Man and guitar become one, or so Sean Penn makes it seem, in Woody Allen's "Sweet and Lowdown," a quirky, tender, splendidly acted fable about a 1930s jazzman who's better at improvising than staying within the lines. His music (beautifully played by guitarists Howard Alden and Bucky Pizzarelli) is transcendent, but his life . . . well, as they say, he needs to get one.

Penn plays Emmet Ray, an unlikely vessel from which sweet music flows. A self-destructive but charming roue, he's as adroit at playing heartstrings as he is those on his guitar. And like movie musicians from "Amadeus" to "Hilary & Jackie," he's felt more pain than pleasure from the touch of the muse.

Emmet's idea of a swell time is getting drunk and watching the trains roll by or getting drunk and shooting rats down at the dump. He has no family, and when his colorful journey begins, the only women in his life are a couple of prostitutes he "manages." Apparently, pimping wasn't that uncommon in Depression-era jazz circles, as talking heads break into the action periodically to point out.

Allen is one of the faux pundits who reminisce about Emmet, a composite of sundry jazz greats who were similarly flummoxed by the demands of ordinary life. That can be said of all of Allen's characters, it's true. But Emmet is not just another of the filmmaker's neurotic nudniks. He's a hepcat. At least he thinks he is, though he's really a lonely guy beneath all that booze and bravura.

His two fears: settling down and and being shown up by his idol, Django Reinhardt, a real-life French Gypsy guitarist whose bittersweet recordings are heard in the film. He wrestles with both of these while he makes his way through the 1930s jazz scene. Reinhardt haunts him without ever making an appearance, but there are groupies aplenty. Emmet marries one of them (slinky Uma Thurman) and truly loves another--Hattie, a mute laundress played with extraordinary sweetness and vulnerability by 22-year-old Briton Samantha Morton.

One of the things Emmet likes about her: She never interrupts when he's patting himself on the back, which is more or less all the time. And unlike the other women in his life, she isn't bugging him to get in touch with his feelings.

But that's between Emmet and his guitar. When he caresses that bit of wood and wire, he's so carried away by the ecstasy of the moment, writhing to the rhythm, face a blissful grimace, that he can barely control his emotions. He's making love to his one and only mistress.

"Sweet and Lowdown," which was written earlier in Allen's career, is less cynical than many of his later films and much more compelling than such slight, recent offerings as "Everyone Says I Love You" and "Manhattan Murder Mystery." It's also a lot more fun.

Sweet and Lowdown (95 minutes, at the Shirlington Outer Circle) is rated PG-13 for sexual innuendo, drug use and language.

CAPTION: Sean Penn and Uma Thurman in "Sweet and Lowdown."