In "Down and Out in Beverly Hills," a bum in despair hurls himself into a rich man's pool. After the rich man saves him, the bum becomes part of the household. He's taken to a fashionable coiffeur (who pronounces his "vagabond" style "tres chic"); the rich man, on the other hand, joins the bum and his friends at the beach, and the next morning, announces triumphantly to his wife, "I ate garbage last night."

It would be a mistake to see "Down and Out in Beverly Hills" as anything but a Paul Mazursky movie -- it bears his signature, satire without acid, his large (perhaps too large) humanity. You couldn't call it a formula movie -- it's too Mazurskyish -- but in its outlines, at least, "Down and Out" epitomizes the reigning formula in movies today: the fish-out-of-water story.

"I've known a lot of fish out of water," says Mazursky. "Their uncomfortableness gives them a kind of poetry. When they become aware of it, it makes them less poetic. But at the beginning stages, there's something innocent about it."

Put simply, the fish-out-of-water story takes someone out of his environment and puts him someplace foreign. "Splash" was, quite literally, a fish-out-of-water story (mermaid in Manhattan); so was last year's biggest hit, "Back to the Future" ('80s kid in '50s); so was "Beverly Hills Cop," the biggest hit of the year before (inner-city cop in posh Beverly Hills). "Witness," another of last year's favorite movies, placed a violent cop among the peace-loving Amish. "Tootsie" took a man and placed him in a woman's world. "E.T." put an alien in the American suburbs.

You can find fish-out-of-water stories among smaller movies, too. "Desperately Seeking Susan," which sent a suburban housewife to the East Village, and a downtown demimondaine to Fort Lee, was a kind of double fish out of water. Albert Brooks' "Lost in America" took the quintessential yuppie and threw him into the hinterland. John Sayles played with the form in "Brother From Another Planet," in which a black alien landed in Harlem, pursued by white space police.

Hollywood is always looking for a guaranteed formula -- there is one producer today, apparently, who is convinced that "uniform" movies, in which the heroes rebel against men in uniform, are automatic gold -- but unfortunately, nothing's foolproof in this crazy business. It's best to think of the fish out of water as a kind of yardstick to hold movies against, a way to think about why some stories work, and others don't.

On some level, of course, almost every yarn is a fish-out-of-water story. War or mere adventure, for example, always has an element of the formula -- an environment of flying shrapnel is generally a foreign one. "I don't think it's new or unique," says one studio executive who's worked widely in the genre. "It's inside every single well-told story."

Yet the fish-out-of-water stories have other, more precise similarities:

They are about diametrically opposed cultures. The essence of "Beverly Hills Cop" is a conflict between poor and rich. And it's important to stress that the clash takes place at the level of values, whether it be "Beverly Hills Cop's" code of the rule book versus code of the street, or "Witness' " conflict between John Book's belief in "whacking" versus the Amish belief in nonviolence.

They are comedies. The jokes grow out of the way an outsider's perspective makes the things we've grown used to seem strange and nutty again. "Splash," for example, peeled away the familiarity of Crazy Eddie -- you were inured to this babbling madman, but, through the mermaid's eyes, he looked ludicrous again. A down vest is a down vest, but in "Back to the Future," it's "Hey, kid, what's with the life preserver?" Even in a drama like "Witness," the culture clash is played for comedy -- it's hard to forget how silly Harrison Ford looks when he first gets dressed up in the plain black coat and broad-brimmed hat of the Amish.

They are didactic. Comedy is more congenial to the genre because, at its heart, it's not simply about the conflict of values, but about teaching them. Through contact with the alien culture (and a shared ordeal), the hero learns something about himself, something that allows him to transcend his environment. In "E.T.," Elliot learns the meaning of friendship from the extraterrestrial, and the neighborhood kids, who used to bully him, band around him in saving E.T. In "Splash," Tom Hanks learns how to love. In "Tootsie," Dustin Hoffman has to be a woman to learn how to be a man.

It's tempting at this point to put on my Siegfried Kracauer autograph-model Tyrolean pundit hat and divine in the vogue for the fish-out-of-water story a larger trend in the culture and politics of the nation; certainly, in a movie like "Desperately Seeking Susan," you can see the appeal to the yuppie desire to play dress up, and in "Lost in America," the antidote to that.

Admittedly, the fish-out-of-water story has been around for a long time. Mark Twain, for example, was awfully fond of it -- he had a Connecticut Yankee in King Arthur's court, and a prince and a pauper trading places. Gulliver was a fish out of water. So was Prince Hal.

Among early movies, "City Lights" put Charlie Chaplin's tramp in a rich man's house; in Buster Keaton's "Steamboat Bill, Jr.," Joe College found himself embroiled in the rough-and-tumble steamboat wars on the Mississippi. And Mazursky's "Down and Out" is based, in part, on Jean Renoir's "Boudu Saved From Drowning."

