"Back to the Future Part III," the third and final installment in the time-travel adventures of Marty McFly, is a winge`d thing, full of ingenious pop dazzle and jazzy high spirits. From its opening shots, the film is like an invigorating elixir, a movie pick-me-up that delivers thrills and races your pulse but keeps your head in gear too. It's divinely frivolous, nearly perfect fun.

　　 The story line -- which director Robert Zemeckis worked out with his partner, Bob Gale -- is gorgeously tangled, and the director sets it all up as if his pants were on fire. Even before the credits have rolled, our heads are spinning. The situation at the outset is this: It's 1955 and, as the clock approaches the crucial moment, Doc (Christopher Lloyd) packs Marty (Michael J. Fox) into his DeLorean and sends him back to the future. BUT! ... before the smoke clears, Marty trots up claiming that he's back from the future, this time with a telegram from the other Doc -- the one stranded in 1885 at the end of "BTTF II" -- detailing where the damaged time machine can be found and how it can be repaired.

　　 As far as Doc is concerned, the machine is to be used once more, to take Marty back to 1985; in fact, he goes to great pains to insist that no one come to rescue him in 1885, where he is living a happy life as a blacksmith. But when a tombstone is discovered showing that unless something is done, Doc will be shot in the back in less than a week, Marty is left with no alternative but to fire up the DeLorean and once again head back into the past to rescue him.

　　 No one is expected to swallow these mazelike twists and double-back detours all in one gulp. The more you hang on to the pieces of the puzzle, though, the more pleasurable it is. When Marty crosses the plane of the fourth dimension and moves into the past, the picture becomes a kind of surrealistic western. The Hill Valley of 1885 is a woolly Wild West outpost with a saloon (which occupies the same spot as the soda shop and its later incarnations) and a sheriff (played hilariously as a Buffalo Bill look-alike by James Tolkan) and a town bad guy who turns out to be "Mad Dog" Tannen, the ancient ancestor of the obnoxious Biff (played again by Thomas F. Wilson) from "I" and "II."

　　 The filmmakers work hilarious variations on the cliches of the western genre. Some of the bits come blissfully out of nowhere. The best riffs, though, are at the expense of Clint Eastwood, whose name Marty takes to disguise his true identity from the members of his own McFly clan. (There's a delightful scene in which Marty gets to hold a tiny infant who is, in fact, his own great-grandfather.) This brand of droll, kinetic humor is a Zemeckis and Gale trademark. The gags in "BTTF III" mark a new high for pop inventiveness. The jokes here are intricately constructed and precision-timed, like the Rube Goldberg gadget Doc constructs to produce a single ice cube (more precious than gold in the dusty Old West). The filmmakers are movie smart too, and because they're constantly playing around with time -- and with our knowledge of what happened in the earlier movies -- their punch lines almost always work on more than one level.

　　 Seamlessly efficient, the movie almost never lets up. Because Marty has ruptured the gas line on the DeLorean, spilling out all the gas and making it impossible for them to cross time zones, Doc is forced to devise an elaborate scheme using a locomotive to push the car up to speed. 英文影评 film builds up a head of steam too, and the momentum is dizzying. The conclusion is basic in design, as classic as a silent movie heroine inching toward a buzz saw, but it's a masterfully executed bit of pure cinema.

　　 But headlong velocity for its own sake isn't the filmmakers' only concern. The picture isn't nerve-jarring and frenetic the way the previous two were. It doesn't slam forward mindlessly; it pauses occasionally to allow the characters to interact. There's even a place, it seems, for a lyrical moment or two, for sweetness and romance.

　　 As a result, this is the most human, the most character-based of the "Back to the Future" films. Whereas before Marty has been the front man, the focus here is more on Lloyd's Doc. As an actor, Lloyd is an extravagant eccentric, an American version of Ralph Richardson, but with just the slightest touch of acid damage. Spiritually, Lloyd is as close as a living person can come to being a cartoon, and in the past I've felt pushed back against a wall by his aggressively animated style. But here his playing seems more modulated, less strained; this time, he's left room on screen for the other actors.

　　 It would have been inconceivable before for Doc to have fallen in love; it wasn't within the character's range, or the movies', really. But that he falls in love -- and not only that, cuts an almost dashing romantic figure -- is an indication of how much the filmmakers have opened up their premise. His inamorata here is Clara (Mary Steenburgen), a doelike schoolmarm, newly arrived in town. From the moment their eyes meet, they're smitten. The romance isn't allowed much screen time, but the conversations they have about their love for Jules Verne give the film a science fiction context the other pictures lacked, plus a blossoming innocence.

　　 It's wonderful to see Steenburgen get this kind of comic showcase. Her presence has a leavening effect; she gives the movie a touch of twitterpated elegance. As for the rest of the regulars, everybody is better this time out, Fox included. By now, Fox must be entirely bored with the series' commonplace hero; there's not much in the writing for an actor to latch on to. But there's something about Fox's averageness that takes hold in this installment. He's such an unspectacular kid, so normal and like every other Valley Boy, that his lack of stature -- especially when he straightens his Eastwood poncho and hat, and heads for his showdown on main street -- is itself heroic.

　　 When Verne is evoked, you immediately nod. "BTTF III" has the expansiveness of a classic fantasy; it's a big, sprawling adventure, full of wonderments and rich surprises. Not only is it junk transformed, it's junk redeemed.