D-

The enormous success of the first “Transformers” movie two years ago guaranteed there would be a sequel, and Michael Bay provides one that’s even longer and noisier than its predecessor—as well as dumber and more incoherent, if you can believe that. “Revenge of the Fallen” is like a big, unsightly, clattery toy powered by a battery that just won’t quit, even though you devoutly wish it would.

In the juvenile script cobbled together by Ehren Kruger, Roberto Orci and Alex Kurtzman, earth once again is endangered by Decepticons, which aim to recover the pieces of the Allspark cube that was shattered in the last installment but, if reconstituted, could not only release their leader Megatron (voiced by Hugh Weaving) from captivity but insure their victory over the good-guy Autobots and lead to the planet’s destruction. Once again the key lies with teen Sam Witwicky (Shia LaBeouf), who’s no longer a high school nerd but a college freshman being introduced to the campus nightlife—and being turned into some sort of key to the location of an ancient weapon as a result of the effect of one sliver of the cube he’s retained as a souvenir. Fortunately when the villainous machines come calling, in the form of a slinky, seductive coed (has anybody out there ever seen “Decoys”?), Mikaela Banes (Megan Fox), his svelte girlfriend from home, is visiting and saves him from her siren call; one would certainly have missed the sight of her accompanying Sam as he serves as the vessel for mysterious clues revealing the weapon’s whereabouts. The culmination of the first act, in which Megatron and his cohorts destroy scads of locales trying to track him down, is a tumultuous battle in which Optimus Prime (Peter Cullen) winds up fodder for the scrap heap.

That leads into the second half of the picture, in which Sam’s hunt for the device is made more urgent by the need to restore Optimus, and more dangerous by the arrival of a new menace, the Fallen (Tony Todd), a bruiser that makes even Megatron look like a wuss and whom—according to another robot, a Decepticon turned ally, only a Prime—like Optimus—can vanquish. The search takes all and sundry—including Lennox (Josh Duhamel) and Epps (Tyrese Gibson), the soldiers from the first picture now promoted to a special unit that tracks Decepticons, as well as Simmons (John Turturro), former gung-ho agent now demoted to private life, Sam’s loopy dormmate Leo (Ramon Rodriguez), and his doofus parents (Kevin Dunn and Julie White)—to Egypt for the big final clash that will determine—gasp!—the fate of humanity itself. Suffice it to say that in the ensuing chaos, pyramids get blown up. (Ever since the destruction of the White House in “Independence Day,” it’s obligatory that some landmark bite the dust in these apocalyptic cartoons.)

Bay obviously wants the movie to be an exhilarating thrill-ride in the “Indiana Jones” mode, but like the last “Jones” movie, it’s chaotic and exhausting rather than fun. The incessant explosions and constant racket are an assault on the audience, and the interludes of quiet and repose are so infrequent and short-lived that they’re like desert mirages. And even in them the actors are encouraged to shout and scream so much that they come across even more irksome than the machines. The worst effect is on LaBeouf. You have to sympathize that he made much of the picture suffering the effects of an auto accident (and his hand is prominently bandaged in the later stages). But that’s no excuse for showing none of the charm and charisma that he did in his younger days. His smug turn in “Crystal Skulls” was only a hint of the frantic, sweaty performance Bay gets from him here, and when he’s imperiled at several points in the story (most notably toward the close), one feels no emotional reaction. Fox is very attractive, of course, but she gets little opportunity to do anything but run around and look concerned, and the director’s use of close-ups of her and LaBeouf (especially one near the beginning where he has Ben Seresin’s camera whirling around them as though they were redoing a scene from “Judgment at Nuremberg”) is oppressive.

The comic-relief supporting players, meanwhile, chew down the scenery while generating few laughs. Turturro rants his way through his one-note part, and Rodriguez, who’s apparently supposed to be endearingly idiotic, comes across as merely idiotic. (You want to cheer when Turturro finally shuts him up with an electric shot. Tase him, bro!) The less said about the sub-sitcom stuff that Dunn and White have to deliver, the better. And even Rainn Wilson, in a cameo as an astrology professor, has nothing to give. (His scene is also one of LaBeouf’s most embarrassing moments, as he shows the first signs of prophetic possession.) Hunkering down in more restrained roles are Duhamel and Gibson, although they have to deliver some of the most tedious lines, like the inevitable “This is bad,” referring to their situation rather than the movie but applicable to both. The only unfortunate individual who has even worse dialogue is Tony Todd, the voice of The Fallen, who has to intone portentous lines like “Revenge is mine” and “The time has come.” But at least he has the good fortune to remain unseen.

Of course, there are always the effects, but for the most part they’re so messy and repetitive that you eventually just tune them out (something you can’t do, unhappily, with the bombastic soundtrack, for which one Geoffrey Patterson is responsible, though Steve Jablonsky also deserves blame for his score, which bludgeons the ear mercilessly). The long, long battle sequences in which the clanking robots smash one another to bits are, once again, little more exciting than watching a couple of figures fashioned from legos have at it. In fact, the single best special effect is definitely Fox’s tight-fitting white pants, which seem never to get the slightest bit soiled as she capers and rolls through the red desert sands. That’s more amazing than a machine that will eat the sun—the fearsome threat of the final reel.

“Transformers” isn’t the worst big Hollywood movie of the summer. That distinction still resides with “Land of the Lost,” which has a surrealistic awfulness Bay’s movie can’t match (even though the self-referential poster of “Bad Boys II” is a particularly crass touch, as are the references to Barack Obama, given the contemptuous characterization of the presidential National Security head). Like “Terminator Salvation,” this is just generically, unimaginatively bad—a sort of routine disaster. But a very real one nonetheless.