A wacky ride in Spielberg's time machine. `Back to the Future' zips from '80s to '50s with irresistible glee

With a lot of help from his friends, the nonstop fantasy factory called Steven Spielberg has done it again. ``Back to the Future'' is a freewheeling blend of sitcom and science fiction, teen-age energy and grown-up nostalgia. Its music zooms from Van Halen to Chuck Berry; its personality is cheerfully split between H. G. Wells and ``Revenge of the Nerds.''

For a movie obsessed with time, it starts slowly and goes on too long. But it's great fun most of the way and makes up in humor what it lacks in sense. It also suggests a way of attracting older audiences to a fantasy aimed largely at youngsters: The action zigzags between the '80s and the '50s, turning the adults into kids and the hero into a buccaneer on the high sea of time travel.

The main character is Marty McFly, who lives with his wimpy parents in a rundown town. Life would be awfully dull if not for his friendship with Doc Brown, the local weirdo -- a loony genius with a dog named Einstein and a Harpo Marx hairdo. His mind-reading gadget was a flop, but his latest project is a dandy: a time machine built into the steely chassis of a De Lorean sedan. Set the controls, hit 88 miles an hour, and you'll zoom into your pick of the past or future.

It's a heady prospect, and the movie uses it conservatively. Taking off in a hurry -- to save his life from Doc Brown's enemies -- Marty finds himself not at the dawn of civilization or the end of the world, but a mere 30 years in the past. It's 1955, the Four Aces are crooning ``Mister Sandman'' on the jukebox, and the town is actually clean.

He starts to explore and runs into none other than his own parents, now teen-agers like him. And do they ever need coaching by a savvy '80s kid. Dad's a wimp even as a high-schooler, and Mom's a boy-chaser with lots of bad habits.

On top of this, Marty has tampered -- right after landing in the '50s -- with an important detail of his family history, changing the circumstances of his parents' first meeting. Now, in this revised version of 1955, they aren't falling in love -- which means they won't marry and have children, so Marty himself will vanish as if he were never born!

Trying to figure this out is like wearing a Mobius strip as a headband; the story's logic turns back on itself with every new event. It's part of the movie's charm that Spielberg and company couldn't care less. They hurtle past the small loopholes and charge straight through the big ones. The situations get goofier by the minute, until Marty finds himself dating his mother and prodding his father to steal her away.

In his spare time he introduces heavy-metal rock to a baffled '50s prom, astounds everyone by predicting the end of a ``Honeymooners'' episode, and impersonates a spaceman with the hilarious help of a Walkman radio -- not to mention saving his family line and setting a number of personalities on new courses.

``Back to the Future'' doesn't exactly leap out of the starting gate, and some scenes are strung out by gimmicky editing. But the story picks up steam as it goes along, and the last third is especially full of speedy surprises. With its quick pace, young characters, and fantastical slant, the picture flaunts all of Spielberg's trademarks even though it was directed by Robert Zemeckis from a Zemeckis and Bob Gale screenplay. Dean Cundy did the smart cinematography, and the snappy visual effects were produced by trusty Industrial Light and Magic.

Among the cast, I have reservations about Michael J. Fox as the hero. Although he's an experienced actor -- being the heartthrob of ``Family Ties'' on TV -- he doesn't live up to the story's wacky momentum or eccentrity. But any shortcomings are overwhelmed by Christopher Lloyd's show-stealing performance as Doc Brown, who hams it up with delirious glee, bugging his eyes and beetling his brow in every direction he can find. Lea Thompson and Crispin Glover couldn't be better as Marty's parents, and the makeup that transforms their ages is just short of miraculous. In its visual illusions, ``Back to the Future'' is a time machine in itself -- and a more diverting one it's hard to imagine.