Although it simmers too long during the buildup phase, Ridley Scott's epic crime drama "American Gangster" eventually pays off during the final reels.

Steve Zaillian's screenplay, based on the true story of a 1970s Harlem drug lord, initially suggests a series of familiar events in search of a story line. As scenes reminiscent of "Scarface," "Serpico" and "Bugsy" drift by, you sense the ambition of the film if not the achievement.

And then it all begins to click: Denzel Washington's spookily unpredictable performance as the homicidal drug dealer, Frank Lucas; Russell Crowe's warts-and-all portrayal of Richie Roberts, the conflicted but committed cop who took him down; and especially Zaillian's insistence on drawing parallels between Lucas' rise and fall and the end of the Vietnam War and demise of the Nixon administration.

The frequent use of television clips of Vietnam turning points seems gratuitous at first, but the movie eventually makes its case. There was, after all, a direct connection between Lucas and the Southeast Asia drug trade. He insisted on going to the source and cutting out the middle man — and when the American presence ended in Vietnam, his business model was threatened.

Scott's use of "Amazing Grace" on the soundtrack, as Lucas faces this inevitable crisis, will offend some and delight others. It's one of many devices Scott and Zaillian use to keep "American Gangster" from becoming just another tale of murderous drug dealers and the cops who try to outwit them.

The movie begins with a particularly nasty demonstration of Lucas' ruthlessness, as a gasoline-soaked victim is torched and shot, and it includes frequent reminders of the character's willingness to use violence to establish his authority.

Washington pours on the charm in several scenes in which Lucas is dealing with his bride and family, attending a landmark prizefight ("It's not boxing but politics," he points out) and playing a role as Harlem's Robin Hood. But the actor makes it perfectly clear that Lucas is always capable of brutality even within his frightened family.

Scott guided Crowe to an Oscar in "Gladiator," and once more he brings out the actor's vulnerable side. Crowe is particularly affecting in an episode in which Roberts confronts a barking, racist militarist and in a divorce-court scene in which he acknowledges that he's not much of a parent.

The exceptional supporting cast includes Cuba Gooding Jr. as a rival dealer; Armand Assante as an aggressive mob boss; Josh Brolin as a corrupt police detective; Chiwetel Ejiofor as Lucas' nervous brother; and Ruby Dee, who's sublime as their troubled mother.

"American Gangster" may not have reached its final form. Like so many of Scott's movies (the current reworking of "Blade Runner" being the most extreme example), it's likely to improve as we get to see more of what he left on the cutting-room floor. (Surely there's more to Dee's performance than we see here.) Call it a most promising work in progress.

John Hartl: johnhartl@yahoo.com