In Houston, Texas, a man in a Mercedes-Benz automobile swerves to avoid an accident and runs into a field of Christmas trees. A fireball hits the earth nearby. “Talec,” a tall alien with cloudy eyes, arises from the crater and says, “I come in peace.” Meanwhile, two members of the “White Boys” drug gang break into a federal warehouse building, kill a guard, and take his uniform and nametag. They put dozens of packets of heroin from the property room into a suitcase and stop at the duty officer’s desk to check out of the building. Leaving a bag of explosives in the doorway, they drive away as the building explodes. They take the heroin to Jocko’s, an empty nightclub, where White Boys boss, Victor Manning, sits with undercover narcotics officer, Ray Turner, who holds a suitcase filled with $500,000. In a car nearby, detective Jack Caine listens to the transaction via Ray Turner’s hidden microphone. When Jack sees two robbers pull up behind him and run with guns into a liquor store, he hurries after them and gets involved in a shootout. Meanwhile, Manning tells Turner they know he is an undercover cop, and the White Boys kill him. Manning and Warren, his right-hand man, leave with the $500,000, telling the other White Boys to clean up the mess. Suddenly, Talec, the alien, enters and fires a silver disc that richocets around the room, cutting the throats of three White Boys. He walks away with the suitcase of heroin. Jack Caine runs into the club and finds his partner and three criminals dead. Later, Captain Malone blames Jack for losing the money and orders him to take a vacation. Federal Bureau of Investigation (FBI) commander Switzer and Jack are both puzzled as to how three armed men sustained deep cuts to their throats. Switzer tells Captain Malone to put Jack back on the case. The next day, Switzer asks Captain Malone to assign protocol-obsessed FBI agent Harwood Smith as Jack’s new partner in the investigation. Jack’s girl friend, coroner Diane, arrives at headquarters to inform the captain that the weapon that killed the White Boys was a razor-thin object. Jack tries to rekindle their romantic relationship, but Diane complains about him disappearing for days on end without telling her where he was, and she is sick of it. Meanwhile, a second alien lands in an empty building, scaring a homeless woman. Elsewhere, Talec injects heroin into a bail bondsman and a warehouse worker, and punctures their foreheads with a pointed instrument. At a pool hall, Jack talks to Boner, an informant, and learns that Victor Manning and the White Boys think Jack stole the heroin. Watching the cue ball carom around the pool table, Jack gets an idea and takes Smith back to Jocko’s. Following marks on the walls left by the disc, Jack finds it stuck, still vibrating, in a stereo speaker. When Smith pulls it out, the disc bounces around the room and flies into the speaker again. Jack visits Diane’s apartment, but she is angry and demands better treatment in their relationship. Jack spends the night with her. Returning to his apartment in the morning, Jack finds the place torn apart. Agent Smith is waiting, having found the door open. When Smith asks what happened to the disc, Jack takes him to Dr. Bruce, a scientist who has neutralized the metal object by placing it amid three magnets that create a neutral electromagnetic field. Bruce explains that the disc is a “self-contained electromagnet” programmed to use the “reverse polarity” of other magnetic fields, including humans, as a means of propulsion. At the morgue, Diane shows Jack and Smith several heroin-overdose victims found with punctures in their hearts and foreheads. None of the victims had needle-tracks on their arms or a history of narcotic arrests. Jack receives a package from Victor Manning in Rio De Janiero, Brazil, warning that Warren, Manning’s assistant, will deal with him over the stolen heroin. Later, Jack sees two White Boys following him in a sports car and explains to Smith that the gang consists of “yuppie criminals”—college-educated drug dealers who dress like business executives. After a quick but uneventful shootout, Jack and Smith drive to Manning’s office building. Jack deliberately triggers the alarms of expensive sports cars parked in front, then slips into the building as guards rush outside. Entering a staff room, Jack holds a White Boys board meeting at gunpoint and tells them someone else, not him, took their heroin. He surrenders his gun when thugs drag the half-conscious Smith in from outside. Warren orders Jack to make a special delivery. He gives Jack a suitcase and sends him to an Asian storefront. When the proprietor opens the suitcase and finds nothing inside, he runs to the back of the building. Jack follows, but finds the man dead. Suddenly, Talec appears and tries to kill Jack, but the other alien appears, shoots fireballs at Talec, and chases him away. Smith escapes from his captors as explosions rock the building. When Jack returns to the morgue, Diane explains that the killer injected heroin into his victims’ hearts, and when the drug released natural pleasure-hormones, called endorphins, into their brains, he punctured the victims’ frontal cortex to extract it. Meanwhile, Talec synthesizes endorphins from his latest victim into a fluid and places the vial among other vials in a kit. Elsewhere, Jack explains to a disbelieving Smith that there are two aliens, but one seems to be in pursuit of the other. When they return to Dr. Bruce, the scientist has been beaten up and the disc stolen. Elsewhere, the good alien interrupts the bad alien injecting heroin into a victim at a supermarket. Talec escapes, leaving the good alien badly wounded. Jack reports to Captain Malone, but without the magic disc as evidence, the chief does not believe his story about space aliens. He places Jack on leave again. Jack asks Diane to go out of town with him, and she agrees. When Jack returns to his car, the good alien sits in the back seat, badly hurt. He tells Jack that if Talec returns to their planet with the human endorphins, thousands of drug couriers will come to Earth and slaughter mankind. When Smith gets into the car, Jack identifies the alien as a lawman. The alien tells them Talec is at a storage building near the river, which Jack identifies as the Fletcher Building. Suddenly, the alien glows and explodes. Jack picks up the fireball-shooting space weapon, but Smith wants to show it to Switzer. When Jack tells him Switzer cannot be trusted, Smith draws his gun on Jack and takes the weapon. He runs to join Switzer at a plaza among tall buildings and turns the weapon over to him. Switzer thanks Smith for bringing the weapon and leading them to Dr. Bruce, but now his usefulness is over. As Switzer prepares to kill Smith, Jack, hiding nearby, shoots the FBI commander. Jack and Smith hurry to the Fletcher warehouse, but Talec ambushes them. In the ensuing firefight, Smith knocks Talec down with fireballs from the weapon, but the alien is undaunted. In the confusion, Talec loses the kit containing the endorphins, and Jack finds it. He and Smith return to Jack’s apartment just as Diane arrives. As they leave the apartment, White Boys shoot at them, but Smith blows up their car with the space weapon. Talec arrives and kills the White Boys, while Jack, Smith, and Diane flee in Jack’s car. After killing a policeman, Talec pursues them in a squad car. During a prolonged fight in a warehouse, Talec tries to inject Jack with heroin, but Jack grabs the syringe, turns it against Talec, then impales him on a jagged pipe. The alien moans, “I came in peace,” and explodes. Jack answers, “And you go in pieces.” Diane asks Jack if they can finally go on vacation.