In "Sugarbaby," Percy Adlon's bubbly, off-the-wall romantic comedy, Marianne (Marianne Sagebrecht), an obese undertaker, lives a life of quiet desperation. By day, she prepares the bodies of the dead for burial; by night, she gorges herself on sausages till the TV turns to a test pattern. Her only "recreation" is a morning float in the community pool: motionless, spread-eagled, alone.

But voices call her out of her anomie; first, the refrain "Sugarbaby" in a song on a golden oldies radio show; and second, the voice of a real-life sugarbaby, a lanky blond subway driver, Huber 133 (Eisi Gulp), whose appearance, as he steps outside the train to announce the next stop, galvanizes her to action.

Slyly, doggedly, she stalks the driver, using her wiles to secure the driver's schedules, buying a pair of binoculars and spying on his home life (his wife leaves for two weeks for a funeral), ordering sexy lingerie and spike heels. Part of the fun of "Sugarbaby" is watching the imposing Sagebrecht break into little-girl giggles, jigs and pouts -- she may be built like a sorority house, but she's just a sorority sister at heart.

But "Sugarbaby" isn't a movie about how fat people are, deep down, like everyone else; it insists that fat people are different, and moreover, it celebrates them. Marianne worships life, and the driver responds to that -- he loves her pampering, the jellyrolls she stuffs in his mouth, the bubble baths and massages, the elaborate meals. Marianne is a woman who laments that "noboby wants to believe that someone who is dead needs tenderness," and the young sugarbaby responds to that. Her soul is as big as she is.

What's fine about "Sugarbaby" is how director Adlon gets you inside Marianne's head, though he makes what might seem bizarre, or even a little sordid, into a carnival world. Cinematographer Johanna Heer bathes the frame in garish colors, violets and blues, lime greens and eye-shocking magentas; Marianne's apartment is a jester's motley of golds, greens, violets and blues. And the camera is alive, off kilter as often as not, panning relentlessly like a child searching for Easter eggs. The music (by Dreieier), a combination of homey harmonica motifs and disco percussion solos, bounces "Sugarbaby" right along -- the movie literally pulses with life.

Love, alas, may be most vivid in anticipation -- once Marianne finds her sugarbaby, and he her, the movie slows into a more conventional style, as they share with each other their childhood secrets, their family histories and quotidian troubles. The movie is at its best when you don't know which end is up, when you think you're laughing at this woman, and find yourself laughing with her.

Sugarbaby, opening today at the Key, is unrated but contains nudity in sexual situations.