In Los Angeles, on January 15, 1947, LAPD Detectives Dwight 'Bucky' Bleichert and Lee Blanchard, investigate the murder and dismemberment of Elizabeth Short, soon dubbed 'The Black Dahlia' by the press. Bucky learns that Elizabeth was an aspiring actress who appeared in a pornographic film. Through his investigation, Bucky learns that Elizabeth liked to hang 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 his silence, she promises him sexual favors. Continuing his relationship with Madeleine, Bucky meets her wealthy parents, Emmett and Ramona.
Bucky's partner, Lee, also becomes obsessed with Elizabeth's murder. Lee's obsession leads him to become erratic and abusive towards his long-time girlfriend Kay Lake, who is also one of Bucky's close friends. 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 a recently released convict, Bobby DeWitt. Bucky goes to the location and gets into an altercation with DeWitt in the atrium of the building. DeWitt is gunned down by Lee, standing on the stairs across the atrium. Bucky sees a man sneak up behind Lee, wrapping a rope around Lee's neck. Lee fights back while Bucky, paralyzed with shock, watches from across the atrium as a second shadowy 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. It is then that Bucky is helped by Millard and Morrie Friedman; a friend of Lee's whom Bucky saw with Lee at the New Year's party in 1946.
Dealing with the grief of losing Lee propels Bucky and Kay into a sexual encounter. The next morning, Bucky finds money from a bank robbery hidden in Lee / Kay's bathroom. Kay reveals that she had been DeWitt's girlfriend, that DeWitt had mistreated her, and that DeWitt had done the bank robbery; stealing a large sum of money from one of Benny "Bugsy" Siegel's nightclubs. Lee had rescued Kay and stolen DeWitt's bank robbery money. Lee needed to kill DeWitt now that he was out of prison; leading to the encounter that resulted in Lee's death. Bucky leaves, furious with Lee and Kay for their actions and lies. He returns to Madeleine's family mansion and continues his intense relationship with her. Kay is furious when she discovers the relationship, especially with the fact that Madeleine bears a striking resemblance to the same girl Lee obsessed over before he was killed, and leaves the scene.
Watching an old movie one night, Bucky notices that a bedroom scene matches the set in Elizabeth's pornographic film. The credits at the end of the film includes the statement "Special Thanks to Emmett Linscott", Madeleine's father. Bucky's search for answers leads him to an incomplete housing project that Madeleine's father had started just below the Hollywoodland sign. In one of the empty houses, Bucky recognizes the set that was used to film Elizabeth's pornographic movie. In a barn on the property, Bucky finds where Elizabeth was killed and her body butchered, as well as a drawing of a man with a Glasgow smile. The drawing resembles a painting in Madeleine's family home and matches the disfiguring smile carved into Elizabeth's face during her murder.
Bucky confronts Madeleine and her father in their home, accusing them of murdering Elizabeth. Madeleine's mother Ramona reveals that she was the one to kill Elizabeth, who looked so much like Madeleine. She confesses first that Madeleine was not fathered by Emmett but rather by his best friend, George. She further reveals that George had been on set when Elizabeth's pornographic film was made, becoming infatuated with her. Finally, she felt that Elizabeth looked too much like Madeleine, was bothered that George was going to have sex with someone who looked like his own daughter, and decided to kill Elizabeth first. Upon finishing her confession, Ramona kills herself.
A few days later, remembering something Lee had said during the investigation, Bucky visits Madeleine's sister Martha with some questions. He learns that Lee knew about the lesbian relationship between Madeleine and Elizabeth and was blackmailing Madeleine's father to keep it secret. Bucky finds Madeleine at a seedy motel, and she admits to being the shadowy figure 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 later goes to Kay's house. Kay tells him to come in and closes the door as the film ends.
Output: Why did Lee need to kill DeWitt?