''This is important. Don''t blow it," wife Lisa (Charlotte Ayanna) tells her rookie cop husband Jake (Ethan Hawke) at the outset of his first day as neighborhood narc-to-be. Eager to nail a coveted spot as a detective on the LAPD ladder, Jake hustles out the door toward meeting with the man who will initiate him into the undercover cop''s vicious world, Alonzo Harris (Denzel Washington).

Mama probably told Jake a lot of things, but days like this were almost certainly not among them. Jake spends the rest of the day and night alternately tooling around in Alonzo''s low-riding Monte Carlo, picking up increasingly disturbing tips from his new partner, and fearing for his life. That fear is generated as much by the seemingly sociopathic Alonzo as by the assorted gangbangers and street denizens the pair encounter. Jake, the idealistic rookie, and Alonzo, the hardened pro, are two characters who flirt with cliche. It''s positively exhilarating then to see these archetypes pushed through the meat-grinder in a film that restores our faith in the power of a great performance while offering some pointed social commentary and a downbeat ending that just plain smarts.

Washington has always been a solid actor capable of astonishing range and depth of emotion, but in Training Day he flat-out seethes. It''s doubly disturbing to see this actor, who is so frequently cast as the stand-up, do-right man, portray a cop so dirty he''d need a sandblaster to even dent his rotten emotional carapace.

Within a half hour of rolling out of bed, Hawke''s Jake is tricked by Alonzo into smoking PCP, beaten by street thugs, and has his rookie-boy world-view irrevocably shattered. Acting-wise, Hawke is no slouch here either. His face seems to sag a bit more with every conflict and we can almost hear his dreams shatter like glass in the background.

Director Antoine Fuqua made his mark in hip-hop videos and commercials; his first film, the Chow Yun-Fat thriller The Replacement Killers was blunted by stylistic excess. Training Day has little of that showy panache. It''s operating from a tight, streamlined script (by The Fast and the Furious'' David Ayer) instead of flash-in-the-pan urban anti-glitz, and although Fuqua can''t help but toss out a few camera curveballs, the film is less an exercise in good-cop/bad-cop theatrics than a portrait of idealism on fire, and the cost of following such idealism to its dangerous conclusion.

Alonzo, on the outs with a group of Vegas-based Russian mafiosi, has reason to rope in a patsy. Jake, wide-eyed and eager, is a prime rube until he glances back to find his personal rubicon lying in the gutter behind him. High marks go also to Scott Glenn, as one of Alonzo''s drug-dealing cronies, and rapper/porn-maestro Snoop Dogg, whose cameo as a wheelchair-bound ''banger rivals Samuel Jackson''s riveting turn as Gator in Jungle Fever for sheer "who knew?" surprise.

Far better than the usual cops and robbers fare, Training Day is a razor-wire-taut (and extremely violent) exploration of what happens when good guys go bad, badder, baddest.