The concert version of "Purlie" running through Sunday at the Kennedy Center is marvelously sung, and choreographer Louis Johnson has provided some astonishingly rousing dances considering how scaled back the show is (no set, minimal costumes, actors on book in front of the onstage orchestra).

The only drawback at all to the evening is, as so often in the case of musicals, the book: The story, which purports to be about the triumph of the resourceful black preacher Purlie (Dorian Harewood) over the racist old coot Ol' Cap'n (Larry Storch) is wan and disappointing.

Based on Ossie Davis's 1961 play "Purlie Victorious," "Purlie" opened on Broadway in 1970, and in terms of race relations it's very much an optimistic period piece. Racism is basically seen as something that people of good will and good sense are no longer going to stand for. Purlie has hatched a scheme with his reluctant brother Gitlow (Reginald Vel Johnson) and his enthusiastic sister-in-law Missy (Ebony Jo-Ann) to get the $500 legacy Ol' Cap'n owes to Purlie's late cousin Bea. To this end, Purlie has recruited Lutiebelle (Stephanie Mills) to impersonate Bea, so they can collect the money and Purlie can start a church.

Purlie is restless, defiant and impatient, the kind of guy who attacks a problem head-on and ends up with a concussion. The pragmatic Gitlow (played with sly humor by Johnson) prefers to maintain a surface amity with Ol' Cap'n while working behind the old man's back to get what he wants, a philosophy he espouses in his delightful "Skinnin' a Cat." (Built like a teddy bear, Johnson turns out to be a quick, elegantly minimalist dancer, very light on his feet.)

Ol' Cap'n's son Charlie (Jim Stanek) is a hippie type who believes in integration and writes protest songs, which he then tries out on the long-suffering family servant Idella (the wonderful, dry Clarice Taylor). At one point his irate father snarls at him, "You tryin' to get nonviolent with me, boy!" Storch is really, really funny in the part, an inspired crank and physical clown. He takes almost all the sting out of the role, which is a good thing, because Ol' Cap'n, with his smug racism and his bullwhip, is fundamentally way too ugly a character for this sweet musical to contain.

You settle in with some satisfaction to watch Purlie outsmart the stupid Ol' Cap'n, but, alas, things don't work out that way. Oh, he gets outsmarted, all right, but by his own son. The bad white guy is defeated and the good black guys are saved by the good white guy. This takes the air right out of the story.

On the whole, Purlie is disappointingly conceived. He's described as a man "who always did irritate the white folks," but then he's depicted as a braggart full of hot air. That straight-arrow actor Cleavon Little originated the role; Harewood brings it more weight and authority than it perhaps deserves. In general, the seriousness of the issues "Purlie" deals with sit oddly with its light tone. It might help if the big male ensemble number, the anthem of discontent "First Thing Monday Mornin'," had come early in the show, darkening the tone right at the top. But it's been used as an opening for the rather truncated second act (five numbers to the first act's 11).

So it's best to ignore the story and enjoy the actors, George Faison's bright direction, and the singing. Among the standouts, Mills reveals a powerful, purring voice, and Viola Bradford, in her gospel solo, brings down the house.

Purlie, music by Gary Geld, Lyrics by Peter Udell, book by Peter Udell, Ossie Davis and Philip Rose, based on Davis's play "Purlie Victorious." Directed by George Faison. Music director, Kay Cameron. Set and lights, Roger Morgan. Costume coordinators, Theoni V. Aldredge and Wallace G. Lane Jr. Concert adaptation, Steven Suskin. Orchestrations, Luther Hendersen and Garry Sherman. Sound, Otts Munderloh. Dance arrangements, Luther Hendersen. At the Kennedy Center through August 2. Call 202-467-4600. CAPTION: Marvelously sung: Dorian Harewood as the resourceful preacher Purlie, and Stephanie Mills as his recruit, Lutiebelle. CAPTION: Larry Storch, left, plays the racist coot Ol' Cap'n, and Reginald Vel Johnson plays Purlie's reluctant brother Gitlow in "Purlie," now playing at the Kennedy Center.