In 1947, 22-year-old Elisabeth (Mia Kirshner) arrives in the bustling city of Los Angeles. Like many young girls, her biggest dream is to become a movie actress. But she is not yet beautiful enough to attract attention at first sight. She has long hair, likes to go to bars and nightclubs, believes in love and romance, and spends a lot of time on makeup. Soon she became a budding starlet. But after her death, her life remained a huge mystery. She said her husband died in World War II, but that was only part of her fictionalized story, and her name was a secret, dubbed "The Black Dahlia" because of her black hair and black clothes.

One day in 1947, a pedestrian found the body of the Black Dahlia in a vacant lot near Hollywood. Her naked body had been brutally hacked in two and showed signs of severe beatings and sexual abuse. There were several chilling rumors about the details of her death. Strangely, when the murder was revealed in the news, many people turned themselves in to police stations and admitted that they had killed the Black Dahlia.

Detectives Butch (Josh Hartnett) and Lee (Aaron Eckhart) are tasked with tracking down the murders, and as the case unfolds, Lee and his fiancée, Kay (Scarlett Johansson), are terrorized by an unknown person, while Butch is intrigued by the daughter of a large Los Angeles family, Madeleine (Hilary Swank), and is shocked to discover that Madeleine may have something to do with the case. So Butch and Lee follow the trail and gradually discover the inner workings of a larger conspiracy at the police station.