We’re told that on any given day there are at least seven performances of the longtime Broadway and West End hit Mamma Mia! around the world. The task of the reviewer is to figure out why.

No clue, really.

The new production on the Walnut Street Theatre’s main stage through July 15 does little to clear up the mystery. To the lighthearted mix of an improbable book stitched around tunes – some catchy, some not – by the Swedish pop group ABBA, the Walnut adds Richard Stafford’s deftly comedic choreography and touching performances by two of the leads.

But the show’s longueurs are considerable. Mamma Mia! relies on clichéd dialogue and stock character tropes (the thrice-divorced cougar, the gay man thrilled at the prospect of fatherhood, the feminist icon secretly longing for love). And Stafford’s unsubtle direction only amps up the caricatures. None of which means that the younger-than-usual Walnut audience didn’t have fun; there is fun to be had, if you keep your expectations modest.

For those unfamiliar with the show, as well as the 2008 movie adaptation starring Meryl Streep, the plot is easy to summarize. On an unidentified Greek island, 20-year-old Sophie Sheridan (Laura Giknis) invites three of her mother’s past lovers as guests to her wedding. Based on her surreptitious reading of her mother’s diary, she figures one of the three must be her father.

Unaccountably, and to mother Donna’s chagrin, all three show up at her taverna, sporting different accents (American, British, and Australian) and bland life stories. Over two and a half hours, songs are sung (music and lyrics by Benny Andersson and Björn Ulvaeus, with additional contributions by Stig Anderson), dances proliferate, hearts are bared, some resolution occurs.

The show’s creation with Johnson taking ABBA songs, including such hits as the title song, “Dancing Queen” and “Take a Chance on Me,” and building a plot around themrepresents a step back, or at least sideways, from the traditional book musical. Mostly generic, the songs don’t do much, if anything, to deepen character or advance the plot. (On opening night, muddy sound, particularly in ensemble numbers, made the lyrics hard to understand.)

But Stafford’s dances (and the show’s dancers) can be exhilarating. His comic touch is particularly evident in the beach number, “Lay All Your Love on Me,” featuring a male chorus in snorkel gear striking deliciously ludicrous poses.

Anne Brummel (most recently Elphaba in Wicked on Broadway and the national tour) brings a touching authenticity to Donna, the one-time disco singer whose past suddenly materializes. Eric Kunze is charismatic and strong-voiced as the American architect, Sam Carmichael, who left her two decades earlier. Giknis has the requisite blond good looks for Sophie, but she’s too often strident rather than poignant.

Jack Mehler’s cerulean and purple-hued lighting endows Peter Barbieri’s island set with atmospheric beauty. Costume designer Gail Baldoni supplies lovely, flowing gowns for Donna and feathers and sequins for the rousing three-song finale. 

Theater

Mamma Mia!

    • Through July 15 at the Walnut Street Theatre, 825 Walnut St. Tickets: $20-$97. Information: 215-574-3550, walnutstreettheatre.org.