On the surface, everything looks promising for The Black Dahlia: Brian De Palma knows his way around the crime genre like few directors do, and with The Untouchables on his CV, the idea of him adapting James Elroy’s 1940’s-set crime drama seems a safe bet. And for the first few minutes, The Black Dahlia is note-perfect, beginning with one of the best opening scenes in recent memory – a long series of pans over a brutally choreographed riot – and plunging from there into the complicated career of our narrator, tough-talking police officer “Bucky” Bleichert (Josh Hartnett).

Ex-boxer Bucky gets a swift promotion by agreeing to go ten rounds with Aaron Eckhart’s Sgt. Leland Blanchard, also a former prize-fighter, on condition that he throw the fight for reasons of departmental politicking. After the match (another stunning set-piece from De Palma) the two become friends and partners, and as Bleichert spends more time with Blanchard, so he also becomes more acquainted with his girlfriend Kay (Johansson), until the lines of their relationships begin to look distinctly blurred.

The central players fit their roles well, and are good enough to make the cliché of a love triangle fairly interesting. The all-important forties atmosphere is spot-on, not perhaps in any historical sense, but in creating down to the smallest detail a bleached, almost sepia and yet distinctively noir vision of LA.

But as the scene-setting drags on and on, so it becomes harder to avoid the nagging question of, isn’t this supposed to be a film about the Black Dahlia? And by the time the Dahlia murder finally occurs, it’s hard to remember what movie you’re supposed to be watching. What should have been the thematic centre arrives more as an afterthought, and stays resolutely sidelined for much of the film’s course.

Both Bleichert and Blanchard eventually become obsessed with the murder and its victim, Elizabeth Short. But all that happens is that more characters, plot threads and distractions are introduced. Bucky, for example, discovers a wealthy and deeply eccentric family, the Linscotts, and starts an affair with the oldest daughter Madeleine (Hilary Swank). Said eccentricity makes for some blackly comic scenes (which are very funny, if slightly inappropriate amidst the rest of the film), and Swank is entertaining in the role of femme fatale; but its hard to see what relevance the Linscotts have to the Dahlia case.

The reason, it turns out, is that all the crucial information has been saved for the denouement. When things finally come together, it’s in a dizzying flurry of twists and surprises that do more to confuse than clarify – not because the muddle of competing plot strands is particularly complicated, but because screenwriter Josh Friedman handles it so messily.

In the end, The Black Dahlia is worth watching for a reasonably on form De Palma, who is obviously having fun and manages some jaw-dropping sequences. There are plenty of great moments – enough to hint at the classic neo-noir that this might have been. But the whole is nowhere near as good as the sum of its parts, and you’ll likely end up enjoying The Black Dahlia more for what it might have been then what it actually is.