LAPD Detectives Dwight "Bucky" Bleichert and Lee Blanchard are paired as partners after engaging in a boxing match to raise funds for the department. Lee introduces Bucky to his girlfriend Kay Lake, and the trio becomes inseparable. Bucky is shocked when Kay tells him she isn't sleeping with Lee, and later tries to seduce him, but he refuses. He also discovers that Kay has been branded with the initials "BD", for Bobby DeWitt, the gangster whose arrest and conviction for a big bank robbery made Lee's career.
Soon after, on January 15, 1947, Elizabeth Short's dismembered body is found and she is dubbed "The Black Dahlia" by the press. Both detectives become obsessed with solving the case.
Bucky learns that Elizabeth was an aspiring actress who appeared in a pornographic film and hung out with lesbians. He goes to a lesbian nightclub and meets Madeleine Linscott, who looks very much like Elizabeth. Madeleine, who comes from a prominent family, tells Bucky that she was 'very close' with Elizabeth but asks him to keep her name out of the papers in exchange for sexual favors. She introduces him to her wealthy parents almost immediately.
Lee's obsession leads him to become erratic and abusive toward Kay. After Lee and Bucky have a nasty argument about a previous case, Bucky goes to Lee and Kay's to apologize, only to learn from Kay that Lee was responding to a tip about Bobby DeWitt. Bucky finds DeWitt in the atrium of a building before he is gunned down by Lee, then sees a man garrote Lee before a second figure steps out and slits Lee's throat. Lee and the man holding the rope fall over the railing to their deaths several floors below.
The grief of losing Lee propels Bucky and Kay into having sex. The next morning, Bucky finds money hidden in Lee and Kay's bathroom. Kay reveals that she had been DeWitt's girlfriend and that he abused her. Lee rescued Kay, stole DeWitt's money, and put DeWitt behind bars. Bucky realizes Lee was there to kill DeWitt and leaves, furious, to return to Madeleine, where he notices a painting of a leering clown. Kay follows him and she is appalled to see Madeleine's striking resemblance to the Dahlia.
Bucky starts putting the pieces together and remembers props in another movie matched the set in Elizabeth's pornographic film. The end credits thanked Emmett Linscott, Madeleine's father, and Bucky digs deeper into a story Madeleine told about him using old film sets to build cheap firetrap housing. In an empty house below the Hollywoodland sign built by Emmett, Bucky recognizes the set that was used in Elizabeth's film. He finds evidence in a barn on the property that Elizabeth was killed and butchered there, as well as a drawing of a man with a Glasgow smile. The drawing matches the painting in Madeleine's home and the gruesome smile carved into Elizabeth's face.
Bucky confronts Madeleine and her father in their home and Madeleine's mother, Ramona, reveals that she killed Elizabeth. She confesses that Madeleine was not fathered by Emmett but rather by his best friend, Georgie. She says Georgie became infatuated while watching Elizabeth film the pornography. Ramona was disturbed by the idea of George having sex with someone who looked so much like his own daughter, and lured Elizabeth to the house and killed her. Before Bucky can decide what to do, Ramona shoots herself.
A few days later, remembering something Lee said during the investigation, Bucky visits Madeleine's sister Martha with some questions. He learns that Lee knew about Madeleine and Elizabeth, and blackmailed Madeleine's father to keep it secret. Bucky finds Madeleine at a seedy motel, and she admits to being the one who slit Lee's throat. Although she insists that Bucky wants to have sex with her rather than kill her, he tells her she is wrong and shoots her dead. Bucky goes to Kay's house and she invites him in and closes the door.
Josh Hartnett as Detective Dwight "Bucky" Bleichert / Mr. Ice
Scarlett Johansson as Katherine "Kay" Lake
Aaron Eckhart as Detective Lee Blanchard / Mr. Fire
Hilary Swank as Madeleine Linscott
Mia Kirshner as Elizabeth Short / Black Dahlia
Mike Starr as Detective Russ Millard
Fiona Shaw as Ramona Linscott
Patrick Fischler as Deputy District Attorney Ellis Loew
James Otis as Dolph Bleichert
John Kavanagh as Emmett Linscott
Rose McGowan as Sheryl Saddon
Troy Evans as Chief Thad Green
Pepe Serna as Tomas Dos Santos
Anthony Russell as Morrie Friedman
Angus MacInnes as Captain John Tierney
Rachel Miner as Martha Linscott
Richard Brake as Bobby DeWitt
Victor McGuire as Sergeant Bill Koenig
Gregg Henry as Pete Lukins
Jemima Rooper as Lorna Mertz
William Finley as Georgie Tilden
Kevin Dunn as Cleo A. Short, Elizabeth Short's Father
Ian McNeice as The Coroner
Fatso-Fasano as Dealer
Claudia Katz Minnick as Bartender At The Frolic Club
Musician k.d. lang has an uncredited cameo as the floor show performer at the lesbian nightclub Laverne's Hideaway.
an adaptation of the 1987 novel of the same name by James Ellroy, in turn drawn from the widely sensationalized murder of Elizabeth Short.