The film opens with the following prologue read by a narrator: "On our planet, hundreds of species of animal, plant, and insect life protect themselves by using their ability to change their color or shape, to blend into their surroundings. Many other species are capable of killing their prey by use of electric shocks, acids or poisons. Of the millions of planets capable of supporting life in the universe, it is a certainty that hundreds of thousands have developed species of life with characteristics similar, and possibly more dangerous, than those found on our own planet earth. It is also a certainty that not all alien encounters will be friendly!"One night, a young woman, Shelly Warner (Kathy Richards), is attacked and beheaded by an unseen and super strong being while walking down a dark sidewalk in Santa Monica, California. Meanwhile, at a yacht party, aspiring actor Randy Morse (Jeffery Reese) approaches a psychic named De Renzey (Jacquelyn Hyde), who foresees a deadly incident in his future, but does not alert the young man.The next day, Roy Warner (William Devane), an author who writes under the alias Steve Dupree, is informed of his daughter's murder. When Roy arrives at the morgue to identity the body, Detective Dave Mooney (Richard Jaekel), the officer in charge of Shelly's case, recognizes him. Mooney tells his partner, Detective Jack Bresler (Biff Elliot), that Roy was released from prison three years ago after serving time for murdering his wife's lover; Mooney was the officer who arrested him.Meanwhile, KFVI television newscaster, Zoe Owens (Cathy Lee Crosby), who is tired of reporting on fashion, convinces her boss, Sherman Moss (Keenan Wynn), to assign her Shelly's murder. On the evening news, she relates the unusual brutality of the crime while also mentioning that the victim's father is an ex-convict and a successful author of crime fiction. De Renzey watches the broadcast with apprehension, sensing that the killer will strike again soon.That night at an ice company, employee Henry Lydell (Vernon Washington) arrives for his evening rounds. When the lights flicker, he enters the basement to investigate and encounters a large mutant creature, whose eyes become laser weapons that incinerate Henry.Meanwhile, at a bar, Mooney meets with Roy to discuss the case, and Roy voices his skepticism of police competence. Roy declares that he will be watching closely to make sure the detective is pursuing every lead, but Mooney warns him not to interfere. Later, Mooney and Bresler investigate the ice company crime scene and find no evidence, except for another mutilated body. As they leave, a crowd of irate citizens demands better security from the police. After installing a police scanner in his car, Roy hears reports of a second slaying, but Mooney refuses to share details.At police headquarters the next morning, De Renzey informs Mooney and Bresler about the young actor she met on a yacht, whom she believes will cross paths with the killer. However, without a name or other substantial details, the detectives dismiss the psychic lead. Meanwhile, reports of a "mangler" on the loose dominate newspaper headlines. Based on evidence found underneath Shelly's fingernails, the pathologist (Casey Kasem) reveals that the killer has gray skin and no blood vessels, prompting one officer to joke that they are looking for a zombie. Captain Speer (Warren Kemmerling) warns his staff to keep this information out of the media so the public will not panic, then pressures Mooney to find a lead.Later, at a restaurant, Roy invites Zoe to his table and tries to obtain additional details on the case. Although, she is intrigued by Roy's roguish charm, Zoe declares that the police are in charge and walks away. That night, a man leaving a bar becomes the third victim when the large creature-zombie attacks him in an alley and rips his head off. At the same time, De Renzey senses that the beastly creature has struck again.In front of police headquarters, a group of demonstrators gather to protest law enforcement's lack of progress. Zoe interviews a man who claims the police are more interested in harassing innocent citizens than keeping the streets safe from a mangler. Later, Zoe accepts Roy's invitation for a drink. With skepticism, she tells him that a psychic named De Renzey confronted her that morning about a potential victim, an unknown actor, who will lead them to the killer if Zoe can track him down. Although Roy is intrigued by the possibilities of the occult, he is more interested in seducing Zoe.That night at her home, De Renzey receives a hurricane-like premonition of evil that disables her and smashes the furniture. Meanwhile, at the airport, the bloodless creature (John Bloom) kills a fourth victim, a flight attendant returning to her car, which the creature incinerates with its laser eyes. Arriving on the scene, Mooney and Bresler are frustrated by the lack of any discernible pattern to the murders other than their nightly occurrence.The next morning, Roy tracks down De Renzey at home in bed, recovering from a stroke. She warns him that the alien killer must be stopped that night or the creature will be nearly invincible, as it grows stronger with each murder. With Zoe's help, Roy searches the city to find the unknown actor from the yacht. They inquire at various nightclubs without any luck, until a young woman, Camille, overhears their conversation at a bar. She says that her boyfriend, Randy, was recently bragging about attending a fancy yacht party and is currently in the parking lot intoxicated and fumbling for his car keys. When Zoe and Roy run outside, Randy speeds away, and a car chase ensues, ending with Randy crashing into an old monastery and running away. When Roy and Zoe look for Randy, they encounter the alien creature as it throws Randy's dead body through a brick wall.Meanwhile, Mooney, who was following Roy's car, arrives on the scene and requests backup as soon as he realizes the mangler is on the premises. Zoe and Roy are able to evade the alien until police arrive and barrage the creature with gunfire. However, the alien uses his laser eyes and super-human strength to kill many of the officers, including Bresler. When Zoe is threatened, Roy fends off the creature with a torch and then sets it on fire, which causes the alien to explode and disappear without a trace.Afterward, Roy and Zoe contemplate Earth's first encounter with a being from another planet. The voice-over narrator explains that man has its first encounter with an alien form of life. A mysterious blind man (glimpsed throughout the film) is seen walking down a dark street. It is never explained who the blind man is or if he has any connection to the alien.