Miss Saigon

, now at the Walnut Street Theatre, was written by the

Les Miz

guys, Alain Boubil and Claude-Michel Schonberg, in collaboration with Richard Maltby Jr. It's a knockoff of Puccini's

Madama Butterfly

, transposed from Japan to Vietnam.

The opera was itself a knockoff of a play by David Belasco, which was a knockoff of story by Philadelphian John Luther Long. For more than a hundred years this basic plot has been wildly popular. It goes like this:

A young Asian woman and an American soldier fall in love. When he ships out, he leaves her behind, not knowing she is pregnant. Several years later he returns, with his blond American wife, to find she has been faithful to their love. To provide her son with a good life, she kills herself so the soldier will take the boy home with him.

Miss Saigon is about betrayal, personal and political. We watch Kim (Melinda Chua) forced into prostitution and sold for the night to Chris (Eric Kunze). Thuy (Mel Sagrado Maghuyop), the man she was promised to, comes to claim her; he is then betrayed in complicated ways. Kim and Chris' son, a bui doi, has been betrayed by a culture that hates mixed-race children; John (Philip Michael Baskerville), an ex-soldier, has made a post-military career out of rescuing these kids and sings about them with a schlocky backdrop of photos of orphanages; it's surprising the number isn't followed by a charity pledge drive.

The show's politics are troubling, mingling scorn and sentimentality. The famous helicopter does make a spectacular appearance (you'll feel the wind!) in Act 2. When Saigon falls, the soldiers pile on, leaving behind all the South Vietnamese who are desperate to escape. The North Vietnamese are portrayed as a robotic army, faceless and ruthless.

The Engineer (Bobby Martino), who runs a sleazy nightclub where he sells girls, is the show's spectacular central role. He lives a vile, street-rat life, driven by longing for the American dream. He has to be not only shocking, vulgar, and repulsive, but seductive as well. Martino lacks the necessary charm.

The confrontation between Chris' wife, Ellen (Kate Fahrner), and Kim is powerful, as each discovers how she has been betrayed by the man she loves. Another high point is the love duet between Kim and Chris, "The Last Night of the World," with its "solo saxophone," sung about an inch from each other's lips.

But no one in the cast gives full-throated voice to any of the songs. Under Bruce Lumpkin's direction, everyone uses big Broadway voices, straining to create lots of passion and intensity but with few genuine emotional moments. What a waste that Jeff Coons and Ben Dibble, two very full-throated guys, have nearly invisible roles in the chorus.

Miss Saigon

Through July 17 at the Walnut Street Theatre, Ninth and Walnut Streets. Tickets $10-$95. Information: 215-574-3550 or www.WalnutStreetTheatre.org.EndText