An Irish busker falls for a waifish Czech flower seller on the streets of Dublin. The Guy and the Girl, as they are known, come together to make music in the bittersweet romance "Once."

This quirky and fresh Broadway musical based on the low-budget Irish 2007 film has struck a chord with theatergoers and critics. "Once" transports Glen Hansard and Markéta Irglová's wistful folk-pop score into a participatory pub setting where the actors all play their own musical instruments in a freewheeling ensemble. Theatergoers can also belly up to the onstage bar before the show and join in the party.

Lest that sound thoroughly subversive, rest assured that hotshot British director John Tiffany has always delighted in breaking down the boundaries of conventional theatrical tropes. Setting "Once" in a cabaret removes the fourth wall and lets the audience become part of the show.

"There's nothing out there like it," says "Once" choreographer Steven Hoggett, who also collaborated with Tiffany on "The Black Watch." "It casts a spell on the audience. It's one of the most immersive shows I've ever worked on. The audience never feels like they are watching a show. The characters are so vulnerable and fragile that it feels real."

Unexpected smash

Perhaps most unexpected of all, this understated and offbeat Tony winner for best musical has become an international blockbuster. The bittersweet musical, which scored eight Tony Awards, makes its regional debut Tuesday through July 13 at San Francisco's Curran Theatre as part of the SHN series. Critics have called the musical "as vital and surprising as the early spring." Indeed, not since "Spring Awakening" and "American Idiot" have the themes of young love and loss been told in such an unselfconscious manner.

Many have been smitten with the musical's incarnation of this enchantingly low-key love story, an antidote to the bloated special-effects juggernauts that usually dominate Broadway.

—‰'Once' uses song and dance in a way I've never experienced in an American musical," raved the New York Times. "When the violins begin to play -- and the accordion and the mandolin and the guitars and the cello -- the instruments swell into a collection of distinctive voices melded into a single, universal feeling. 'Once' massages that feeling until it hurts quite exquisitely."

The unconventional feel of the romance, its embrace of piercing regret without any hokey Hollywood schmaltz, was precisely what seduced Tiffany, who is always drawn to life's poignant rites of passage, from the Scottish soldiers in a famed regiment in "The Black Watch" to the scruffy street musicians in "Once." His characters lose their innocence in such a potent way that the audience cannot help but feel their pain.

"I am interested in telling the kind of stories that have not been told before," says the eclectic director, who is currently working on the stage version of "Let the Right One In" in London, "and the kind of stories that are best told live. The trick is to find the right theatrical vocabulary to tell the story."

He pauses, then adds he knew he succeeded in creating a sense of interactivity with "Once," when "people began telling me that they felt they had watched the show with everyone they have ever loved."

His love affair with the project began with Hansard and Irglová's gem of a score, which includes the Oscar-winning song "Falling Slowly," the cathartic "When Your Mind's Made Up" and the anthemic title song.

"Putting this kind of gentle, low-fi music on stage seemed quite crazy," admits Tiffany, also hailed for a Broadway revival of "The Glass Menagerie" starring Cherry Jones and Zachary Quinto. "I had never heard music like that on stage before. It's delicate, it's folk, not rock, and it was very hard not to bash that delicacy to bits."

Subtlety is key to the magic of "Once," which eschews the usual brassy brand of Broadway slick packaging and pumped-up special effects. The need for depth and nuance is one reason Tiffany enlisted his old pal Enda Walsh, the Irish playwright best known for the demented masterpiece "The Walworth Farce," to write the adaptation for the musical. Not an obvious choice by any means.

"It's a bit like having Charles Manson adapt 'It's a Wonderful Life,' I know," chuckles Tiffany, "but despite the fact that he has written so many dark, violent and twisted plays, he's a real softy on the inside, and that's what we needed for this piece."

Respect for stillness

The impulse on Broadway is to crank everything up to 11, but "Once" demands a respect for stillness, from the score to the choreography.

"It's a very quiet and unassuming piece, and that's part of its charm," says Hoggett, who just received a Tony nomination for "Rocky." "So many Broadway shows are smash-and-grab technical juggernauts. For us, this show was about trying to capture what was essential about the story. It's sparse and distilled."

Hoggett found himself in the unusual position of fighting to not choreograph certain numbers because he felt the songs needed no razzle dazzle in order to shine.

"It was a very sexy challenge for me," says the choreographer perhaps best known for "American Idiot." "Half of the songs stand on their own and needed no movement at all. They needed to be given to the audience in a very pure way. You had to hold the line with a show like 'Once,' because there is so much beauty, so many moments that you can't embellish all of them. You have to be strong and keep your focus and stick to the minimalism."

Ironically, the more restrained the performances in the show, the more deeply the audience seems to be moved. It's as if the honesty of the stagecraft feeds the catharsis for theatergoers, many of whom have trouble fighting back a tear at the finale.

"It's so refreshing and honest and truthful, even though the ending is not particularly uplifting," says Tiffany, who is in talks to direct the play version of "Harry Potter." "It connects to the audience very deeply."

Navigating the line between art and life, dance and movement is the key to fresh and innovative shows like "Once," which bend genres and bust expectations. The originality of "Once" is something you won't soon forget.

"It's a truly thrilling time to be working in this field," says Hoggett, "you feel you have the license to run wild."

Contact Karen D'Souza at 408-271-3772. Read her at www.mercurynews.com/karen-dsouza, and follow her at Twitter.com/karendsouza4.

'Once'

Book by Enda Walsh; music and lyrics by Glen Hansard and Markéta Irglová
Through: June 17-July 13
Where: Curran Theatre,
445 Geary St. San Francisco.
Tickets: $45-$210, 888-746-1799, www.shnsf.com