No one has sketched the psychology of the baby-boomers, or found the comic potential beneath it, as acutely as Albert Brooks. His hilarious "Lost in America" gives us a slapstick of the mind -- it's not the hero, but the hero's ego that gets splattered with pies and knocked down the staircases of his own mind.

On the eve of an expected promotion, David Howard (Brooks), an advertising executive, lies awake in bed. He's climbing the corporate ladder, but he's stuck in a rut, too. His wife (Julie Hagerty), personnel director in a big retail store, feels the same way.

So when David gets fired, it's like school's out for them. Modeling themselves on "Easy Rider," they decide to drop out of society and head cross-country in search of the real America.

"We have to touch Indians!" David says excitedly.

So "Lost in America" zooms off as a clever parody of postcounterculture nostalgia for sweaty, work-shirted reality. The easiest of easy riders, they buy a Winnebago, complete with a microwave oven (with browning capability), and they're fueled by a $100,000 "nest egg." They want to "find themselves," but the self is the only thing they know -- they've spent their lives within its four walls.

A teddy bear with hair like a Persian lamb coat, Brooks is the most self-lacerating of comedians. He seeks out the person inside himself that we all fear most -- the whining brat who in some sense deserves to be a victim. The character he has portrayed in his own movies, and in a series of memorable supporting roles, is intellectually accomplished, but emotionally infantile. Words intoxicate him. A brilliant, limitless spieler, he paces the room in a fog of his own rhetoric -- he doesn't really want to touch Indians, but he loves talking about it.

The consummate rationalist, Brooks is all voice -- he has the diction of Demosthenes, and when he wants to emphasize a phrase, he seems to be pulling it with his teeth like a piece of taffy. When he matter-of-factly recites his situation to strangers -- "Well, you see, my wife and I have dropped out of society" -- he's the only one who doesn't get the joke. But he's also an accomplished physical comedian -- at times his limbs stutter like the pistons of an engine that won't ignite on a cold morning. Like everything else in this character's life, his body never works as well as his mouth.

"Lost in America" is clumsily edited and routinely shot -- technically, it's Brooks' least accomplished film. Drama depends on the conflict between characters, but in Brooks' stand-up comedian tradition, other people are just props in the monologuist's daffy odyssey. "Lost in America" lacks balance. What it needs is a skilled comedienne to play against Brooks, someone in the Carole Lombard mold, but Hagerty's performance is as sweet but thin as the glaze on a doughnut.

The narrative is equally thin. Brooks is strong on character (specifically, his own), weak on story -- the twists of the plot never match the dazzling inventiveness of the lines. What makes "Lost in America" so pleasurable is its sheer reality. Many of its scenes play like a documentary (particularly Brooks' encounter with a casino owner, an impeccably crass Garry Marshall). For Brooks, the funniest stories are true ones. Lost in America, opening today at area theaters, is rated R and contains some profanity.