The film opens with Mischa Barton as Shelby driving through a dark night, crying and smoking, when she drops her cigarette and hits Elizabeth, a woman who steps into the road. Nearby, Matt Long plays Mike, Shelby’s ex, and his new girlfriend, Jessica Stroup as Elizabeth, as they head toward Mt. Bliss to celebrate the retirement of Mike’s football jersey. Shelby is shown grappling with a bank dispute over a bowling alley she inherited from her mother, while clinging to the belief that she and Mike might still be together. After a game, Mike and Elizabeth decide to visit Shelby’s bowling alley, where Shelby greets Mike with a kiss, only for him to remind her that he’s moved on with a new relationship.
Shelby befriends Elizabeth, and after a few tequila shots, Elizabeth blurts out that she intends to meet Mike’s parents. Shelby warns that Mike’s parents can be judgmental, which makes Elizabeth increasingly nervous. To make a strong impression, Elizabeth decides to sober up at a motel, but Homecoming weekend leaves every room booked and the nearest motel four miles away. Mike returns home to his mother, who whispers that a friend claimed Elizabeth got drunk at the alley; Mike insists Elizabeth wanted to make a good impression and that she’ll stay at the motel. His mother presses him about why Elizabeth would feel that way, hinting that Shelby has fed her a false story.
Elizabeth begins walking along the road, hoping to find a ride to the motel, while Shelby, now shown crying and smoking again, drives on. A passing driver ignores Elizabeth, and Elizabeth is the very person Shelby hits in the crash. Shelby brings Elizabeth back to her house and tends to her injuries with medical supplies from her mother’s illness. The next morning, Mike tries Elizabeth’s phone, but it goes to voicemail. Elizabeth wakes in Shelby’s house, confused, and Shelby lies, telling her she found Elizabeth in the road as a victim of a hit-and-run. Elizabeth has a bruised face and a broken ankle, and Shelby claims she left a message on Mike’s voicemail to explain where Elizabeth was. Shelby then sedates Elizabeth.
Shelby relocates Elizabeth’s car, then returns to her room to find it plastered with photos of Mike and Shelby’s past together, marked with “SM + MD.” Elizabeth, who had been confined to Shelby’s house, discovers the photos and realizes how deeply Shelby has surveilled and manipulated her. In retaliation, Shelby dislocates Elizabeth’s bad foot. Mike and Billy go to the motel where Elizabeth was dropped off, only to learn she never checked in. They discover Elizabeth’s car is missing and later find that Shelby has hidden it in a barn. Shelby also rifles through Elizabeth’s suitcase and takes a gift intended for Mike, planning to pass it off as her own.
Billy urges Mike to take Shelby back, arguing Shelby still loves him despite the earlier rejections. Mike agrees to go out for drinks, but in a bar Shelby appears and the two share a passionate moment in the bathroom. Mike, however, insists he cannot cheat on Elizabeth, and Shelby leaves distressed.
The next day, Elizabeth is discovered locked in Shelby’s bathroom. A careful search uncovers Polaroid photos of Shelby’s dying mother taped to the cistern lid, a document titled “Poisonous Plants,” and a form indicating no autopsy had been performed at Shelby’s request—evidence that Shelby had killed her own mother. Fearing for her life, Elizabeth searches for a way out and eventually uses a screwdriver to break free.
Shelby returns home, only to find Elizabeth gone from the bathroom. Elizabeth retrieves Shelby’s jacket and limps down to the barn, where she finds her own car and attempts to escape, but Shelby recaptures her and ties her up with duct tape. While bound, Elizabeth concedes she knows about Shelby’s mother’s death and pleads for mercy, offering money if Shelby would release her. Shelby ignores this, prunes Elizabeth’s Achilles tendons with garden shears, gags her, and locks her in the basement.
Meanwhile, Mike prepares for the jersey-retirement ceremony, and Billy stops by Shelby’s house for a ride. With the power cut, Elizabeth uses the moment to cut the power and instructs Mike’s departure as chaos unfolds. Billy goes down to the basement with a flashlight and finds Elizabeth bound; Shelby silently approaches and attacks Billy with an axe, killing him and burning his body in a barrel.
At the high school ceremony, Mike discovers Elizabeth’s initials on his jacket, realizing Shelby may be holding her captive. He races to Shelby’s house, where he hears Elizabeth’s voice from the basement door. A tense confrontation follows, and Shelby shoots Mike in the leg. A brutal struggle ensues, and Shelby is knocked down; Elizabeth fights back using Mike’s helmet, delivering blows until Mike can lift her away from the danger. In the end, the screen fades to a home video of Shelby and Mike kissing and laughing behind the camera, as a cruel reminder of their past. The final shot, however, reveals Shelby’s eyes opening in the dark—an unsettling sign that she is still alive.
