I had to stop trying to figure out who killed the Dahlia,” an investigator says in the third act of Brian De Palma’s disappointing, often ludicrous noir, The Black Dahlia. For the fictionalized detective, it’s a matter of concentrating on another angle of the real-life 1947 Elizabeth Short murder. For the audience, however, this decision will likely be motivated by the same attitude taken by the LAPD after an exhaustive probe into the still-unsolved case: surrender.

Based on the book by James Ellroy (L.A. Confidential) and scripted by War of the Worlds writer Josh Friedman, The Black Dahlia doesn’t quite begin promisingly, either. Josh Hartnett—do I really need to continue?—is Bucky Bleichert, a champion-fighter-turned-cop who’s partnered with another former pugilist and sometime opponent, Lee Blanchard (Eckhart again). Though voice-over is a standard and often compelling component of film noir, the duty here is left to Hartnett, who, both in voice and performance, doesn’t seem to know the difference between cool and just plain flat. Instead of playing the young hotshot to Lee’s older, unflappable cop, Hartnett weighs the film down with Bucky’s initial stoicism, which doesn’t make his later turn as passionate crime-solver and ardent lover terribly believable, either.

We find that Bucky and Lee are not only work partners; Lee’s apparent girlfriend, Kay (Scarlett Johansson, again self-assured and sophisticated beyond her 21 years), takes a shine to Bucky, and the three of them develop an odd friendship in which they spend the bulk of their free time together. And though the guys start off as regular fuzz, they’re soon promoted by the district attorney after he organizes a boxing match between them to attract positive publicity. Here, the story finally takes off: Their first case, which turns into a messy shootout, is quickly overshadowed when the mutilated corpse of a young woman is found nearby. They discover that it’s Short, who had been nicknamed the Black Dahlia because of her fondness for dark clothing. She was an aspiring actress—and likely prostitute—who’d ended up taking roles in black-and-white porn films (with a usually unsatisfied, unseen director voiced by De Palma). Her body was cut in half at the torso and drained of blood, her bowels and reproductive organs removed, her arms bruised, her mouth sliced to either ear in a bloody smile.

Despite the murder’s gruesomeness, De Palma, director of such acclaimed blood baths as Scarface and Carlito’s Way, doesn’t linger on it, mostly giving the details as a coroner’s report and allowing only glimpses of the body. The investigation itself takes interesting directions, leading Bucky to discover that Short (played by Mia Kirshner in flashback) was possibly a lesbian and involved with a similar-looking vamp, Madeleine Linscott (Hilary Swank). Meanwhile, Lee goes, as Bucky says, “all squirrelly” over the case, obsessing about it 24/7, becoming cruel to Kay, and furiously leaving a meeting in which the department is watching one of Short’s old films for clues.

From there, The Black Dahlia goes spectacularly wrong. All sorts of extraneous characters, some only represented in name, get tangled in the case. Noir-slick and period-appropriate dialogue such as “no dice” degenerates into such nonsense as this exchange between Bucky and Madeleine: “I don’t get modern art.” “I doubt modern art gets you, either. I do!” And Bucky’s latent passion is absurdly ignited—see him sweep dinner off the table for a melodramatic, out-of-nowhere embrace or rattle off the confusing chain of events he’s unraveled that might lead to Short’s killer. (Mark Isham’s mournful-jazz score, mostly stylish, only adds to the soapiness of such scenes.) And in addition to Hartnett’s instability, Johansson is ’40s-era beautiful yet uncharacteristically disappointing, barely registering as she’s given little to do but be a housebound girlfriend.

Eckhart, though, completes his 2006 leading-man trifecta—including roles in Thank You for Smoking and Conversations With Other Women—with a terrific performance as the increasingly unhinged Lee: Watch his face go from amused to uneasy to downright menacing in seconds as he sees Bucky plant one on Kay at a New Year’s ball. Kirshner is wide-eyed and doleful; also worth mentioning is Fiona Shaw as Madeleine’s nutty mother, who turns a family dinner hilariously awkward with her drunken comments and goes far, far over the top in a second scene that is laughable yet oddly compelling. Besides its few good performances, the best thing about The Black Dahlia is its lushness, with sunny days and gleaming, golden homes, as well as nights and old-fashioned indoor set pieces nearly sepia in color. Like the Dahlia herself, it’s all too pretty to end up such a wreck.