and turn around and head back in the same direction that George and Frank took. They find the empty telephone truck on the side of the road. John retrieves his gun from the car and they begin their search, starting with the truck. John notices blood on the truck door and the shimmery trail, a telltale sign of the xenomorph. George appears to them, but is behaving very strangely. John notices Frank's arm on the ground, unmoving, the rest of his body is hidden behind a boulder. John and Ellen head back to town to get the Sheriff. John states his conclusion that Frank is dead, and it appears that George was the perpetrator. The xenomorph, now masquerading as George explains things to the human George and Frank.
The Sheriff reluctantly agrees to accompany John and Ellen back out to where they encountered George and Frank, but when they arrive the truck and any evidence is gone. The three drive back to town. The Sheriff returns to his office, but John notices George and Frank walking through town. John follows and confronts them. They warn John not to interfere.
Back at the Excelsior Mine, miners Sam (Edgar Dearing) and Toby (Warren MacGregor) are building a fire just outside the mine opening. They are waiting for Tom (George Selk) to return with food. Tom and his mule are just about back to the mine when the mule is spooked, and runs off. Tom turns around and is confronted by the xenomorph and is "globbed" or absorbed. Sam and Toby think they hear something in the mine and enter it. As they exit the mine they are met by the creature and are also a globbed.
John and Ellen are summoned to see the Sheriff that evening. Frank's wife (Virginia Mullen) and George's girlfriend Jane Dean (Kathleen Hughes) have reported them acting strangely and now missing. Their clothes are also missing. The sheriff questions John because he appears to be the last one to have spoken to either of them. Ellen takes Mrs. Daylon and Jane home in John's car. John stays with Matt to discuss the situation; he tells Matt that Frank and George are in fact the creatures from the space craft stuck in the crater. The sheriff tells John that electrical equipment has been stolen from the hardware store and that Dr. Snell and his assistant, Bob, have been reported missing. John and Matt go out to the crater and find Dr. Snell's car, but no trace of Snell or Bob.
Ellen is driving alone on the highway when she encounters Frank standing in the middle of the highway. She stops and he gets into the car. Frank instructs Ellen to take him to the mine. Back at the Sheriff's office, the phone rings. It is for John, and he is informed that the xenomorphs have Ellen. The Sheriff and John drive out into the desert. John exits the car and searches for Ellen. He sees what appears to be Ellen on a ridgeline in a cocktail dress. He follows her. She leads him to the opening to the mine shaft. A voice from inside the mine tells John that they are repairing their ship and will be leaving that night. The missing townsfolk are being held hostage. John begs to see the creature as it really is. It shows itself and John is horrified and repelled. He returns to see Matt who has been waiting by the car. Matt drops John off at his house. John notices the door unlocked and a shimmering trail leading back to an empty closed. All of his clothes are missing.
The Sheriff gives orders to clear everyone away from the crater. John tells Matt that he got a visit and his clothes are missing. Frank appears in town just outside the Sheriff's office. Matt is spoiling for a confrontation with the creature impersonating the lineman, but John restrains him. They fight. Frank drives off in his telephone truck. The Sheriff forms an impromptu posse as John steals his police car and heads off to the crater. The Sheriff intends to confront Frank on the highway. Taking a shortcut through the desert, he forms a vehicle barricade across the highway. As Frank approaches, the posse opens fire. The truck careens off the highway and hits a boulder and bursts into flame. Meanwhile John arrives at the mine entrance and enters. He finds alien Ellen. She tells John they only need a few more hours. She then attacks him with a wand that emits a beam that cuts through rock, but misses. He shoots her with his gun and she falls into a lake below and dissolves. The posse arrives outside as John continues his exploration of the old mine. He hears machinery and finds the aliens in their human form making repairs to their ship. The head xenomorph has assumed John's identity. He explains their mission and the equipment's power. John demands Ellen's release as a sign of good faith. Alien John reciprocates by ordering all the hostages released. As they depart the mine, alien John resumes his alien form. John seals the mine entrance with dynamite to keep the posse away from the aliens. The ground begins to rumble and the ship departs, streaking across the sky.