The father of the contemporary fish out of water, though, is Frank Capra. "Mr. Smith Goes to Washington," for example, fits all the criteria: a zeke from Montana finds himself lost in the sophistication of Washington; the culture clash is played for comedy, as the arch reporters make a sap out of him; the conflict is a moral one, between idealism and corruption; and through contact with the alien, Washington rediscovers the better part of itself.

If the fish out of water has been around forever, though, something is revealed by the form it currently takes. "Some Like It Hot," for example, may be a fish-out-of-water story (male musicians masquerading in all-girl band), but it's not a modern one -- unlike the otherwise similar "Tootsie," there's no exchange of values, no self-discovery.

And in the particular lineaments of the modern fish-out-of-water story lies at least the seed of self-recognition. It wouldn't be controversial to say that the homogenized national culture has become oppressive, so that the central assumption of the fish-out-of-water story -- that there are cultures left to place in opposition to each other -- is intrinsically appealing. And if that national culture leaves people bored silly, the fish out of water is a way to make it seem new and fresh again, to breathe life into a world that is always, everywhere the same. One of the most magical scenes in "Splash" comes when the mermaid looks at a "Don't Walk" sign and exclaims, "Pretty!," and the hero admits, yes, he had never thought of it that way.

There is something unavoidably modern in the story of the outsider. "There would be a lot of psychoanalysts who'd have gone out of business," says Martin Brest, director of "Beverly Hills Cop," "were it not for the fact that almost everyone feels that they are the one who is out of it, and everyone else is in on it." Clearer still is the appeal for a contemporary audience of the search for values that lies at the heart of the genre. It exalts familiar American types (like Eddie Murphy's fast-talking hustler in "Beverly Hills Cop"). It exalts tolerance, for aliens, mermaids, Amish, women and (occasionally) blacks. And, as in "Back to the Future," it says you can remake the world.

In his landmark book on ancient folklore, "The Hero With a Thousand Faces" (which at least one producer has called the essential text of screen writing), Joseph Campbell analyzed the basic features of what he called the "monomyth," the myth essential to all cultures: separation, initiation and return. The hero is somehow torn from the ordinary world, endures trials against supernatural forces and obtains victory, and returns with "the power to bestow boons on his fellow man."

In this light, the fish-out-of-water story is at least a kissing cousin of the monomyth. If the hero is not torn from the ordinary world, the very appearance of an outsider may make the ordinary world extraordinary (as in "Splash"). The struggle may be supernatural (as in "E.T."), more psychological than supernatural (as in "Tootsie") or both ("Back to the Future"), but the outline is the same. And while the details vary, the contact with the alien always results in boons for the natives (by way of metaphor, the stolen bathrobes in "Beverly Hills Cop").

Ancient or not, it's just a good way to tell a story. "It's instant drama," says Bob Zemeckis, who directed two fish-out-of-water hits, "Back to the Future" and "Romancing the Stone." "It's instant conflict." If drama, axiomatically, consists of the conflict of opposites, then the genre is automatically dramatic. "You sort of see the person in position A," says Mazursky, "and then you look forward to seeing how they will handle position B, which leads you to a kind of dialectic. That's what I always presumed drama was about." On the other hand, it's also automatically comic -- it doesn't take a genius to have E.T. break into the refrigerator and drink beer, yet the effect is drolly endearing.

The fish out of water fits easily into the three-act structure (exposition, conflict, resolution) that most screen writers favor -- the fish goes out of water, he flounders, he triumphs and returns. And it automatically illuminates character. What better way to show that Marty McFly is the quintessential '80s kid than to place him in the '50s?

"Down and Out," though, is a failed fish-out-of-water story. And that's what, in the end, is unsatisfying about the movie. Part of what was intriguing about Mazursky's "Moscow on the Hudson," another fish-out-of-water story, was the way it strayed from the formula. It took a Russian musician and placed him in New York, but hero and alien were the same, and the exchange of values took place, not with another character, but inside his own head.

"Down and Out," on the other hand, might have been better as a more rigorous fish-out-of-water tale. From the introduction of the bum Jerry into the household, Mazursky seems intent on tossing aside what's obvious in the setup -- instead of building jokes out of the bum's unfamiliarity with the mansion and its devices, Mazursky has the bum superior to his environment from the outset. Yet the biggest laughs (like a scene where Jerry gets on his hands and knees to eat with the family dog, while the lady of the house looks on in horror) stem from precisely the kind of fish-out-of-water comedy the movie could use more of.

"Down and Out" suggests the kind of conflict of values that the fish-out-of-water story depends on: wealthy Dave is a workaholic, but Jerry doesn't want to work; Dave is a striver, but Jerry's given up. But the idea is never really pursued. And while Jerry communicates values to the rest of the family (the wife has an orgasm, the son finds himself, the daughter finds love, the maid finds politics), there's no thread connecting what he's given them. "I told them what they wanted to hear," says Jerry, but we never know what he wanted to hear. It's not an exchange of values, but an exchange of the absence of values.

Maybe "Down and Out" would have been a stronger movie this way, but in the long run, I'm glad Mazursky didn't make it. Hollywood, too, needs its fish out of water.