Richard Haywood, a wealthy and popular high-schooler, secretly teams up with another rich kid in his class, brilliant nerd Justin 'Bonaparte' Pendleton. His erudition, specially in forensic matters, allows them to plan elaborately perfect murders as a perverse form of entertainment. Meeting in a deserted resort, they drink absinthe, smoke, and joke around, but pretend to have an adversarial relationship while at school. Justin in particular behaves strangely, writing a paper about how crime is freedom and vice versa, and creating a composite photograph of himself and Richard.
The young men kill a woman and set up Richard's marijuana supplier, janitor Ray Feathers, as a psychotic serial killer. They then incinerate all evidence. The case is assigned to detectives Cassie 'the hyena' Mayweather, who carries a sequoia-size chip on the shoulder from her previous life, and her new partner, Sam Kennedy, who just transferred from the vice squad. They work together well, and begin a romantic relationship very early on in their partnership. Footprints found at the crime scene lead to Richard Haywood, who has an alibi. Cassie becomes seemingly obsessed with the idea that Richard is guilty, and refuses to pursue the case from other angles. This displeases Captain Rod Cody and Cassie's vindictive ex, Assistant D.A. Al Swanson, who become nervous because of the clout held by Richard's parents.
False forensic evidence planted on the body by Justin, including a baboon hair, lead police to the janitor, who keeps a baboon in his shed. The baboon attacks Cassie, causing her to be hospitalised. Meanwhile, Richard kills the janitor in a staged suicide. Sam and Cassie fall out over different professional attitudes towards the investigation, and Captain Cody bans her from pursuing the investigation further when she disobeys instructions.
Justin is interested in his classmate Lisa Mills, and ignores Richard in order to go on a date with her. Richard sees them kissing, becomes jealous, and shows Justin a tape of Lisa having sex with Richard. This enrages Justin, who destroys the composite photo, but regains control of his emotions and remains friends with Richard. Determined to prove that Richard and Justin in fact know each other and are guilty, Cassie secretly takes pictures of them together. However, Richard sees her, and attempts to frighten her by seducing her. To protect herself, Cassie hits Richard in the face. Richard's father contacts Captain Cody, who decides that Cassie must see a psychiatrist. However, Sam decides that Cassie may be right, and further investigates the case.
It is revealed that Cassie herself was the victim of attempted murder by a serial killer to whom she was married, and who had been her high school classmate. Various flashbacks show Cassie's ordeal. Although she was found alive, she has scars on her chest from her wounds. The murderer is in jail, and Cassie is trying to escape testifying at his upcoming parole hearing.
Sam and Cassie bring Richard and Justin in for interrogation. Using the Lisa Mills incident, they try to convince the young men to confess, bribing them with a lighter sentence. Although they know that both Richard and Justin were involved in the crime, they are unsure which of them actually killed the woman. The killer will be executed, while the accessory will serve a lighter sentence. However, neither one is willing to talk, and they are released.
When Richard attempts to save himself by making a false "confession" to the DA, Cassie and Sam decide to arrest both Richard and Justin immediately. The two flee to the resort, but Justin first visits Lisa and admits what he did. Lisa calls the police and tells them where to find Richard and Justin. Meanwhile, Richard gives Justin a pistol, and holds one himself, as they will both commit suicide. However, Justin discovers that Richard put bullets into Justin's pistol, but not into his own. Cassie arrives, and tries to arrest them. Justin surrenders, but Richard shoots Justin in the shoulder and flees upstairs.
Cassie pursues Richard, who tricks her into going out onto an unstable balcony. Richard strangles Cassie, but she fights back and pushes him over the railing, and he falls to his death. But, before Cassie can get back inside, the porch breaks. Justin saves her, and she tells him that he will likely get a light sentence.
When Cassie examines the strangulation marks on her neck, she realises that the marks on the dead woman's neck were not made by Richard. Instead of Richard being manipulated by Justin, as she had previously thought, Richard was being manipulated by Justin, who was the actual killer. Cassie and Sam trick him into confessing. Now at peace with her past, Cassie agrees to testify at the parole hearing.
Cast
Sandra Bullock as Det. Cassie Mayweather
Ben Chaplin as Det. Sam Kennedy
Ryan Gosling as Richard Haywood
Michael Pitt as Justin Pendleton
Agnes Bruckner as Lisa Mills
Chris Penn as Ray Feathers
R. D. Call as Capt. Rod Cody
Tom Verica as A.D.A. Al Swanson
