It takes two quick scenes to establish the degenerate character of NYPD detective Jack Mosley, a quintessential broken-down cop in the well-executed thriller 16 Blocks.

Asked to secure an early morning crime scene until forensics arrives, Mosley sits on a couch drinking bourbon and reading the newspaper. Back at the station house at the end of the same overnight shift, the squad secretary automatically hands him breath mints.

Even more than his actions, his appearance is telling. Mosley is strikingly gaunt and scruffy, with bloodshot eyes and a bum leg. Booze plus years on the job have taken their toll.

Bruce Willis has made the weary, troubled cop his specialty (most recently in Sin City) and each incarnation is slightly different. This dispirited, malnourished specimen is a shambles, having no where to go but up. When he wheezes "I'm tired," you believe him.

Hung-over and anxious to clock-out, he's ordered to transport a felon from lock-up to the courthouse sixteen blocks across lower Manhattan. Taking into account rush hour gridlock, it's still a simple assignment. But when Mosley decides to pop into a liquor store en route, all hell breaks loose. As quickly as you learned what a bum he is, you realize he'll prove his mettle and do the right thing.

Mosley's adversaries turn out to be his fellow officers -- led in fact by his ex- partner, Frank Nugent, a poster boy for police corruption played by David Morse. The "X" factor in the equation is motor-mouthed petty criminal Eddie Bunker (Mos Def), who witnessed the men in blue do something very bad and is slated to spill before the grand jury. They are determined to stop him. 

Morse has had a decent career playing the bad cop. When he darkens the door in a movie, you can cue the villain music. Here he's likeably reptilian; his calm rationalizations for shielding police at the expense of the criminal element are made creepier by his Mephistophelean goatee. As venal as Nugent is, part of you wants him to prevail or at least not go down. He and his fellow dirty detectives are just protecting their own; it's about business, and before he burnt out, Mosley was knee-deep in it.

Hip-hop artist Def has done some fine acting work in The Italian Job, HBO's Something the Lord Made, and as a cop in The Woodsman. This role will give him maximum exposure. Apart from an annoying, intermittent nasal whine (presumably a choice made to convey a personality tic of Eddie’s), it's a breakthrough performance that may do for Def what Collateral did for Jamie Foxx -- i.e. prove he's a substantial actor. 

The interplay between Mosley and Eddie is almost as intriguing as that between Tom Cruise's assassin and Foxx's cab driver. A long rap sheet notwithstanding, Eddie is an un-threatening criminal. Constantly yammering, he dreams of baking birthday cakes and carries around a recipe book. Race is handled in an understated manner. The police, Mosley included, call African-American Eddie "kid", a synonym for "boy." And yet Mosley is the one treated like an errand boy.

With renegade cops giving chase, Mosley and Eddie scurry through Chinatown, ducking into the subway, tenements and sweat shops before a hostage stand-off on a city bus. The final confrontation is a model for good cop/bad cop movie encounters. Despite improbable coincidences, the entire scenario is handled with such sure-footed alacrity you'll think you're seeing it for the first time.

Director Richard Donner (Lethal Weapon) freshens the proceedings with big assists from his acting trio and an action script that exploits familiar characters. He delivers the tension of a beat-the-clock plot along with meaningful bonding.

According to 16 Blocks, people really can change and so everyone deserves a second chance. Willis, Morse, and Def don't need one. They should stick to what they're doing.

(Released by Warner Bros. and rated "PG-13" for violence, intense sequences of action and some strong language.)