Rosa Salazar shines as a not-quite-human heroine in this long-awaited manga adaptation produced by James Cameron.

It’s been a while since Robert Rodriguez made us feel like he was living up to the potential he displayed in the early “Desperado” days and then again with “Sin City.” (The less said about the latter film’s long-delayed sequel, the better.) Five years after that misbegotten follow-up, the filmmaker is showing signs of life again — albeit of the artificial variety. “Alita: Battle Angel” is his best film since he brought Frank Miller’s graphic novel to the screen, a sci-fi epic that does something rare in an age of endless adaptations and reboots: lives up to its potential while leaving you wanting more.

An adaptation of Yukito Kishiro’s manga “Gunnm,” the film takes place 300 years after the Fall — one of those sci-fi apocalypses you always hear whispered about in films such as this, usually in vague terms that ensure the big reveal at the end catches you surprise. “Alita” is the kind of movie whose backstories take as long to explain as their actual narratives, but Rodriguez does well to keep the overt exposition to a minimum — in part because his heroine is a cipher who doesn’t even know her own name when first we meet her.

Like a female cross between Pinocchio and Frankenstein’s monster updated for the space age, Alita (Rosa Salazar) is discovered in a scrapyard one fateful day by Dr. Ido (Christoph Waltz), a cyber-surgeon with a soft spot for innocent creatures whose bodies — whether real or mechanical — have failed them. She couldn’t tell you who she is or where she hails from after being brought back online, but quickly realizes that she can kill anyone who crosses her path with her bare hands.

That’s a useful skill to have in Iron City, a near-dystopia living under the long shadow cast by the aerial city called Zalum. The hoi polloi are kept earthbound in the overcrowded metropolis, never allowed to ascend to the supposed paradise above them — unless, that is, they become the final champion of an absurdly dangerous sport called motorball. Featuring a roster of mechanically enhanced competitors who’d just as soon kill one another as put a point on the scoreboard, it leads to some of the film’s most kinetic sequences — and a chance for Alita to meet her maker.

Fox

As fate would have it, the skills that make one an elite killer lend themselves well to motorball. Alita discovers these abilities quite by accident and, pure soul that she is, longs to use them as a means of discovering her own secrets and truths. What few details we do see are glimpsed in brief flashbacks that show enough to fill in the most important blanks while also keeping us guessing as to the bigger picture slowly being revealed. The two central mysteries — Alita’s past and what exactly is going on up there in Zalem — are, of course, connected, but even when adhering to familiar tropes (which is often) “Alita” carves a unique space for itself on the strength of its endlessly likable protagonist.

The large, almond-shaped eyes that dominated early discussions of the film, a window to both Alita’s soul and the uncanny valley in which much of the film resides, are a constant reminder that she’s not quite human. She’s something else, something different and in many ways superior, but as ever in these stories what we’re most drawn to is her capacity for human emotion — we want her to feel love, even if it causes her pain along the way. In that way, Alita’s eyes are ultimately an advantage: Not only do you quickly get used to them, but they convey a depth of feeling that’s rare in this kind of film. “It’s all or nothing with me,” she says at a pivotal moment. “This is who I am.”

Salazar, whose human form most recently materialized in “Bird Box” and the English-language remake of “The Kindergarten Teacher,” has graduated from supporting act to leading lady with grace and grit. Her motion-capture performance is one of the most impressive we’ve seen this side of Andy Serkis, with a physicality that’s enhanced by digital trickery but a spirit that’s all her. Salazar is backed by Jennifer Connelly, Jackie Earle Haley, and Ed Skrein in the ensemble cast, but it’s Mahershala Ali who most compels in a small role — as stylish here as he is at award shows, the actor brings his one-of-a-kind presence to the role of a sophisticated baddie who’s constantly being possessed as an unwitting vessel by the film’s largely unseen villain.

James Cameron, who first became interested in directing “Alita” nearly 20 years ago, co-wrote and produced the film that “Avatar” made him too busy to helm himself. It clearly benefited from his involvement, but Rodriguez was the right man for the job. He seems invigorated by the material, delivering his best film since “Sin City” and creating a fleshed-out world that’s vivid enough to make you wish you could see it through Alita’s eyes.

Grade: B+

“Alita: Battle Angel” will be released in theaters by 20th Century Fox on February 14.

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