Every time Woody Allen miscalculates and makes a movie as weak as last year's "Celebrity," I start to wonder if he's down for the count. I should know better.

Once again, Allen has come roaring back with "Sweet and Lowdown," a buoyant, saucy and deftly original faux documentary that purports to be about a fictitious jazz guitar legend named Emmett Ray (Sean Penn).

According to the old-timer radio jocks and jazz historians (writer-director Allen among them) that populate the movie's modern interview interludes, Emmett was a neurotic (no, really?), weasely egoist of a 1930s lounge lizard louse, whose curt and cocky facade barely masked a belly full of wild insecurities, the main one being that he was the world's second greatest jazz guitarist.

As the "documentary" gives way to the fictional narrative, Penn brings Emmett to life in a piquant, pappy performance, so joyful of this plucky music that his eyelids flutter and his knees dance when he plays -- and so naked in his vulnerabilities that, like all Woody Allen best heroes, he becomes sad and sympathetic in spite of being a royal cad.

Immodest ("They say I'm a great lover."), ill-mannered and misogynistic, he nonetheless goes to pieces whenever he hears the music of his rival, his torment, his god, Django Reinhardt (a real, legendary '30s guitarist), and it exposes his human underbelly. The two times he came face to face with Django, Emmett fainted.

Of course, Allen says in one of his interview segments, "Like all Emmett Ray stories, you never know what's true and what's exaggerated."

Some of those stories include yarns about Emmett's tendency to live beyond his means (he has a jones for expensive rumble-seat convertibles), his dashed dreams of Hollywood recognition, his perverted fixation with shooting rats at the dump to get his jollies, and his brief marriage to a sassy, vampy, Marlene Dietrich-Algonquin Roundtable wannabe (Uma Thurman), who amuses herself by digging around in Emmett's neuroses. ("You keep your emotions all bottled up," she complains. To which Emmett replies, "You say that like it's a bad thing!")

Inspired by Allen's adoration for the '30s jazz scene, "Sweet and Lowdown" is his most effortlessly enjoyable comedy in ages. Penn's performance as the arbitrary Emmett Ray is impeccable. Even the often austere Thurman gives one of her most warm-blooded performances (although Allen should have dropped her bombastic voice over).

But the real revelation of "Sweet and Lowdown" is the wonderfully moody and expressive performance of Samantha Morton (Harriet in PBS's "Emma" import) as the mousy, malleable laundress Emmett becomes attached to in spite of himself.

Morton is a sweet, shy, grinning innocent in her role as Hattie, a girl that is everything an insecure egomaniac needs -- a little simple, completely smitten, a born doormat, and mute.

Yet in that irrepressible Woody Allen way, she still manages to frustrate Emmett no end.

Morton is the movie's heart, and as soon as the credits roll, you'll be dying to see more of her.