Earl Brooks (Kevin Costner), a wealthy and respected businessman, lives a secret life as a serial murderer known as the “Thumbprint Killer”. For the past two years he has attended 12-step meetings for addicts to curb his “killing addiction”. When Brooks’ id, Marshall (William Hurt), urges him to resume his murderous compulsion, Brooks kills a young couple having sex in their bedroom. As part of his psychopathology, Brooks leaves each victim’s bloody thumbprint on a lampshade. He follows a meticulous modus operandi, including fastidious preparation, cleansing the crime scene, posing the bodies, and even locking doors before departing. Brooks realizes later that the bedroom curtains were open during the murder, the window facing an apartment building.
Brooks’ daughter, Jane (Danielle Panabaker), unexpectedly arrives home, having dropped out of college; she wants a job at her father’s company. The same day, a man calling himself “Mister Smith” (Dane Cook) turns up at Brooks’ office. He lives in the apartment across from the murdered couple and blackmails Brooks with photos he took during the crime. Brooks reluctantly agrees to Smith’s demands to accompany him a murder, warning Smith that killing can become addictive.
Brooks’ wife, Emma (Marg Helgenberger), says that Jane left college because she is pregnant. Soon after, the Brooks are visited by Palo Alto detectives investigating a murder in Jane’s former campus dorm. Brooks realizes that Jane committed the murder and initially considers allowing her to be caught to “save her” from becoming like him though later changes his mind. Assuming an alternate identity, he flies to Palo Alto and commits a similar murder to provide Jane an alibi.
Detective Tracy Atwood (Demi Moore) interviews Smith and several other residents living across from the latest crime scene. Brooks learns Atwood is undergoing a difficult divorce from her husband, Jesse Vialo (Jason Lewis). He decides that Vialo and his lawyer/lover, Sheila (Reiko Aylesworth), will be Smith’s first “victims”. At the scene of the Vialo murder, Smith wets his pants in a fit of panic, leaving his DNA to be discovered later.
After leaving the scene, Smith pulls a gun on Brooks, which Brooks anticipated might happen. Brooks says he is unable to stop killing and plans to commit suicide to spare his family the shame of his being caught. He takes Smith to a cemetery. Brooks wants Smith to shoot him and place his body in an open grave that will be filled the next day.
Smith attempts to shoot Brooks, but Brooks had previously disabled Smith’s gun on the off-chance he changed his mind about suicide. Brooks now wants to live to see his grandchild and instead kills Smith, hiding his body in the open grave. Smith’s urine has provided the only DNA evidence tied to the Thumbprint Killer, allowing Brooks to remain undetected and return to his normal life.
Knowing he is in the clear, Brooks calls Detective Atwood, whom he has come to admire, and asks why she became a police officer. She says her wealthy father had wanted a son, and she needed to prove herself. Brooks hangs up before the call is traced. Atwood realizes the voice was not Smith’s, and someone else may be the Thumbprint Killer. That night, Brooks has a nightmare in which Jane murders him, suggesting he fears she will become like him.