In a sunbaked desert, a green-skinned figure wanders with a mysterious laser cannon strapped to his arm, and two reptilian aliens aboard an approaching spacecraft step out brandishing weapons. After a tense firefight, the aliens disintegrate the man and, once they spot an airplane in the sky, they retreat to their ship, leaving behind the cannon and a pendant the man wore.
Back in town, Billy Duncan, Kim Milford, wakes up to a morning that quickly sours: his mother has been invited to a trip to Acapulco and, despite his protests, she leaves him behind. He seeks out his girlfriend Kathy Farley, but her grandfather, Colonel Farley, Keenan Wynn, is a grizzled, conspiracy-spouting veteran who warns Billy to stay away. The teen world around him feels heavy and judgmental, with a bully named Chuck Boran, his friend Froggy, and a pair of sheriff deputies harassing him and giving him a speeding ticket.
In the desolate heat, Billy stumbles across something unbelievable: a laser cannon nestled with a strange pendant. He learns he can wield the weapon only while wearing the pendant, so he experiments, firing at things around the desert and letting his fear and excitement drive reckless bursts of firepower. At the same time, the alien crew on their ship converse in an unsubtitled language, and their leader orders them to return to Earth to recover the cannon they left behind. The tension between the human and alien sides begins to braid together as the town’s quiet routine starts to crack.
A pool party becomes a flashpoint. Chuck makes unwanted advances on Kathy, and Billy steps in to defend her, sparking a confrontation with Chuck and Froggy. That same night, Billy uses the cannon to explode Chuck’s car, an act that leaves the teens shaken and the authorities curious. Enter Tony Craig, a government official who arrives to investigate both the explosion and the broader desert mystery; he warns the sheriff that information about what happened must stay contained, suggesting a cover-up in the works.
Billy’s body begins to change, producing an unusual growth on his chest. Kathy urges him to see Doctor Mellon, and Roddy McDowall surgically removes a metallic disc from Billy’s sternum and arranges for it to be evaluated at the police laboratory. That night, a green-skinned Billy — now visibly deranged and threatening — attacks Mellon’s car and kills him in a fiery explosion. The next day, Tony investigates the wreckage and brings back a sample of unfamiliar material, which Mike London, Rick Walters, tests and identifies as inorganic and not of Earthly origin.
The violence intensifies as Billy, under the growing influence of the pendant and the cannon, kills two sheriff deputies who previously taunted him. He and Kathy share a night together in the aftermath, and when Kathy discovers the alien pendant, she cautiously places it on Billy’s chest. The result is a shocking transformation: his skin turns green, his face deforms, and a full-blown rampage ensues, with Billy firing indiscriminately with the deadly weapon. A small law-enforcement plane opens fire, but Billy destroys it. He continues to target Chuck and Froggy, obliterating their car in a blaze of laser power.
Tony Craig keeps pressing Colonel Farley for information as the two aliens land on Earth and join the hunt for Billy, moving through the landscape in a hastily stolen hippie van. They kill a passerby and commandeer his vehicle, while Billy roams an empty city block, firing at will. Kathy and Tony finally corner him in an alley just as one alien fires a beam from a rooftop, striking Billy down. The laser cannon and the pendant are destroyed in the aftermath, and the aliens depart aboard their spacecraft. Kathy collapses in tears over Billy’s corpse, while Tony looks on, left to reckon with the planet’s violent collision of human fear and otherworldly power.
