Stephen Temperley's "Souvenir: A Fantasia on the Life of Florence Foster Jenkins," now at Geva Theatre Center, may not be much more than half a play, but the long-admired Judy Kaye as the real-life Jenkins is all that a playwright and an audience could hope for. A serious actor and singer with a talent for broad comedy, she transforms herself into both a lonely woman and an aging constant-motion-machine who pouts and flirts like a conniving child, bounces as if gravity's a mere nuisance, and screeches. Mainly she screeches. Kaye gives the bravura performance of a great clown in pursuit of the belly laugh. She's a combination of Margaret Dumont on a sugar high and Harpo Marx, who apparently learned how to yowl once they strapped him into a corset.

Jenkins, who died in 1944 after becoming a celebrity for her godawful singing, believed deeply in the purity of her own voice. Rich enough to live by her deepest passions, she studied classical singing, gave numerous recitals in New York City, and eventually performed at Carnegie Hall shortly before her death. Yet she never hit the right note except by accident, and never realized that her audiences were laughing at her. The glorious music she heard inside and the whooping and wailing that came out of her mouth had very little to do with one another.

Set during the Great Depression and World War II, the play's extremely funny first act develops the touching relationship between Jenkins and her accompanist, Cosme McMoon. An aspiring composer, he is clearly gay and needs the money; she is a wealthy but lonely widow. Despite his comic horror at her singing and her obliviousness, a deep friendship grows between them.

That's the story. Pretty much all of it. And it all unfolds in the first act. Temperley handles the problem of writing a one-joke play by having McMoon. played sympathetically by Donald Corren, narrate the story, almost always a clumsy device. He's the sane anchor for her boundless enthusiasm. Corren, who like Kaye originated the role on Broadway in 2005, has a gentle touch with the character and a pleasing singing voice, but the hint of effeminacy he gives Cosme kept disappearing and reappearing.

Temperley also structures the play around several popular songs of the day, most notably the 1928 standard "Crazy Rhythm," whose lyric provides a clever running commentary on the state of Jenkins and McMoon's relationship. The scene in which he tries to teach her to sing the tune off the beat is simply hilarious.

But the problem of the second act remains. It lacks character development and adds nothing to the plot; it actually begins by sagging. It doesn't even have much dialogue until a crisis in confidence emerges to be solved just before the end. What it does have is an old-fashioned star turn as Kaye tosses the story aside to recreate Jenkins' Carnegie Hall recital, bouncing from aria to art song, every number in a different - and funnier - costume. At the end, Cosmo explains that the music his dear Madame J hears in her soul is the purest music of all.

It's hard to overstate the ironic grace and elegance of designer R. Michael Miller's music room, where this sweet madness unfolds, or especially Tracy Christensen's dozen and a half costumes and marcelled wig for Kaye. She changes from a proper Park Avenue dowager to a Swiss milkmaid to an unlikely angel in the push-and-pull of a strip of Velcro. The costumes are the perfect match for Kaye's comic rambunctiousness. That same rambunctiousness is Temperley's - and his play's - saving grace.

Souvenir: A Fantasia on the Life of Florence Foster Jenkins.

Through October 4

Geva Theatre Center, 75 Woodbury Blvd.

$22-$59 | 232-4382, gevatheatre.org.