It's no surprise to see Bernardine Mitchell giving an infectious, powerhouse performance in "Mahalia," the musical about gospel legend Mahalia Jackson that opened Saturday night at MetroStage. Nothing less will do, of course, and Mitchell has a proven flair for this sort of thing, stretching back through acclaimed turns at Arena Stage ("Blues in the Night") and Studio Theatre (as Bessie Smith in "Bessie's Blues" nearly a decade ago).

What's unexpected in this "Mahalia" is the strength of Mitchell's sidekicks, S. Renee Clark and William Hubbard, a pair of double-threat talents who sing beautifully and coax glory from their keyboards. As actors they aren't in Mitchell's class, but then the story Tom Stolz has written about Mahalia Jackson's life is embarrassingly slight. The overly cute, platitude-laden book shuns drama and attempts no insights into one of the larger figures of 20th-century culture. Stolz provides some wan punch lines to land, but nothing to act.

The underdeveloped show is redeemed by the vibrance and integrity of its musical performance, for which music director Clark gets credit. Everything from soft a cappella spirituals to foot-stomping, organ-driven anthems is handled expertly under Clark's guidance. (The score is largely a collection of traditional spirituals.) Rarely do you see a musical director perform, but it's a treat here to watch Clark lead by example; onstage, she is the engine of a fair amount of joy. When Hubbard sings the up-tempo "Jim Crow Blues" early in the first act, Clark accompanies him on piano, and the jolly rumble in her playing kindles a giddy twist in Hubbard's hips and a cry in his rising, supple voice.

Hubbard provides the same sort of propulsive accompaniment, more often from the organ that sits on the opposite side of the stage from the piano. Not that the keyboards are His and Hers: Hubbard and Clark both play whatever is needed, piano or organ, frequently teaming up to create a full-bodied sound to match Mitchell's from-the-toes singing.

"You sound more like Bessie Smith than Bessie Smith!" gushes Hubbard (as Mahalia's cousin, one of several roles he's called on to play) at the top of the show. The script describes Jackson shepherding her talent away from jazz and blues and toward the Lord despite the impressive secular musical influences that abounded in the New Orleans and Chicago of her youth. This sort of thing, like most of Jackson's life, is simply talked about between songs in Jackson's blunt narration and humorous prayers, and in the flat facts that pass for dialogue.

Clark, playing longtime Jackson accompanist Mildred Falls, is obligated to say, "Down Beat just voted you the world's most acclaimed vocalist!" at one typical point. During the textbooklike passage on Jackson's civil rights involvement, Hubbard reads vast passages from Martin Luther King Jr.'s 1963 speech.

"Tell about the dream, honey," Mitchell-as-Jackson prompts, getting laughs.

Right: The dreamy part here finds Mitchell as a young Jackson sweetly singing an a cappella version of "Hand Me Down Yo' Silver Trumpet, Gabriel," her hands clasped demurely, face beaming, the lilt of the song ebullient as Clark and Hubbard harmonize. It's there in Mitchell's forceful rendition of "I'm Going to Live the Life I Sing About in My Songs," the dusky melody swelling into ripe temptation thanks to Clark's driving piano. It's in the precise, delicate sliding notes the trio sings in "I'm on My Way" and "Let Us Go Down to Jordan," in Mitchell's unflagging light touch with the skimpy book scenes, and in her carefully considered, exciting singing throughout.

Carol Mitchell-leon's production is sensibly spare, with the keyboards framing a raised platform and a few chairs that are enough to suggest everything from car seats to dressing-room furniture. It seems reasonable to wonder when the popular formula of poorly written, robustly sung blues and gospel musicals will wear thin in Washington, but this show proves a core truth: When you render the music this well, much can be forgiven.

Mahalia, by Tom Stolz. Directed by Carol Mitchell-leon. Lights, Adam Magazine; costumes, Michael Reynolds; set consultants, Tracie Duncan and Carl Gudenius; original sound design, Mimi Epstein; sound design, Matt Rowe. Approximately 2 hours 10 minutes. Through July 11 at MetroStage, 1201 N. Royal St., Alexandria. Call 800-494-8497 or visit www.boxofficetickets.com.

Bernardine Mitchell's voice overcomes the shortcomings of the MetroStage musical.Bernardine Mitchell as gospel singer Mahalia Jackson, center, with S. Renee Clark and William Hubbard in the MetroStage production of "Mahalia."