Loosely based on a true story, “The Black Dahlia” is adapted from a book by James Ellroy, most famous for “L.A. Confidential,” and directed by Brian De Palma, most famous for “Scarface.” The awkward marriage of the two will satisfy fans of neither.

Pugilists-turned-policemen Leland “Lee” Blanchard (Aaron Eckhart) and Dwight “Bucky” Bleichert (Josh Hartnett) become partners after the two deck it out in a staged fight, part of a shady departmental campaign that gets way too much screen-time. Along with Lee’s disaffected girlfriend, Kay Lee (Scarlett Johansson), the cops live it up: drinking, dancing and catching criminals.

The merry m?nage ? trois is broken up by the titular body – brutally murdered Elizabeth Short (Mia Kirshner). Lee, who has a thing for damsels in distress, becomes obsessed with the crime, leaving Bucky and Kay to take care of each other. At the same time, Bucky is getting involved with Madeline Linscott (Hilary Swank), a licentious heiress who may be connected to the crime.

Like “L.A. Confidential,” “The Black Dahlia” takes place in a ’40s Hollywood filled with beautiful people and ugly deeds. There might be some message about corruption and fame here too, but De Palma doesn’t seem to really care and neither should anyone else. The only times the movie has any focus is when the camera is following a beautiful woman or a bloody death. Naturally, there are several of both. But “The Black Dahlia” is too fey for “Scarface”-lovers: no rapper is going to be seen wearing a leather jacket with Josh Hartnett’s face on the back.

As the body count rises, De Palma loses his grip on the noir style. The music is too insistent, the love scenes too intense, the murders too gory (one character even falls off a railing into a pool of water, but this time he’s impaled on the fountain). A plot this convoluted needs a cool hand at the wheel. Instead, De Palma makes each scene into a denouement until neither the plot twists nor the hardboiled dialog rings true.

As the movie slides from drama into camp, few of the actors survive. Hartnett tries vainly to stay serious and almost succeeds, but Eckhart outdoes him with every steely stare. Johansson and Swank turn in shallow impersonations of old Hollywood stars – neither has any humanity. Kirchner, seen in old screen tests as a pathetic Hollywood hopeful, is the most believable of the leading ladies.

But the most memorable performance is Fiona Shaw as Madeline’s snooty, unhinged mother. Her scenes are entirely unrealistic and the only genuinely entertaining part of the movie.

Shaw sees “The Black Dahlia” for what it is: dressed-up trash. The costumes and the camerawork are pitch-perfect, but as any character in a real noir will tell you, a pretty face can’t hide a rotten core.