The makers ofBack to the Future Part III must have been exhausted by all that hurtling back and forth along the space-time continuum that Michael J. Fox did inBack to the Future Part II. (If only he’d been racking up frequent-flier mileage!) InPart III, they simply plop down Marty McFly (Fox) and Doc Brown (Christopher Lloyd) in the Old West and leave them there. This last installment in the series is also the first dud. More than anything, it recalls those campy ’60s sitcoms that always featured an Old West episode — you know, a character would get conked on the head, the story would turn into a dream sequence in 10-gallon hats, and the whole thing would be filmed on generic Western sets.

Back to the Future Part III has that same sort of studio back-lot clunkiness. Only this time it’s the audience that gets conked — by the sheer desperation of the whole enterprise. Writer-producer Bob Gale and director Robert Zemeckis trot out all the frontier cliches. Cowboys gallop through Monument Valley to the beat of thumping Ponderosa music. The bullying Biff Tannen (Thomas F.Wilson) shows up as a distant ancestor, ”Mad Dog” Tannen, a black-hatted gunslinger who fires bullets at Marty’s feet to make him dance. The climactic shoot-out is a dim parody of spaghetti Westerns.

Meanwhile, the plot hinges on how to get the DeLorean time machine back to 1985 without the aid of gasoline. What a hip and funny invention that magical sports car seemed in the first movie, and what a laborious device it has become.Part III opens with a 10-minute onslaught of Doc Brown’s ranting metaphysics. Throughout the film, he keeps shouting ”Great Scott!” and launching into another soliloquy. He’s like a speed freak reading from a physics textbook. It’s just about impossible to decipher what he’s saying, or to care.

Part II was crammed with so much manic invention that it was like a roller coaster that wouldn’t stop. Zemeckis certainly held your attention, but in an aggressive, pointless way. Gone was the soul, the cross-generational emotionalism, that made the first movie a pop classic.

InPart III, Zemeckis continues his transition from moviemaker to glorified mechanic. There’s hardly a spark of life left in this material (it’s just a compendium of gimmickry now), yet Zemeckis and his executive producer, Steven Spielberg, seem content to trivialize their own accomplishment. The fact that they shotII andIII back-to-back was widely publicized. Now thatIII has arrived, it’s obvious that they put their passion into the heady, money-saving logistics of filming two episodes in rapid-fire succession. They’ve turned the Back to the Future series into a marketing blitzkrieg, releasing this installment less than a year after the previous one and bringingPart II out on videocassette a day beforePart III hit the theaters.

Spielberg and Zemeckis may be marketing geniuses, but it’s hard to believe today’s young audiences will want to sit through a blandly synthetic Western parody.Back to the Future Part III might have worked had Zemeckis mixed up the present and the past more, injecting futuristic elements into the antiquated setting and turning the whole movie into a delirious sci-fi hodgepodge. On the few occasions he tries this sort of thing — the flying hoverboard fromPart II makes a crucial reappearance — the results are amusing. But the only true surprise of this movie is how lazy it is.

This time, Marty is more of a bystander. The story centers on Doc Brown — a crucial mistake, considering that Lloyd, a very funny character actor, is best in small doses, and that Michael J. Fox’s crack timing is one of the strongest elements in the series.

Doc falls in love with a pretty schoolmarm (Mary Steenburgen) who shares his passion for Jules Verne; it’s a romance of eggheads. At the same time, he’s desperately trying to prevent himself from being shot and killed. Marty and Doc, you see, have stumbled upon a tombstone that features the exact date of Doc’s death. Unless they can change the course of history, Doc has less than a week to live. The time-travel logic seems shakier than ever. After all, if Doc (as it stands now) died in 1885, then how could he have invented the time machine that first brought him back to 1885? And how could Marty have joined him?

Oh, well — thePlanet of the Apes sequels never made much sense, either. Here, though, Zemeckis’ slipshod plotting results not just from carelessness but from his trying to wrap up the series with an upbeat message. For all its screwball buoyancy, the originalBack to the Future was rigorously deterministic. It held fast to the idea that one tiny event could alter your destiny, for better or worse.Part III is full of sappy homilies about how ”the future is what you make it” — i.e., regardless of the past, if you just work hard enough to change things, everything will turn out fine. At one point, Doc, exasperated by all the trouble his DeLorean has gotten him into, talks about how he wants to ”destroy that infernal machine.” Traveling through time, he says, ”has become much too painful.” At this point, many in the audience may well agree.