LAPD Detectives Dwight "Bucky" Bleichert (Josh Hartnett) and Lee Blanchard (Aaron Eckhart) are paired as partners after engaging in a boxing match to raise funds for the department & to generate publicity for a city vote on a proposal to increase policemen salaries by 8%. After gaining the upper hand and flooring Lee, Bucky ends up losing the fight and 2 of his front teeth.Lee is well known detective in the department and introduces Bucky to Deputy District Attorney Ellis Loew (Patrick Fischler) & Chief Thad Green (Troy Evans). Lee introduces Bucky to his girlfriend Kay Lake (Scarlett Johansson), and the trio becomes inseparable. The duo arrest many known mob bosses and become famous. Lee notices the growing closeness between Kay and Bucky. 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 (Kay testified against Bobby). Bobby is scheduled to be released from Prison in late Jan 1947.Soon after, on January 15, 1947, Elizabeth Short's (Mia Kirshner) dismembered body (all her internal and reproductive organs have been removed and her mouth was carved in the manner of a smile) is found and she is dubbed "The Black Dahlia" by the press. Both detectives become obsessed with solving the case. On that day, the duo were apprehending Baxter Smith, the associate of a known sexual predator Nash in a nearby building and were involved in a shootout. Lee saved Bucky's life, and Baxter ends up dead. Nash didn't kill Elizabeth as his victims were all young black girls. Cleo A. Short (Kevin Dunn), Elizabeth Short's Father works at a local diner and has an alibi for the day and time on which Elizabeth was killed. Cleo says Elizabeth was fascinated by the military types and was a bit of a tramp.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 (Hilary Swank), who looks very much like Elizabeth. Madeleine, who comes from a prominent family, tells Bucky that she was 'very close' with Elizabeth (They had sex once) but asks him to keep her name out of the papers in exchange for sexual favors. She introduces him to her wealthy parents (Emmett (John Kavanagh) and Ramona (Fiona Shaw), sister Martha (Rachel Miner)) almost immediately, to confirm her alibi.Ramona doesn't like Emmett and says he married her for her father's money. She calls Bucky one of Madeleine's male whores and says she wont be insulted like this, in front of a common policeman. While having sex, Madeleine tells Bucky that her dad is corrupt (using old film-sets to build firetrap housings) and that's how he made his money in LA.. It is suggested that Madeleine is very concerned about her image and that she knew Elizabeth looked like her and that she acted in Porno films. That could be a motive to kill her.Lee's obsession leads him to become erratic and abusive toward Kay. After Lee and Bucky have a nasty argument (Lee locates the sexual predator Nash and ends up killing him) 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 (Richard Brake). 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 garrote 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. When Dewitt was about to be released, his snitch, who introduced Kay to Lee, demanded $10,000. The snitch was Baxter Smith. Lee had eliminated him, in the garb of the Nash shootout. Bucky now realizes Lee was there that night to kill DeWitt and leaves, furious, to return to Madeleine, where he notices photo of Georgie, Emmett's business associate, and 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 (William Finley). 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 (Rachel Miner) 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.