It sounded like an exercise in the unstageable: a show set in a factory and a brothel, in the sewers beneath Paris and behind a barricade. Other challenges included a bloody battle, a leap into the Seine and a story that spanned 17 years. Oh, and it needed to be a musical.

These were the challenges that Cameron Mackintosh took on when he decided to produce "Les Miserables," which is on its final tour after more than 20 years onstage, making its eighth and last Washington stop at the National Theatre.

"Les Miz" has proved its staying power over the years. As Mackintosh puts it, the musical "seems to have this extraordinary ability to keep coming back." So, will this final tour -- rather like Cher's -- be the first of many? No, says the British producer by phone from New York: "At the moment, it's planned that this is the final tour."

The show, as practically everyone knows, is based on Victor Hugo's novel and chronicles the fate of Jean Valjean, who serves a long prison sentence for stealing bread. Upon release, he breaks parole and changes his name, becomes a small-town factory owner and mayor, and spends years evading police inspector Javert. When Valjean moves to Paris with his adopted daughter, Cosette, he finds himself caught in the middle of a revolt against King Louis Philippe, even as Javert draws ever closer.

The story of the birth of "Les Miserables" the musical is equally full of twists and turns. Apparently, it was one of Mackintosh's earlier musical productions that inspired French composer Claude-Michel Schonberg and lyricist Alan Boublil to set Hugo to music. Mackintosh explains, "I discovered several years into the ['Les Miserables'] run in London where the idea to do the show came from. Alan told me that in 1978, he went to London to see the revival of 'Oliver!' While he was watching it, he was reminded of the Hugo story. As soon as the show was over, he called Claude-Michel and said, 'I've got an idea for our next show.' "

The duo penned a musical tableau adapted from the book, in French, which was staged at the Palais des Sports outside Paris in 1980. Two years later, a young director, Peter Ferago, gave Mackintosh a recording of the show. "I never got to see it [the show]; I'd vaguely heard of it," Mackintosh says. "But I heard the album, and I really liked it. So when I met with Boublil and Schonberg, I told them I wanted to rebuild it."

And rebuild it they did, with English lyrics by Herbert Kretzmer, and with Trevor Nunn and John Caird adapting and directing. The show opened in London's West End in 1985 and on Broadway in 1987. It was an instant hit with audiences (despite mixed reviews early on) and has since been performed in 38 countries in 21 languages before more than 51 million people, according to the official "Les Miz" Web site. No one has been more surprised at the popularity of "Les Miz" than Mackintosh.

"I would never have thought it was going to have this level of success. I thought it was a serious musical" -- a tale of revenge, revolution, poverty and pain. Mackintosh claims the show's appeal has "as much to do with Hugo as the music."

"The characters of 'Les Miz' were timeless. . . . This is not just a French story for the French." In fact, he says, the book spoke to American rebels, too: "Indeed when I had come to do a production in Washington [a pre-Broadway run of "Les Miz" at the Kennedy Center in December 1986], I had found, coincidentally, that" during the Civil War, "it was the hot new book of Lee's army." He adds, "It was and remains still a book which inspired people and the story which is about the eternal nature of human nature."

While one show winds down, another is just starting its life onstage: "Mary Poppins," which Mackintosh is co-producing with Disney, is a smash hit in London; it opens on Broadway next fall. Adapted from the novel by P.L. Travers and the 1964 Disney movie starring Julie Andrews, the stage version includes such classics as "A Spoonful of Sugar" and "Chim Chim Cher-ee," plus a half-dozen new songs by George Stiles and Anthony Drewe. Mackintosh says, "It's something I've been working with for 25 years," trying to make it "a really strong, traditional musical, like 'My Fair Lady.' "

This time, instead of barricades, Mackintosh will take us to the rooftops of London, and the rioting French will be replaced by a riotous flying nanny.

Is "Mary" the new "Miz"?

"Les Miserables" has been on stage for more than 20 years and has been performed in 38 countries in 21 languages, according to its Web site."Les Miz" producer Cameron Mackintosh.