United States: The whole country is anxiously awaiting the arrival of two astronauts aboard a spacecraft that will land off the eastern coast. Meanwhile, in the studios of a television station in San Diego (California), while waiting to broadcast the landing live, a journalist interviews Thelma, a young speleologist who has just returned from an exploration and is about to conduct another one. During the interview, the girl suffers a crisis, sensing an imminent danger. The broadcaster's director is forced to close the link and broadcasts live images of the spacecraft. Thelma immediately decides to call her psychiatrist, as she had already had similar seizures in the past, being endowed with strong telepathic perceptions. After meeting her, the psychiatrist manages to calm her down.Thelma and her boyfriend Roy meet up with the other cavers in their group in a bowling alley, then head to the next expedition location. During a layover, Burt gets out of the car to buy candles and Thelma decides to take a stroll to a nearby beach. While Roy is catching up with her, she suffers another crisis. Meanwhile, pulsating bluish rocks begin to appear in the area near the shuttle ditching. A little girl on the beach picks one up and when her mother finds it she discovers that something has torn her daughter's face to shreds. As they drive to the cave, the group hear on the radio that the authorities have not found any surviving crew inside the craft. Arriving in the vicinity of the cave, the group stops at a diner to change and refuel; Burt collects a shard of bluish rock and gives it to Thelma.The group of speleologists begins the descent between the deep tunnels of the caves, moving more and more inland. The next morning, as Thelma and Jill begin their exploration, Jill notices that the stone has begun to throb inside Thelma's bag; she picks it up and is suddenly attacked by a monstrous creature. Thelma immediately runs to ask the other friends for help and reports that Jill's face is disfigured. Roy immediately orders them to join Jill with a stretcher. As soon as they find her they discover that her face is intact and that the girl has just passed out. Ron drops the stretcher and Jill is loaded into it. As they begin to climb one by one, suddenly an alien creature emerges from Jill's face, attacks Ron and decapitates him. The cavers flee and after Burt manages to retrieve the sonar they split: Burt and Maurine head to a cave, Roy and Thelma head to an underground river while Cliff and Bill head north. During the search, Thelma reflects on what is happening and wonders if the events that have transpired are connected to disappearance of the astronauts.Roy, Thelma, Cliff and Bill meet, while Burt and Maurine split up to look for a ride. Maurine falls into the trap of the alien, who also attacks Burt as he attempts to save her. Meanwhile, the four survivors continue to search for a way out and Roy falls and injures his leg. Thelma and Bill stay close to Roy, while Cliff wanders off to find Burt and Maurine. Thelma and Cliff keep in connection via a walkie-talkie, but suddenly the connection is broken and Thelma through her telepathic powers tries to guide him to a safer place. Just as he manages to make contact with him, the alien attacks and kills him. The three remaining friends start walking again, but suddenly they find themselves in front of Cliff and so Thelma realizes that the alien takes possession of human bodies to obtain heat and physically develop its shape. Thelma attempts to telepathically confront the creature, but suddenly the alien detonates Cliff's head and attacks Bill, killing him.Thelma and Roy, forced to leave Bill for dead, eventually escape the cave. On their way back to the city, they find a police car, but no police officers are in sight. Even Roy's attempt to use the police radio fails as there is no answer. Our heroes stop at the diner again, only to find it abandoned. Roy tries to call for help on a payphone outside the diner, but no operator is available. They get back into the car and continue to drive home to San Diego. During the drive, Roy tries searching the radio stations, but there is no reception.Thelma and Roy finally make it back to the city, but strangely, no one is present. They go back to the bowling alley, only to find that it's also vacant. Roy goes to the back and is killed off-screen by an alien, which is waiting in the area, as he yells for his girlfriend. The alien proceeds to chase after Thelma until she dodges its attack by knocking it with a bowling pin and escapes the bowling alley. Thelma runs through the now-deserted city yelling for help, but there is no answer. There's no signs of carnage anywhere, yet no one seems to have survived this apparent invasion. She eventually calms down and sits at an intersection. Suddenly, text shoots at the screen, warning the audience "You may be next!"By the time the end credits finish rolling, it's also implied that Thelma will be inevitably killed by the aliens. With that, there are no more humans alive in the area. The aliens have won and taken over the Earth.