NOTE: This spoiler was submitted by MovieLover
The movie starts off with Katie Burke (Katie Holmes) sitting on some stairs. Her friend Sam (Zooey Deschanel) goes with her for a walk. Katie is going to a workshop for some sort of agency that she wants to work at when she gets out of college. We see that Katie is also under severe pressure to complete her thesis.
At the same time, Detective Wade Handler (Benjamin Bratt), an ex-alcoholic, is given a new case by his lieutenant (Fred Ward). It concerns a missing person named Embry Larkin (Charlie Hunnam). Embry was a rich orphan. Handler finds out that Embry was Katie’s boyfriend when he disappeared. Handler contacts Katie, who says she’s not sure if Embry disappeared, left school, or both. She also seems to be generally irritated by Handler.
Katie, Sam and some friends including Harrison (Gabriel Mann) and Amanda (Gabrielle Union) go to a party one night, so that Katie can relax from constant stress and exhaustion, mostly because of her thesis and wondering if she’s going to get her job. All of them get drunk/stoned, and during a cool strobe light effect scene, Katie thinks she sees Embry lurking. A few days later, she goes outside and thinks she sees him again, driving away in a car. Handler tells Katie that he thinks Embry is dead, because he had two airplane tickets to Greece that he has never used, not to mention never having taken any money or credit out of an account or anything.
One night while Katie is studying hard for her thesis, she falls asleep, completely exhausted. When she wakes up, a number has been inscribed in the desk where she was sitting. It’s a number for a library book. She goes to shelf where the book is, titled “The Inferno”.
Suddenly, she sees what are presumably Embry’s eyes staring at her from the other side of the shelf. She slowly backs out. He’s gone.
Handler talks to Sam, Amanda, and Harrison about Katie and Embry. Sam tells him that Embry didn’t like Katie’s male friends. Harrison, who has long had a crush on Katie but doesn’t seem to realize the feeling isn’t mutual, tells Handler than Embry once put him in the hospital because he poked him with a glass bottle. Harrison confesses to Katie that he loves her, but she says he’s a great guy, but she doesn’t feel the same.
The next day, Katie is again in the library, hard at work. An odd girl in the library, Mousy Julie (Melanie Lynskey) bothers her. Julie tells her that Harrison has disappeared. Katie is convinced that Embry is responsible for Harrison’s disappearance. Katie has lunch with Harrison’s parents who are trying to account for his disappearance. While in the restaurant, Embry sits at another table. Katie storms outside and yells at him. He tells her to come to his country house, which was a nesting spot for the two before Embry disappeared.
Katie goes there and falls asleep again. When she wakes up, Embry is playing the piano. She tries to leave, but Embry follows her. Eventually, he pins her against the wall, but she knees him in the groin and escapes. She goes over to Handler’s apartment. Katie and Handler have sex, and in the middle, she imagines Handler transforming into Embry. She gets this out of her head, and she realizes that for the first time in a while, she’s happy, sitting there with Handler. She also completes her thesis.
Handler goes to a DNA expert, who tells him that a note that Embry has supposedly just written Katie was written 2 years before. Handler gets in his car, and buys an alcoholic drink, stressed. He drinks, but quickly spits it out. Katie is waiting on some steps for Handler to pick her up. They are going to go off somewhere together for a little. Embry suddenly attacks her, telling her that wherever she goes, he’ll find her. Handler shows up, and Katie tells him what just happened.
Katie and Handler go into an old tunnel that Katie and Embry used to hang out in a lot.
Handler has his gun and flashlight. Katie continues to see Embry, but Handler doesn’t. Suddenly, we flash back to Katie and Embry, 2 years before, in that same tunnel, they’re in now. Embry tells Katie that he’s leaving and that he never wants to see her again. Katie gets upset, talking about the trip to Greece they’re supposed to take together. Suddenly, Katie picks up a rock and repeatedly smashes Embry over the head with it, killing him. Katie is crazy because her father did the same thing to her when she was little. He abandoned her.
Handler realizes that Katie is a psycho and that he can’t go off with her. Katie picks up another rock and hits Handler over the head with it, plunging him to the water where he ultimately dies. We cut forward a few months. Katie is at her job that she wanted when a man she is supposedly seeing who works with her tells her that he’s going to get promoted, and he doesn’t think they should be dating each other.
Katie gets a look on her face…