Unknown / Provided Photo

Movie mysteries have an implicit duty to their audiences. In exchange for two hours of our time — most of which are spent puzzling over false leads and leaning over asking, “Who’s that guy again?” — we are rewarded with a conclusion that justifies all the nail-biting and eyebrow-furrowing.

Brian De Palma’s The Black Dahlia provides solutions to its mysteries, but they are the sort of schlock soap opera characters dream for a season while blacked out on the bathroom floor. It doesn’t help that little else in the movie works, either — a shame since the source material is so luridly compelling.

In 1947, the body of an aspiring actress named Elizabeth Short was found in a field in Hollywood, the corpse cut in half, most internal organs removed, face sliced from ear to ear. In real life, the still-unsolved murder, nicknamed The Black Dahlia, became one of the biggest media sensations of the century’s first half. In the mind of literary icon James Ellroy, the murder became a complex web of police corruption and seduction set in glamorous post-war Hollywood, all detailed in his 1987 bestseller.

For a while, Dahlia is diverting but dull detective fluff. Officer Bucky Bleichert (Josh Hartnett) narrates his working relationship with buddy/rival Sergeant Lee Blanchard (Aaron Eckhart) and establishes some tame triangular sexual tension with Blanchard’s gal, Kay Lake (Scarlett Johansson). Playing the classic 1940s dame — in love with her man, mad about martinis — to perfection, Johansson is one of the few redeeming qualities of the movie.

De Palma, a director not known for subtlety, initially restrains his penchant for overkill, but once Short’s body is discovered in the field, he throws caution to the wind and away he goes. The movie becomes an unseemly mishmash of De Palma’s over-the-top visual style and enlarges its labyrinthine web of supporting characters (“Who’s that guy again?”). The biggest misfire among all the supporting players is a haughty heiress, played by Hilary Swank, with connections to the dead girl. A well-respected two-time Oscar winner, Swank gives one of the worst and most miscast performances of recent years, though Hartnett’s mopey non-charisma trails in second. Her embarrassing attempt at a Scottish brogue comes and goes and she parades around oozing as much sexuality as the corpse in question. A vixen Swank is not.

And a movie The Black Dahlia is not. A cruel joke is more like it, especially as its “twists” finally arrive in a creaky scene that throws in flashbacks galore, attempts to explain the rest and prays to God we’re too confused to notice how insultingly preposterous it all is.

Hey, at least it’s over.