LAPD detectives Dwight "Bucky" Bleichert and Lee Blanchard are partnered together following a boxing match aimed at raising public support for their department. Lee introduces Bucky to his girlfriend, Kay Lake, forming a tight-knit trio. Bucky is taken aback when Kay reveals she hasn't been intimate with Lee, leading her to make advances toward him, which he rebuffs. Complicating matters, Bucky learns that Kay has the initials "BD" branded on her, which signify Bobby DeWitt, a notorious gangster whose arrest and conviction for a major bank heist boosted Lee's career.
Tragedy strikes on January 15, 1947, when the dismembered remains of Elizabeth Short, infamously known as "The Black Dahlia," are uncovered. The grim discovery captivates both detectives, driving them into an obsessive quest for answers.
Bucky discovers that Elizabeth aspired to be an actress and had connections in the underground scene, even starring in an adult film. During this investigation, he visits a lesbian nightclub where he encounters Madeleine Linscott, who bears a striking resemblance to Elizabeth. Madeleine, who hails from an affluent family, claims to have been close to Elizabeth but requests that he keep her name out of the media in return for sexual favors, leading to a swift introduction to her wealthy parents.
As Lee's fixation grows, it turns him volatile and abusive towards Kay. Following a heated confrontation over a past case, Bucky seeks to make amends, but upon visiting Lee and Kay, he discovers that Lee had been pursuing a lead on Bobby DeWitt. In a harrowing sequence, Bucky witnesses an armed confrontation where Lee is brutally killed, only for Bucky to subsequently lose Lee as another mysterious figure murders his partner.
In the aftermath of Lee's death, grief binds Bucky and Kay in a transient moment of passion. The following morning, Bucky stumbles upon cash hidden in the couple’s bathroom, prompting Kay to disclose her past with DeWitt, analyzing the cycle of abuse she endured and how Lee heroically intervened by imprisoning DeWitt and absconding with his money. Realizing the reason for Lee's final movements deepens Bucky’s anger and drives him back to Madeleine, where he is struck by the resemblance she bears to the recently deceased Elizabeth.
Piece by piece, Bucky connects the dots, recalling that the props from an old film, "The Man Who Laughs," coincided with those used in Elizabeth's adult film. His investigations reveal links to Emmett Linscott, Madeleine's father, who embarrassingly would compromise safety standards while building inexpensive firetrap housing. He uncovers a chilling scene below the Hollywoodland sign, recognizing a set from Elizabeth's film where he discovers evidence of her murder and a grotesque drawing reflecting the unique Glasgow smile that marked Elizabeth’s face.
Confronting both Madeleine and her parents reveals dark secrets; her mother Ramona confesses to murdering Elizabeth, citing that Madeleine was not only her daughter but bore a deeper connection to Elizabeth’s looks. Driven mad by these revelations, Ramona takes her own life before Bucky can process the horror.
Days later, recalling a significant statement from Lee, Bucky visits Madeleine's sister Martha, leading to revelation after revelation about Lee's pernicious knowledge of Madeleine and his blackmail of her family. Ultimately, Bucky tracks down Madeleine to a rundown motel, where she admits to the heinous crime of murdering Lee. As he wrestles with his own intentions, he refrains from her seductive allure and, with a heavy heart, ends her life. Bucky then seeks solace at Kay's home, taking a step further down a dangerously dark road when she invites him inside, sealing their fates together.
