Celia Jamerson is an English-major student out cycling when she is ambushed by a psychopath who tries to run her over in his van. The driver doesn't look where he's going and crashes his van and dies.
Jake Corey is a police officer investigating the crash when he notices a trail of blood exiting the van and disappearing into the fields. Suspecting that the driver had an accomplice, he goes to the nearby hotel known as the Oakwood Inn and warns the owners Ron and Peggy that they should close early. Peggy doesn't listen and tells Ron to help her fix up the inn after dark. Jake notices this and goes in to tell them off, only to see Ron has killed Peggy and is eating her flesh. Ron tries to attack Jake but Jake shoots him in the head.
Roland is a college student with a fetish for gore and horror whose roomate Jason is dating Dana. Dana despises Roland due to the fact that he has some pornographic photos of her that Jason took. Roland tries to blackmail Dana into giving them back, but Dana turns the tables on him and says that if he can spend a night in the now-deserted Oakwood Inn, he can keep the pictures. If he chickens out, he'll have to give Dana the pictures.
Alison is a roomate of Celia's who is dating an English grad-teacher named Evan. Evan only wants to be with Alison for sex. Alison feels that they should try going without sex, but Evan gives her the cold shoulder. Feeling rejected, Alison hangs out with her friends Celia and Helen. The three friends spend the next day shopping. Alison, despite wanting a more chaste relationship, buys a see-through negligee with the possible intention of seducing Evan. At the food court, the three run into Roland, who is buying some supplies for his night in Oakwood Inn. Roland visibly lusts over Alison, earning him the dislike of all three girls.
That night at the Oakwood Inn, Roland handcuffs himself to the bar to ensure that he won't leave by dawn. Roland sees a snake-like creature near his feet. Roland is terrified at first, but when the creature gets under his skin and inserts its head into his brain, Roland feels a sense of ecstacy. Dana sneaks into the inn planning to spook Roland, but Roland kills her at the behest of the creature and eats her flesh.
Jason, who was out of town for a few days, comes home wondering where Dana is. Roland tells Jason that Dana was at the Oakwood Inn but disappeared, implying that she may have been kidnapped and raped. To help catch this supposed rapist, Roland tells Jake to bring another girl to the inn and use her as bait. Jason reluctantly asks Celia out on a date. As he spends time with Celia, Jason begins to realise he's falling in love with her. Still feeling guilty about Dana, Jason gets Celia drunk and takes her to the Oakwood Inn. Roland then kills Jason and eats Celia, getting a buzz from the creature in his back as he does so.
Jake has the bodies of the mad driver and Don examined at the morgue. The autopsy uncovers that both men have a large gap between their spines and the skin above it, along with exit wounds in their abdomens. The mortician suggests that some symbiotic creature had possessed the two and made them kill by probing their brains. Jake investigates the Oakwood Inn, uncovering a nest in the basement carrying all the creature's offspring. Jake destroys the nest and all the eggs and goes on the hunt for the parent.
Jake finds out from his ex-wife that their 4-year-old daughter Kimmy has run away from home. The two start fighting before breaking down in tears. Jake patrols the area surrounding his ex-wife's house, but finds nothing. Returning home, Jake finds Kimmy hiding in his room. She tells Jake that the reason she ran away was because her mother hit her. She also says that she encountered scary man who obviously wanted to kidnap her. Getting a description of what this person looked like, Jake has a suspect on the identity of the creature's host.
Alison and Helen are concerned that Celia never came home. Alison receives a note from Evan beggin for forgiveness. Alison decides to go to his place and address their problems, but reminds herself that they shouldn't have sex. Alison arrives at Evan's place and they have dinner and drinks. While watching a movie, Alison falls asleep but is gently woken up by Evan. Evan offers to rub Alison's shoulders but says he won't try to overstep his boundaries. Once he touches Alison, she feels her willpower slipping away, allowing Evan to take her blouse off. When Alison notices a food stain on her bra, she realises that Evan had been feeling her up while she slept. Furious, Alison storms out.
When Alison goes to bed, she is attacked by Roland who broke in while she was out. After gouging one of his eyes, Alison runs to Helen, only to find that she's been murdered. Alison runs outside with Roland in hot pursuit. He pins her down but she grabs his knife and stabs him in the chest. Jake spots Roland attacking Alison and shoots at his hand, allowing Alison time to get away. Alison manages to find a police car and rams Roland just as he catches up to her.
At the police station, Alison relays the night's events to Jake. He offers to let Alison stay at his place while the police investigate the area. Alison asks why she needs to be so careful if Roland has been fatally injured, so jake tells her that Roland was possessed by a symbiote that made him try to kill her. He also warns that the creature might very well be looking for a new host and that Alison should stay indoors until they catch it. Jake also begins to develop an attraction to Alison, but keeps his distance out of decency.
Jake goes out and finds Roland's body. He sets it on fire to make sure the creature isn't waiting to leap out and attack him before calling the mortician and having him examine the remains. They find that the symbiote had already left Roland's body.
The next morning, Alison gets a call from Evan, who somehow managed to get Jake's phone number. He says he wants to have one more chance with Alison and is she feels the same way, they could meet up. Alison decides that if she just goes out this one time and ocmes straight back she should be okay. Jake scours the dorm area where Alison was attacked and finds blood stains in Evan's room, making him realise that Evan had become the symbiote's new host. Returning home, Jakefinds that ALsion has already left to meet up with Evan. When Evan picks Alison up, he takes her to Oakwood Inn and they make out. As Alison runs her hands up Evan's back, she feels an abnormal bulge in his spine. Evan then attacks Alison, planning to do what Roland couldn't. Alison finds a screwdriver and stabs Evan, killing him. The creature then erupts out of Evan's back and goes after Alison. Jake rushes to hte Oakwood Inn, knowing that the creature would feel safest there. He sees Alison rushing towards him with the creature's head already buried in her lower back. Jake grabs its tail, stabs Alison in the back where its head is and cuts it out of her. Jake then beats the creture to death before burning it.
In the epilogue, Kimmy now lives with Jake and Alison, who have formed a steady relationship with an even balance of sex and mutual respect.