LOS ANGELES – Composer John Kander and lyricist Fred Ebb were in the throes of writing “Curtains” – their 12th Broadway musical – when Ebb died of a heart attack on Sept. 11, 2004.

A legendary showbiz partnership of 40 years – one that spawned such classic musicals as “Cabaret” and “Chicago” – was suddenly over.

About a month after Ebb’s death, the other collaborators on “Curtains” – book writer Rupert Holmes and director Scott Ellis – had to ask Kander a difficult question: Would he continue working on “Curtains” without Ebb?

“We wanted to go on, but we didn’t know what John was feeling,” recalls Holmes. “Whatever he said, we would have respected.”

Kander did not hesitate. Ebb loved the show, he said, and would want it to reach the stage.

With Kander and Holmes now writing additional lyrics, “Curtains” was back on track.

It opened here in L.A. last week at the Ahmanson Theater. Judging by the reaction of audiences and critics, Ebb’s faith in his final show was not misplaced.

“Curtains,” said the Los Angeles Times, “does what musicals rarely do anymore: entertain.” The paper noted flaws in the show but confidently predicted “multiple Tony Awards” in its future.

Budgeted at $10 million, “Curtains” is expected to arrive on Broadway in the spring at the Al Hirschfeld Theater.

Variety had quibbles as well, but concluded the musical “has so much going for it . . . that one can forgive its [flaws].”

Audiences here have certainly embraced it. Michael Ritchie, the artistic director of the Ahmanson, says the show is “doing tremendous business” and will be “very profitable for us.”

A backstage musical whodunit, “Curtains” stars David Hyde Pierce as a detective investigating a series of murders on the set of a Broadway musical that is in serious trouble out of town.

When he’s not tracking down the killer, the detective, a devoted theatergoer, is helping to fix the show.

Pierce, who last appeared on Broadway in “Spamalot,” gives a performance that’s sure to earn him a Tony nomination.

Kander calls him “the soul of the piece.”

He’s backed up by a cast of top-tier Broadway performers, all playing familiar theatrical types: Debra Monk, hilarious as a cynical producer (the character is said to be based on real-life producer Fran Weissler); Edward Hibbert, devouring the scenery (and practically the audience) as a camp director; Karen Ziemba as a writer who’s really a performer at heart; and John Bolton as a ruthless and powerful critic.

The score contains three signature Kander and Ebb tunes that lovingly poke fun at showbiz. One – a showstopper about critics – poses the question: “What kind of man would take a job like that?”

Kander and Ebb and their original book writer Peter Stone (“1776”) began work on “Curtains” 30 years ago.

“It was a musical within a musical with a lot of murders – so many we lost count – that evolved into a Pirandello-esque piece about various forms of illusion and reality,” Kander says. “We were having a good time, but the show got to be so complicated it was unproduceable.”

Stone died in 2003, and Holmes, who wrote the musical “The Mystery of Edwin Drood” and the Broadway thriller “Accomplice,” took over the book.

He kept Stone’s irreverent attitude toward show business but turned the musical into a traditional whodunit.

He also set the show in 1959, when the musical theater was still in its golden age and Kander and Ebb were an up-and-coming songwriting team.

“John would never say this, but ‘Curtains’ is a love letter to the world that he and Fred helped create and extend over the years,” Holmes says.

Says Kander: “If there is anything besides entertainment at the heart of this, it is that a life in the theater is a wonderful life. I truly believe that.”

Like their characters, the creators of “Curtains” are working hard to get their show in shape for Broadway. But, on the whole, they’re pleased with the show they’ve got.

Says Kander: “I don’t have that awful feeling, as I’ve had on some shows, when I go: ‘My God, I don’t have a second act!’ “