While driving along a highway in fictional Angel County in
California, sheriff's deputy Martin Gordon and his wife Brett meet Martin's uncle, Sheriff Ben, and together they investigate a reported plane crash site. At the site, the group encounters the abandoned truck of a forest ranger, the ranger's hat and an
alien spacecraft that resembles a
camping trailer. A large, slow-moving, slug-like creature had earlier emerged from the craft and departed prior to the group's arrival. Believing that the absent ranger might be inside, Ben enters the craft by crawling underneath it. Shortly thereafter loud screams, along with growls like those of a lion, are heard from the craft, after which Martin radios for help. In response to Martin's request for assistance, a special unit of military troops commanded by Col. James Caldwell and traveling in the back of a light-duty, civilian truck arrives at the site. Two of the troops enter the craft, examine its contents, and report back to Caldwell the presence inside of a large, tethered creature. The next day, "the world's leading authority on space emissions," Dr. Bradford, arrives to direct the ongoing investigation, which includes an examination of the creature and the spacecraft's analog devices. As the investigation proceeds, the departed creature stalks the countryside and, despite its markedly awkward and slow pace, successfully approaches, attacks and eats a bikini-clad girl, a housewife hanging laundry, a man and his grandson, picnickers at a
hootenanny, several teenagers and couples in their cars at a
lovers' lane. Following the lover's lane incident, Caldwell orders his troops to attack the creature, telling Bradford that the creature should be captured alive if possible. Troops stand close together, walking slowly toward the creature as a unit, and fire their small caliber firearms. The attack proves ineffective and all but two of the troops are devoured. Caldwell then throws a grenade that kills the creature. After briefly examining the dead creature's tissue, Bradford hurriedly returns to the spacecraft and triggers an explosion there. Although the explosion mortally injures Bradford, it does not damage the craft or its instrumentation, and it allows the tethered creature to exit. As the creature prepares to devour Bradford, it is killed in a collision with Martin's arriving police car. Bradford explains to Martin and Brett that the creatures were "mobile" laboratories designed to consume human beings, analyze the bodies chemically to detect weaknesses and transmit the information from the spacecraft into outer space. Although Martin fails to destroy the spacecraft's transmitter equipment, the dying Bradford says that the creatures' home planet might not even exist anymore, concluding that "only God knows for sure." ==Cast==