In downtown Los Angeles, California, middle-aged businessman Jack DeVries robs a bank, killing one man in the process, before calmly walking to his Ferrari sports car and driving away at high speed. Police give chase, and DeVries smashes through a police roadblock, crashing the car. Hit by several bullets, DeVries gets out of the vehicle and collapses before it explodes. He is taken to hospital, where he is placed in intensive care and not expected to survive. Police report that in the past two weeks, DeVries killed twelve people and wounded twenty-three more. He stole six cars, primarily Ferraris, robbed eight banks, six supermarkets, four jewelry stores, and one candy store. Six of the people he killed, he carved up with a butcher knife, two of which were children. Police detective Tom Beck is happy that DeVries will die. At police headquarters, Federal Bureau of Investigation (FBI) agent Lloyd Gallagher from Seattle, Washington, informs Beck that he has been working on the DeVries case for a month and Beck has been assigned to help him. In the hospital, DeVries revives, and approaches cardiac patient Jonathan Miller in the next bed. He opens Miller’s mouth, then opens his own. A large snail-like creature climbs out of DeVries’s mouth and down Miller’s throat. DeVries collapses onto the floor and dies as Miller wakes up, gets out of bed, and walks out of the hospital. Doctors are puzzled, saying that Miller had a massive heart attack a day ago and is in need of a triple bypass operation, yet now is well enough to walk out of the hospital. Since DeVries is dead, Beck tells Gallagher he can return to Seattle. Gallagher insists the search is still on, but now they are now looking for Jonathan Miller. However, police records show Miller has no prior criminal activity. Miller goes to a record store on Melrose Avenue and begins pocketing cassette tapes. When the store clerk tries to stop him, Miller kills him, takes all the money from the cash register, and walks out with a large cassette tape player. When police sketch artists draw a picture of Jonathan Miller based on witness descriptions, Beck questions Gallagher, who explains that when the man they are looking is continually changing his identity, but must be stopped before he moves again. Miller goes to a Ferrari dealership, interrupts a sale, and steals the Italian sports cars, killing three people in the process and taking their wallets. The man buying the Ferrari was an arms dealer, so Miller goes to his office and takes a large cache of guns. Next, Miller goes to the Harem Room, a strip club off Hollywood Boulevard. After watching a performance by stripper Brenda Lee Van Buren, Miller follows her to the dressing room. The snail-like creature leaves Miller���s body and moves into Brenda’s. Meanwhile, Beck invites Gallagher home for dinner, introducing him to his wife, Barbara, and his toddler daughter, Juliet. Gallagher seems socially uncomfortable, has an awkward time using silverware, and becomes intoxicated after just a little wine. Gallagher reports that his wife and young daughter were killed by the same man that killed his FBI partner. Police radio they have found Miller’s stolen Ferrari outside the Harem Room. In Brenda’s dressing room, they discover Miller’s dead body. Brenda has sex with man in a Cadillac, then pushes him out of the car and drives away. Beck and Gallagher spot the Cadillac and give chase until Gallagher shoots out one of the Cadillac’s tires, causing it to crash into an art gallery. Brenda fires at Beck and Gallagher, who shoot her in the arm, but it does not affect her. On the building roof, Brenda fires at Beck, who is hit and falls over the side, but grabs hold of the ledge. Gallagher helps him up, then fires at Brenda, hitting her several times without injury. As he comes closer, Brenda recognizes Gallagher. He pulls out a futuristic-looking ray gun and points it at her. Brenda tells Gallagher that she will not come out and if he thinks it over, he is wrong. Brenda jumps off the side of the building, her body smashing onto the concrete below. When a dog named Jake comes over to sniff at the body, the creature transfers into the dog. By the time Gallagher reaches the ground, he sees that Brenda is dead and realizes the creature has moved to another body. Meanwhile, the coroner wants to know why Brenda’s body has fifteen bullets holes in it. Beck arrests Gallagher, confiscating the odd-looking ray gun. Beck runs a background check that shows Gallagher was killed in a forest fire a month ago along with his friend, Robert Stone. Stone’s body was never found, and authorities assumed it was burned in the fire. However, a photo of Robert Stone shows the face of the man Beck knows as Gallagher. Beck brings Gallagher from lockup, demanding answers. Gallagher says that the creature he is chasing is not human. The alien uses a human body until it is depleted, then transfers to a new body. However, the only time the creature can be killed is when it is between bodies. Now that the creature knows where Gallagher is, it will come after him. Meanwhile, the creature transfers from Jake the dog to the dog’s owner, Lieutenant John Masterson, who comes to the police station. In the evidence room, Masterson gets the ray gun as well as other guns before threatening Beck to take him to Gallagher, killing several police officers along the way. Other officers fire at Masterson and he falls to the ground, but gets back up. Beck runs to lockup to get Gallagher, but Masterson catches up to them. Beck, who grabbed the ray gun when Masterson was hit by bullets, tosses the weapon to Gallagher who holds it on Masterson, inviting the creature to come out. Masterson reveals that Gallagher is an alien law enforcement officer from the planet Alteer and suggests the two of them could rule the human race. Beck shoots Masterson in the head but he gets away. The creature transfers from Masterson to the body of Cliff Willis, Beck’s police partner, and escapes. Beck asks why Gallagher did not use the ray gun on Masterson. Gallagher shoots Beck with the ray gun, but it has no effect on him. Gallagher explains that the weapon does not work on human flesh and it only works when the creature is between bodies. At the Park Plaza Hotel, Senator Holt is planning to announce his candidacy for U.S. President. Cliff Willis comes to the event. Police shoot at him, hitting him multiple times, but Willis does not fall. Willis shoots Beck in the chest then goes after the Senator. Police find Willis’s dead body in a supply closet. Gallagher realizes the creature transferred to Senator Holt. During the press conference, Gallagher rushes in and pulls a flamethrower out of a bag. Police fire at Gallagher, but their shots do not stop him. Gallagher ignites the flamethrower at Holt, setting his body ablaze. The creature comes out of Holt’s charred body and Gallagher shoots it with the ray gun, killing it. At the hospital, Beck is unconscious from his chest wound. Gallagher comes to Beck’s room, but Beck dies. Gallagher transfers his life force into Beck’s body. Doctors find Gallagher’s dead body on the floor. Beck wakes up, greets his wife, and holds his hand out to his little girl.