On a rural highway through the fictional Angel County, sheriff's deputy Martin Gordon [Vic Savage] and his wife Brett Gordon [Shannon O'Neil] pull up to meet Martin's uncle, Sheriff Ben [Al Lewis], and the trio heads to investigate a reported plane crash. At the wreck site they discover the abandoned forest ranger's truck, the ranger's hat, and an alien spacecraft that resembles a camping trailer. A large, slow-moving slug-like creature had already emerged from the craft and slipped away before their arrival. Believing the ranger might still be inside, Sheriff Ben crawls underneath the craft to enter, and soon loud screams and growls echo from within as Martin radios for help.
A specialized military unit commanded by Col. James Caldwell [John Caresio] arrives in the back of a light civilian truck to take charge of the investigation. Two troops go inside the craft, examine its contents, and report back that a large tethered creature remains inside. The next day, Dr. Bradford [William Thourlby], described as the world's leading authority on space emissions, arrives to direct the inquiry, coordinating the examination of the creature and the craft's analogue devices. As the investigation unfolds, the alien creature begins to stalk the countryside, despite its awkward, lumbering gait, feasting on a bikini-clad girl, a housewife hanging laundry, a man and his grandson, picnickers at a hootenanny, and several teenagers and couples in their cars at a lovers' lane.
Caldwell orders his troops to capture the creature alive if possible. The soldiers press forward in a tight formation, firing their small-caliber weapons, but the attack proves ineffective and all but two of the troops are devoured. In a final attempt, Caldwell throws a grenade that destroys the creature. Bradford quickly returns to the spacecraft, examines the dead tissue, and triggers an explosion there. The blast mortally injures Bradford but does not damage the craft or its instrumentation, allowing the tethered creature to slip free. As the creature closes in, it is killed in a collision with Martin's arriving police car.
Bradford explains to Martin and Brett that the creatures are mobile laboratories designed to consume human bodies, chemically analyze them to detect weaknesses, and transmit the findings from the spacecraft into outer space. Although Martin fails to destroy the transmitter equipment, Bradford offers a sobering note about the possible fate of the creatures’ home planet: it might not exist anymore, and, as he concludes with a quiet ache, “only God knows for sure.”
> only God knows for sure.
