With Ken Stack’s production of “Pygmalion,” Acadia Repertory Theatre steps gingerly into classic territory. It’s not just that the story of the pedagogue Henry Higgins and the flower girl Eliza Doolittle is loosely based on Greek legend, or that it is, perhaps, George Bernard Shaw’s most famous stage play. It’s the Lerner and Loewe problem and the fact that their 1956 musical “My Fair Lady” is imprinted so thoroughly, so cloyingly on our collective American psyche.

The result, sitting in a theater watching a serious and fun production of Shaw’s witty script, is an unbridled impulse to see the actors break into “Wouldn’t It Be Loverly?” or “I Could Have Danced All Night” or “Get Me to the Church on Time.”

It’s a shameless – bordering on shameful – response. But it is also inevitable.

If you can muscle past that, however, the story is a powerful reminder of the slippery territory in gender and class politics. That’s Shaw, the literary Irishman who put the “pro” in progressiveness. He certainly didn’t want his story twisted through the Hollywood back lots or the Broadway footlights, and he held onto the rights to nearly his dying day (at age 94). Surely, he turned over in his grave when later adapters ended the show with Eliza picking up Higgins’ slippers rather than sporting off with Freddie.

There certainly is nothing sensationalized in Stack’s production. It’s straightforward and, at its best, spunky. In Act I especially, Stack keeps the action swift and dapper. Act II stumbles into Shaw at his most didactic, and comes off more as a dramatized pitch than a story unfolding. But that’s Shaw, too.

Stack’s set, a star unto itself, easily transforms from St. Paul’s, to Higgins’ study, to his mother’s garden and back again without a hitch. The changes undoubtedly increase the show’s running time (about 21/2 hours) but it’s more pleasant to watch the scene changes than to sit through some of the performance lulls in the second act.

Those lulls, by the way, are created by an uneven cast, guilty of intermittently mugging and overdoing the turgid British stereotypes. Several of the actors are simply too young for their parts, and it steals somewhat from the heft of the show.

Unquestionably, however, Doug Meswarb is a successful Higgins. He’s part mama’s boy, part pompous tyrant, and while Meswarb appropriately has a zero charm factor, he creates a character who is dazzling. In a role that has been played by Rex Harrison, Peter O’Toole and Leslie Howard, Meswarb finds his own unflagging place.

Cherie Willis, as Eliza, is rightly annoying in her flower girl days, but slips in and out of cartoonishness as the show goes on. The role is complicated and demanding, and she’s up for much of it but she never really reaches a convincing level of grace and independence.

Michael S. Miller’s Colonel Pickering is lively, if a little less innately mannered than one might expect. And Alison Cox, as Mrs. Higgins, has the perfect demeanor for the role of a doting but impatient mother – if only she would cut back her facial responses by about half.

Steve Robbins plays Alfred Doolittle with intense determination. When he’s not slowly piecing together lines, Robbins cuts a fine figure of a philosophical and foibled father. On opening night, he was the only actor to receive a warm and appreciative round of applause at his final exit.

Acadia Repertory Theatre will present “Pygmalion” 8:15 p.m. Tuesday-Sunday through July 27 and 2 p.m. July 28 at the Masonic Hall in Somesville. For information, call 244-7260.