At 71, James Cameron still makes movies like he’s got something to prove — despite his many accolades, awards and box-office successes. But that fighting spirit is what makes Cameron’s films feel so alive and so urgent, a spirit that he has infused into his epic sci-fi space Western saga, “Avatar.” For almost three decades, he has been ensconced on the planet Pandora, dedicating himself to this film series that expresses all of his obsessions: technology, the ocean, swaggering Marines, powerful women, guns, a deep skepticism for the military and corporations, and a streak of rebellion.

2022’s installment, “Avatar: The Way of Water” was a massive expansion of the world on Pandora, exploring the oceans of the planet, and his third film “Avatar: Fire and Ash,” is an extension of that already established world-building, just a whole lot more of it. “Fire and Ash” is only five minutes longer than “The Way of Water,” but clocking in at three hours and 17 minutes, it feels much heftier, but perhaps because there’s much more conflict this time around, less frolicking in the beautiful digital sea.

There is function to the bloated running time — by the time the big climatic battle is crashing around us, you care so deeply about these characters: the blended Sully family made up of Marine-turned-Na’vi, Jake (Sam Worthington), his wife Neytiri (Zoe Saldaña), their band of kids, including Lo’ak (Britain Dalton), Kiri (Sigourney Weaver), Tuk (Trinity Bliss), and adopted human kid, Spider (Jack Champion). You care about the Metkayina village where they live, on a beautiful reef, and the society of whales called the Tulkun, that commune with the Metkayina. Cameron builds our emotional connection to these characters, which pays off when he rains fire down upon them.

“Fire and Ash” continues the same conflict of the prior films, as the Na’vi fight to save Pandora from human invasion. But in this chapter, there are shifting alliances, and surprising new foes. Jake Sully’s eternal antagonist is Col. Miles Quaritch (Stephen Lang), whose consciousness has been permanently placed in a Na’vi form. But while Jake adopted the ways of the Na’vi, Quaritch remains hard-charging soldier, and in “Fire and Ash,” he gets his own female counterpart in Varang (Oona Chaplin) the witchy, wild queen of a warlike Na’vi tribe that wields fiery arrows and raids trader caravans for fun. When Quaritch places a flamethrower in Varang’s hands, the two are instantly locked in a lustful dance of destruction, becoming the dark doppelgangers of Jake and Neytiri.

The object of Quaritch’s bloody quest is Spider. He has attained the ability to breathe on Pandora thanks to a mysterious mycelium that Kiri calls upon with her own strange powers. The RDA mining operation begins a frenzied chase to track him down to study him, so that humans might one day inhabit Pandora. Spider becomes the most valuable and most dangerous character in the film, and Jake fears that keeping him around will endanger the entire planet.

Cameron relies on these family ties and easily understood emotions to navigate through the complicated plot, the lore of the Na’vi, and the vertiginous action sequences. The spectacle is absolutely mind-boggling — no surprise there, Cameron can still mount an action sequence that brings together military-grade weapons and fantastical flying creatures at the same time, and it somehow feels right. What also feels right? The extremely corny dialogue that we let him get away with, because it’s Cameron.

If “Way of Water” was Cameron’s nod to his own “Titanic,” with its watery environs and big sinking ship climax, “Fire and Ash” feels more like “Aliens” — and not just because Weaver delivers a riff on her iconic line from that movie’s climax.

It’s good to be back on Pandora.

(“Avatar: Fire and Ash” contains intense sequences of violence and action, bloody images, some strong language, thematic elements and suggestive material)

‘AVATAR: FIRE AND ASH’

Rated PG-13. At the AMC Boston Common, Causeway, South Bay, Alamo Drafthouse Seaport, Landmark Kendall Square and suburban theaters

Grade: A-

Neytiri, performed by Zoe Saldaña, left, and Jake Sully, performed by Sam Worthington, in a scene from “Avatar: Fire and Ash.” (20th Century Studios via AP)