Quick question, but one that's been nagging at me for some time now: who the hell is Sam Worthington? OK, fine; technically I know who he is, so maybe a better question would be: why Sam Worthington? Just a few short years ago he was plucked from relative Aussie obscurity to headline James Cameron's "Avatar," then quickly handed the keys to the kingdom with leading roles in "Terminator: Salvation" and the "Clash of the Titans" revamp. Massive success came Worthington's way despite very little charisma, no obvious talent, and a distracting, curly mullet. Presumably they were scouring Down Under for a more docile Russell Crowe type, but the flavorless Worthington sure ain't that. Kudos to the man, though, for keeping his gravy train on track, though he can take neither blame nor credit for the fact that the preposterously entertaining "Man on a Ledge" is the best bad movie of this young year.

With his problematic Noo Yawk accent and flat line delivery, Worthington stars as former cop Nick Cassidy, who we meet as he's checking into midtown Manhattan's Roosevelt Hotel for a lunch of lobster and fries, followed by a trip just outside his 21st-story window. A flashback to Nick's time in the slammer brings us up to speed as to why Nick might threaten to kill himself, but if you've seen a movie before, or at least this particular movie's amazingly comprehensive trailer, then you know that popcorn action flicks typically don't off their main character. Nick isn't actually suicidal, and through his dealings with troubled police negotiator Lydia Mercer (Elizabeth Banks), we learn that the recent prison escapee claims to have been framed for stealing a mammoth diamond belonging to real estate tycoon David Englander (Ed Harris in hammy evil mode).

Yes, "Man on a Ledge" is another one of those films where someone attempts to convince the world of their innocence by breaking a gaggle of other laws. Because right across the street from Nick, his brother Joey (the always welcome Jamie Bell) is quietly preparing to nick the supposedly missing diamond from Englander's high-tech fortress with the help of his ballbusting girlfriend Angie (Genesis Rodriguez) and her teeny-tiny tank top. Meanwhile, various factions of the NYPD - including the talented Anthony Mackie as Nick's secretive former partner and Edward Burns, a consistently magnetic presence who's turning into a nifty character actor - try to maintain control of the scene, as potential jumper Nick riles up the rubbernecking public in hopes of distracting everyone's attention from the unfolding heist.

The outrageous script by Pablo F. Fenjves piles implausibility upon implausibility, such as convenient paper trails, impossible coincidences, and uncharacteristically dopey moves specifically designed to get one caught. (Fun fact: Fenjves was the ghostwriter for "If I Did It," O.J. Simpson's brazenly tacky fuck-you to the Los Angeles courts system.) "Man on a Ledge" is the live-action filmmaking debut of Danish director Asger Leth, whose first film was the 2006 Haiti documentary "Ghosts of Cité Soleil." Leth's work here is capable enough, but his smartest decision was to enlist cinematographer Paul Cameron; he's shot stuff for action masters like Michael Mann and Tony Scott, and between the often dizzying outdoor work and the DePalma-influenced vault break-in, he offers up the thrills that the clichéd and hilariously expository screenplay never got around to incorporating.

So forget the dull Worthington (at least until "Wrath of the Titans" drops next month) and concentrate instead on his stacked supporting cast. (I haven't yet mentioned Kyra Sedgwick as a gleefully callous reporter.) Even with her woefully underwritten part, Elizabeth Banks is the standout here. For a long time I considered Banks interchangeable with Rachel McAdams - well, after I realized they weren't the same person - but Banks is distinguished by the subtly gritty edge that she lends to her characters, infusing them with a fragile but tough humanity. Never mind the misogynistic boys' club at her job (though dig the deadpan banter between Banks and Burns); Lydia is contending with some pretty substantial demons of her own, and as both she and Nick angle for their respective redemptions, we start to care more about her than about him. Probably not what the people behind the camera intended, but at least we're still watching.