There is one reason and one reason only to see "Training Day" -- watching Denzel Washington sink his teeth into a charismatic role as a chest-thumping, cigarette-chopping, gold chain-wearing, hydraulic low rider-driving, street punk-pounding narcotics cop who is crooked, proud and high on power.

That's not to say the movie doesn't have some other merits -- imposing permanent-sunset photography, a slickly menacing ghetto atmosphere, a strong performance by Ethan Hawke as a patrol officer having his mettle tested for a promotion to Washington's unit of vigilante detectives. But the draw here is seeing a great actor, known for playing roles of incorruptible integrity, go bad.

At once droll and incredibly intimidating, Washington is a knockout as Alonzo Harris, a detective who believes in street justice and enjoys metering out a bit of it himself a couple times a day. His philosophy -- or at least part of it, for the movie is thick with meaty moral dialogue -- is that "it takes a wolf to catch a wolf." And Alonzo fancies himself the Big Bad Wolf.

Just how bad is what beat cop Jake Hoyt (Hawke) is about to find out. Principled and idealistic, yet shrewd and gritty, he has one day to prove he can hold his own on Alonzo's turf -- and you can see bits of his virtue being chipped away for every mile of his revealing ride-along.

Alonzo tells him "a good narcotics agent should have narcotics in his blood," then roughs up some college kids making a street corner pot purchase, takes their dope and forces Jake to get stoned at gunpoint. Later Alonzo stands by as his recruit gets roughed up while saving a high school girl two from rapists in an alley. Then he lets the perps go after giving them a good scare, because they're not the kind of big fish he wants on his arrest record.

When Jake learns that Russian mobsters have put a price on his mentor's head, he starts to get nervous wondering just how corrupt Alonzo is -- and he's about to find out because the man didn't pick Jake at random to roll with him that day.

Directed by flashy Antoine Fuqua ("Bait," "The Replacement Killers"), "Training Day" thrives on the strengths of its two stars, who play beautifully the unbalanced dynamic of their mutual distrust. But while that's enough to carry the movie for several reels, the plot eventually writes Jake into a corner (the best scene in the picture finds him playing poker with gang-bangers he knows are supposed to kill him) and resorts to a wildly convenient coincidence get him out of it.

Once that happens, a wrong-minded degradation begins, hitting several logical potholes on the road toward a terribly out-of-character action movie finale. Hawke even leaps off an apartment building rooftop (after getting the snot beat out of him) onto the hood of a moving car to stop a bad guy's getaway.

This conspicuous departure from the film's early high, in which Fuqua skillfully sidestepped several looming clichés, alters the fantastically tense mood of "Training Day," bringing it down to the level of a video game.