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
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, especially 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 Detective Cassie Mayweather 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 becomes nervous due to the clout held by Richard's parents.
False forensic evidence planted on the body by Justin and Richard kills the janitor in a staged suicide.
Cassie is determined to prove that Richard and Justin in fact know each other and are guilty and, along with Sam, she brings Richard and Justin in for interrogation. 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 resor and Cassie finds them.
The end is near... Who is the killer? Who is the victim?