A quarter-century-old manga provides the source matter for James Cameron’s megaproduction “Alita: Battle Angel.” That explains why, although in its original printed form Yukito Kishiro’s series might have broken new ground, much of the story now feels derivative. As visually extravagant as it is cluttered and silly, the tale of a kick-ass mechanized superheroine is eye-popping in the IMAX 3D format, but otherwise feels pretty stale.

In a dystopian future some three hundred years after a cataclysmic war, the population is divided into two groups. The surface-dwellers live in Iron City, a messy metropolis where battle-ready cyborgs patrol the streets as bounty hunters who receive credits for eliminating criminals. Ordinary folks may lead hard lives (though to tell the truth we don’t see very much of them), but they do have a popular form of entertainment: a roller-derby style competition called motorball, in which more cyborgs skate around a track in a huge arena, furiously trying to toss a metal sphere they fight over into a basket. Above Iron City floats another metropolis called Zalem, where the elites reside under the rule of a mysterious ruler called Nova.

Among the earthbound residents is Dyson Ido (Christoph Waltz), a doctor who treats damaged cyborgs using parts he scrounges from piles of garbage dumped from above. On one of his scavenging trips he discovers the head from a girl cyborg that still shows signs of life and gives it a new body. He calls the revived mechanism Alita, after his deceased daughter; and as played, in motion-capture movements with CGI enhancement, by Rosa Salazar, she soon exhibits remarkable fighting skills—as well as an immediate love of motorball, to which she’s introduced by a street kid named Hugo (Keean Johnson), who hopes to get a ticket to Zalem by collecting cyborg parts as head of a gang that literally rips them from those to whom they’re still attached.

It’s not long before Alita and Hugo are romantically involved, but she is also drawn into the world of the bounty hunters, as well as the motorball contest. The latter is supervised by a powerful fellow called Vector (Mahershala Ali), who has a special connection to Nova, and who’s assisted in his work by Chiren (Jennifer Connelly), Ido’s ex-wife. For reasons connected with that long-ago war, in which super-soldiers from Mars were pitted against earthlings, the duo aim to take down Alita using some of the more grotesque cyborgs as their instruments, both in the motorball tryouts she’s decided to enter and in the streets. She, of course, will fight back, and try to protect Hugo from his enemies as well.

This summary might make the movie sound like a jumble, and it is, not least because there are other plot threads to it as well. It’s arguable that Cameron simply admired the manga too much and brooded over the adaptation too long, reluctant to give up any part of it. But since what he had in mind was a feature rather than a multi-episode anime series, pruning and simplification were needed, not amplification and a degree of fidelity that extends to keeping the dialogue at comic-book level.

The flaws of his screenplay with Laeta Kalogridis are accentuated by Robert Rodriguez’s direction, which is high on CGI pizzazz but low on the human dimension. He seems devoted to the effects, and in fact they’re very good (kudos to the army of VFX workers), filling the huge screen with a grandiosity of vision that sometimes takes the breath away (and at others comes close to being nausea-inducing). The result is reminiscent of his work on “Sharkboy and Lavagirl,” though of course on a far larger scale—which is hardly a compliment.

In the resulting extravaganza, the performers are a secondary concern, but Salazar obviously gave the action her all; it’s a pity that her work is obscured by the comic-look redoing of the character, which makes Alita look as if she’d stepped out of the pages of a manga. And while Waltz brings his usual smoothness to Ido, Johnson is such a vacuous presence that Alita’s romance with Hugo—which extends to a ridiculous cliff-hanging denouement (a rare instance in which the effects don’t quite measure up)—is pure blandness. Apart from one good line allotted to him at the close, Ali is wasted, as is Connelly, and while Ed Skrein camps it up as the oiliest of the bounty hunters, others stuck in those roles, like Jackie Earle Haley, are so covered in CGI that they barely register. The only real exception is Jeff Fahey, who looks bemused in his role as a dog-lover.

The bottom line with “Alita” is that you’ve probably seen most everything in it before, even if in different permutations, but never so vividly depicted as in Cameron and company’s cutting-edge visuals. Whether that’s enough reason to bother with it is up to you.