A young white boy from Chicago is currently singing, dancing and acting all the roles in a kick-butt, rhyme-busting musical drama, during whose run the Kirk Douglas Theater had better keep up the insurance on its roof.

One Richard Maltby lyric that guaranteed a laugh in the 1996 musical “Big” complained, “Nothin’s worse crap/Than a little white Polish boy from Jersey talkin’ rap.” How times have changed. A young white boy from Chicago, by way of upstate New York and the Edinburgh Festival, is currently singing, dancing and acting all the roles in a kick-butt, rhyme-busting musical drama, during whose run the Kirk Douglas Theater had better keep up the insurance on its roof. Matt Sax’s exuberantly entertaining tour de force may even win over older folks to whom the words hip-hop evoke only Easter bunnies.

Plenty of theater pieces have made use of this intense performance art: “The Bomb-itty of Errors” integrating it amusingly with Shakespearean diction; “Russell Simmons’ Def Poetry Jam” demonstrating its variety.

Sax and collaborator/helmer Eric Rosen’s broader ambition is to portray hip-hop as the transformative agent in a disaffected character’s journey, as if to justify proponents’ claims about the tonic effect of a form that many have dismissed as antisocial at worst, doggerel at best. “Come inside/ open up your ears wide … To the beat y’all” is the siren song to the skeptic.

Object of attention is Clifford Keyes, a stumbling, purposeless adolescent burdened with multiple guilts after the ‘rents’ messy divorce years before. In a Brooklyn bookstore, he comes under the wing of Sir John, poetry slam MC and nascent Pygmalion.

The scars of a car crash having taken out his performance dreams, Sir John identifies in the suburban kid a need for self-expression and cleansing to which the free flow of ideas and rhymes is well suited. He’ll mold Clifford’s clay to prove that all the world’s a stage (Walt Spangler’s raised, light-bordered, polished-black trapezoid serving beautifully as private and public platform).

Speaking of the Bard, pretension is evident in the hint of Clifford as a prince and Sir John as Falstaff, but there’s not much “Henry IV” here. The fat knight seeks to pull Hal away from his righteous self, but the Brooklyn mentor’s oft-repeated mantra is “It only comes out truthful if it comes from a truthful place.” If anything, a broken adult’s attempt to redeem his own dreams through a talented, needy youth is more Miyagi than Falstaff.

Yep, you wanted a musical version of “The Karate Kid” and here it is, direct from the DJ’s booth. Hot wax on, hot wax off.

The familiar trope doesn’t detract from “Clay’s” interest. For one thing, Clifford’s is a psychologically rich story about the effect of choices made when one was too young to know better. The dissolution of his parents’ marriage — a dreamily lazy Nancy too inert to satisfy slick Geoffrey of the wandering eye — casts scars no less crippling than Sir John’s.

For another, the purifying effect of processing pain through a sieve of heavy percussion and fly rhyme is exciting to witness: “My face refused to smile as I saw my father’s eyes/It seems they jumped up out his head disguised themselves as mine.” Plays and films have telescoped time to show instant recovery through psychoanalysis; why not through hip-hop, whose beat seems more liberating than months on the couch anyway? Clifford’s sudden ability to echo Sir John’s beatboxing plays like “The Rain in Spain.” (Yo yo, I think he got it.)

But the ace in the hole is Sax, so assured as both writer and performer that he redefines charisma for a hip new age. He invests each character with a psychological gesture — mimed cigarette for Nancy; hair flirtatiously tucked back for stepmom Jackie; a face-hiding hoodie defining Sir John — and then sets them in conflict, the kicks, knee-bends and spins of hip-hop performance proving equally useful for shape-shifting between roles.

Sax’s open, rubbery face draws us in to listen attentively and find unexpected variety within each character’s rap expression. The dialogue, spoken lyrics and song — some artfully prerecorded to blend with the live action — never stop but they do vary and build, Sax and Rosen skillfully maintaining story focus instead of whipping us into a frenzy.

There was no need to take Clifford all the way to superstardom under the name of “Clay” — Ralph Macchio brought us to our feet just by winning a local trophy, after all — though Sax does possess superstar talent, and Howell Binkley’s lighting thrillingly transforms Sir John’s shop into a rock arena. (What an odd lighting grid, incidentally, all painted silver and askew. Looks like the Tin Man’s breastplate in “The Wiz.”)

Some have criticized “Clay” on sociological grounds, questioning not just the unlikelihood of a white rapper’s going so far (as if Eminem, and Beastie Boys before him, hadn’t proved that success is within everyone’s reach given talent and luck), but the fitness of black Sir John’s mentoring to a white, middle-class protege.

But should he restrict his healing efforts to disadvantaged black youth only? If hip-hop is so powerful — as one can’t leave “Clay” without realizing it is — does demonstrating its crossover power diminish it? Is disqualifying Clifford, or Sax for that matter, from seeking self-realization in the genre all that different from denying a black student the chance to discover herself in the poems of Robert Frost? I think not, yo.

Clay

Kirk Douglas Theater; 317 seats; $40 top

Production

A Center Theater Group presentation of an About Face Theater and Lookingglass Theater Company production of a play in one act by Matt Sax, developed in collaboration with Eric Rosen. Directed by Rosen.

Creative

Sets, Walt Spangler; lighting, Howell Binkley; sound design and orchestration, Joshua Horvath; production stage manager, Elizabeth Atkinson. Opened Sept. 19, 2007. Reviewed Sept. 22. Runs through Oct. 14. Running time: 1 HOUR, 20 MIN.

Cast

With: Matt Sax.
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