In the opening scene, a man named Jack DeVries (Chris Mulkey), robs a Los Angeles Wells Fargo bank, and kills all of the security guards inside. DeVries leads the Los Angeles Police Department on a high-speed chase, during which he forces an innocent motorist and several LAPD patrol cars off the road.
In the meanwhile, the hot-tempered LAPD detective Sgt. Thomas Beck (Michael Nouri) questions DeVries' neighbor; the neighbor tells Beck that DeVries was a quiet, decent gentleman. Beck's partner, Det. Cliff Willis (Ed O'Ross) runs up and informs Beck that the LAPD is pursuing DeVries through West Hollywood.
DeVries encounters a police blockade overseen by Beck. DeVries is shot several times, smashes through the blockade and crashes. He staggers out of the wreck, tosses out the bank bags full of money and laughs at the amazed police; the police then shoot at the Ferrari which explodes, knocking down DeVries.
At a local hospital, a doctor informs Beck and Willis that DeVries is not expected to survive the night; Willis tells DeVries' doctor that within a span of only two weeks, DeVries has been on a violent crime spree.
At the police station, Lt. Ed Flynn (Clu Gulager) and Lt. John Masterson (Clarence Felder) discuss reassigning Beck. After Flynn leaves, FBI Special Agent Lloyd Gallagher (Kyle MacLachlan) arrives and informs Masterson and Beck that the latter has been assigned to work with Gallagher. Gallagher states he has been trailing DeVries for the past month all the way from Seattle. When he learns that the spree killer is at the hospital, Gallagher then rushes off to go see DeVries.
At the hospital, DeVries suddenly awakes, disconnects his life-support equipment and walks over to the comatose man in the next bed, Jonathan P. Miller (William Boyett). DeVries rips off Miller's breathing mask, then a slug-like alien life-form emerges from DeVries' mouth and transfers itself into Miller's body; DeVries then drops dead. When a doctor and nurse defibrillate Miller, Miller amazes the staff by awakening and calmly walking out. Gallagher arrives at the hospital and sees DeVries lying dead on the floor; Gallagher then questions Miller's attending physician, who tells Gallagher about Miller and his medical problems. Gallagher has told the doctor that Miller is wanted by the FBI, but the doctor is doubtful, saying that Miller is a good and gentle man.
Miller is caught pocketing heavy metal music tapes by a music store owner; when the irate man tries to stop Miller, Miller beats the owner to death and steals $100 from the cash register, a small revolver pistol and a portable boombox. As Miller walks out of the store, he callously bumps into a young woman who is just walking in and sees the owner's bloodied corpse.
Gallagher asks Beck to put out an alert on Miller, but Beck refuses after running a background check on Miller, who has never broken the law other than receiving a few traffic tickets. Beck investigates the music store owner's murder, but sees that Gallagher is already at the crime scene. Beck takes Gallagher aside and berates him, as Beck believes Gallagher is overstepping his jurisdiction and tells him to go back to Seattle; Gallagher instead follows Beck back to the police station, where the female witness identifies Miller as the murderer. Beck then agrees to put out an APB on Miller and assist Gallagher, who tells Beck that DeVries and Miller were partners, and that neither man has a criminal record since they both frequently change identities. Gallagher and Beck leave the station in Gallagher's Porsche; Beck is amazed that Gallagher can afford such an expensive car, but Gallagher jokes that he stole it.
Meanwhile, Miller is eating at a diner and annoying the customers and staff alike with his loud music blaring from his stolen boombox, but then turns it off when he sees visiting United States Senator Holt (John McCann) on TV; Holt is running for President of the United States. A passing red Ferrari Mondial catches Miller's attention and he rushes out of the restaurant without paying for his meal, leaving the boombox behind as he runs down the street.
Gallagher tells Beck that Miller was at the hospital to see DeVries, and that Miller steals what he wants and kills anyone who gets in his way (Beck notes that DeVries acted in the same way).
Miller arrives at a Ferrari dealership where he shoots and kills three men at dealership and steals the red Ferrari Mondial that he followed earlier; one of the men killed by Miller was an art gallery proprietor named Michael F. Buckley, who just bought the Ferrari that Miller stole. When Gallagher and Beck investigate the triple homicide, Gallagher mentions that Miller likes to steal Ferrari's, which Beck finds odd since DeVries also had a taste for Ferrari's. Gallagher later tells Beck that both DeVries and Miller had killed Gallagher's partner. Beck privately notes Gallagher's odd responses to his questions.
Miller checks the wallet he stole from Buckley's pocket and finds business cards for Buckley's art imports business and a strip club. Miller drives to the art gallery where he then finds that Buckley was also an illegal arms dealer. The Alien realizes that Miller's poor medical condition makes Miller an unsuitable host and unsuccessfully tries to escape Miller's body. Miller recovers and loads as many guns as he can carry into a large bag. Meanwhile, Beck and Gallagher find out that Buckley was an arms dealer and Beck orders police units be sent to Buckley's home and business; by the time the police arrive at the art gallery, Miller is already long gone.
