A green-skinned man (Steve Neill) wanders through the desert with a laser cannon attached to his arm. A spaceship lands and two aliens emerge, one of whom shoots the man, which disintegrates his body. The aliens depart on their spaceship, leaving behind the laser cannon and a metallic pendant the man was wearing.

Teenager Billy Duncan (Kim Milford) wakes up in his bed, seemingly disturbed, and learns his mother is leaving for vacation. He goes to visit his girlfriend Kathy (Cheryl Smith), but her deranged grandfather Colonel Farley (Keenan Wynn) makes him leave before he can see her. As Billy drives around town, he is harassed by bullies Chuck Boran (Mike Bobenko) and Froggy (Eddie Deezen), and by two police deputies who give him a speeding ticket. Billy wanders into the desert and discovers the laser cannon and pendant. He starts playing with the cannon, pretending to shoot things, then realizes he can fire the weapon while wearing the pendant.

Meanwhile, on the alien spacecraft, the two aliens converse with their leader who shows them footage of Billy using the cannon, prompting the aliens to turn their ship around to head back to Earth…