16 Blocks (12A) <!-- none onestar twostar threestar fourstar fivestar -->

Bruce Willis has portrayed his share of desperate cops, but I can't recall any of them being quite the stumblebum he is in 16 Blocks. He plays Jack Mosley, an NYPD detective whose gammy leg and drink problem make him an instant candidate for the boring jobs nobody else wants to do, like delivering petty crook Eddie Bunker (Mos Def) from lock-up to courthouse, 16 blocks away.

A cakewalk, you might think, only Jack decides to stop for breakfast at the liquor store en route and emerges to find his prisoner about to have a great big hole blown through his head. Time was when you'd have felt safe with Bruce as your escort, but one look at his sweaty pallor and heavy gut tells you he's not the superhero of old. It's the kind of reverse makeover Sylvester Stallone endured a few years back for CopLand, playing the tubby, half-deaf but decent lawman who finally stands up to his corrupt colleagues at the station.

And that's exactly what Jack has to do here, because young Eddie is giving evidence that will bring down a whole bunch of cops on murder and extortion charges. So the film charts, in real time, the journey of 16 blocks that Jack and Eddie must negotiate to get to court and send the bad guys down.

And, for about an hour, it's tremendously exciting; not a word I would previously have associated with a movie - or even a scene - by the Lethal Weapon director Richard Donner. Yet the whippy camerawork and lighting look like the work of a proper film-maker, while the location shooting lends downtown New York a humid, edgy feel that puts us right inside the story.

Credit, too, must go to Willis, swapping the famous smirk for a tragic grey moustache and looking so depressed that he can hardly bear to speak. Mos Def, conversely, can hardly bear to shut up, and keeps up a grotesque nasal whine that could offer Philip Seymour Hoffman's Capote a challenge for the most insufferably grating voice of the movie year. I felt a brief shiver of sympathy for those cops who wanted to "silence" him.

As is so often the way, however, 16 Blocks begins to squander its accumulated goodwill in the final third. The plot seems to drive up a cul-de-sac when Jack and Eddie contrive a hostage crisis on a city bus, and the principal bad-apple cop (David Morse) has them cornered. Instead, our dynamic duo are suddenly granted the ability to travel through hyperspace, and the mood takes on the sentimental machismo of a buddy movie as Eddie confesses his dream of opening a bakery in Seattle and Jack finally steps up to do what a man's gotta do.

Donner, having begun the movie with urgency and panache, reverts to Lethal Weapon form and throws in a twist ending that allows the movie to have its birthday cake and eat it too. But, for a while back there, it looked a leaner, meaner proposition than we could have hoped for.