Beck and Gallagher go to Beck's house for dinner. Gallagher becomes saddened when introduced to Beck's wife and daughter. Beck's daughter suddenly becomes withdrawn and frightened in Gallagher's presence. Beck once again finds Gallagher's odd responses and behavior as atypical of a FBI agent.
Miller then decides to visit the strip club that Buckley once frequented, where the Alien jumps hosts once again, this time taking over the body of a stripper named Brenda (Claudia Christian), which allows her to bypass a police search despite carrying the large bag full of weapons. Brenda/Alien then has sex with a man in his car, induces a fatal heart attack in the man and steals his car. Gallagher is able to identify Brenda as the Alien's new host, and he and Beck pursue her in a chase through the streets and to a rooftop, where Brenda opens fire on Gallagher and Beck with a M-16 assault rifle, prompting Gallagher and Beck to return fire, mortally wounding Brenda. Gallagher points a strangely-shaped weapon at Brenda, but before he can fire it, Brenda taunts Gallagher and leaps from the roof. On the ground, Masterson arrives from his house to take charge of the scene; the Alien emerges from Brenda's dying body and enters Masterson's dog, Roy.
Later, when Gallagher refuses to tell Beck what is going on and what he knows about the situation, Beck has him taken into custody and runs his credentials. Beck learns that "Gallagher" is in fact missing and presumed dead following a forest fire during a hunting trip over one month ago.The man with whom he has been working matches the description of Gallagher's friend Robert Stone, who similarly disappeared following the same fire. When Beck confronts Gallagher/Stone with this knowledge, Stone tells him that they are in fact pursuing an Alien thrill killer who has the ability to take over human bodies. Beck disregards the story as insane.
Meanwhile, the Alien takes over the body of Lt. Masterson to get into the police station so that it can find and kill Stone. A shootout ensues, during which it is revealed that Stone is in fact another alien named Alhague, a police officer from a planet orbiting the star Altair. Alhague has been pursuing the Alien ever since it murdered his own wife and daughter and his police partner nine Earth years ago. The weapon he attempted to use on the rooftop is from his home world, and has the capability of killing an alien parasite without harming the host body (due to the Alien's different biochemistry); the weapon shoots an energy beam that causes a cinder-block wall to explode when it is mishandled by an unsuspecting police evidence technician, allowing Beck to free Alhague during the resultant chaos. As Masterson hunts Alhague, the Alien brags that Earth would be an easy target for his kind to conquer. Following an explosion from a M72 LAW rocket launcher fired by Masterson at Alhague in the cell block, the Alien transfers into Beck's partner Det. Cliff Willis (Ed O'Ross) and escapes.
Witnessing the respect shown to Senator Holt and knowing that Holt is running for President, the Alien decides to take over his body; with Presidential power, the Alien could summon others of his kind to Earth, where they could easily conquer humanity. Alhague and Beck follow Holt, but Beck is severely wounded by the possessed Willis during the chase (as is Willis, who is so badly wounded that he must find a new host) and the Alien is able to transfer itself into the Senator before it can be stopped (one of Holt's aides hiding in a pantry witnesses the Alien taking over Holt's body, but she is so traumatized from the experience that she says nothing).
Alhague is forced to attack Holt in the middle of a press conference, just after Holt announces to the media that he wants to be President; despite being shot several times by United States Secret Service bodyguards, police officers and Holt himself (who seizes a revolver, shocking everyone present as he repeatedly shoots at Alhague), Alhague is able to use a homemade flamethrower taken from the police evidence locker, burning Holt to death and forcing the alien to emerge from Holt's charred corpse. Before a crowd of horrified reporters, helpless Secret Service agents and LAPD officers, Alhague kills the Alien with his weapon before himself collapsing.
Beck and Alhague are both taken to the hospital. Although Alhague rapidly recovers from his gunshot wounds, Beck has been mortally wounded. Alhague enters Beck's room, where he sees the effect that Beck's pending death is having on his wife and daughter. Once everyone else has left, Beck dies, but Alhague opens his mouth and emits a yellowish-white light (revealing himself to be an energy being), transferring himself into Beck's body. Stone's lifeless body collapses to the floor, while the alien Alhague (in Beck's body) makes a spontaneous recovery. Beck's wife and daughter return. Beck's daughter initially hesitates when Beck reaches out to her from his hospital bed. (Note: it is strongly implied that Beck's daughter, Betty, may have special abilities to "see" persons to who they are and that she knew from first sight that Gallagher/Stone/Alhague was an alien being, and she now knows that the Alien is inhabiting her father's dead body). But then, Betty smiles and takes his hand. Alhague now has a wife and daughter again and now confined to Earth for the rest of his now-natural human life.