Like all of Jia Zhang-ke’s films, “Ash Is Purest White” is, at its basis, a tale of change in modern Chinese society, in this case using a sort of low-rent “Godfather” scenario as the plot catalyst. Rest assured, however, that it is no conventional gangland saga: it has the same ruminative, moody tone as the writer-director’s previous work.

In common with “Mountains May Depart,” the film has a tripartite structure. It begins in 2001, when its heroine, Qiao (Jia’s wife Zhao Tao), comes into the headquarters of her boyfriend Guo Bin (Liao Fan), a small-time provincial gang boss who enjoys settling disputes among his associates and the locals while holding court in a mahjong parlor. She’s a hard-bitten type who takes no guff from anybody, and joins the crew in a toast to their brotherhood and its code of conduct (something that the original title alludes to with the word “jianghu”).

But times are changing in the crime world, as in society as a whole. We see the larger reality when Qiao visits her father in their coal-mining hometown, where the industry is dying and the population moving away. Transition in her smaller, more closed world is portrayed when an older mob associate of Bin’s is killed, apparently by a gang that represents a new, less disciplined type of criminality, and then he’s assaulted, first by a pipe-wielding thug and then by a bunch of vicious motorcyclists. The person who saves him from the attackers is Qiao, who uses his illegal gun to fire a couple of warning shots that send the attackers packing.

Then she protects him further by claiming that the gun is hers, earning a five-year prison term by doing so. He doesn’t even visit her while she serves the sentence, and when she’s released, he’s disappeared.

That begins the second section of the film, set in 2006. Qiao travels the Yangtze past the Three Gorges region so prominent in Jia’s earlier “Still Life,” past towns soon to be submerged by the massive reservoir’s rising water, in search of Bin. Old acquaintances tell her that he’s moved on and doesn’t want to see her. Jia provides Qiao with a series of encounters during her odyssey that reveal, with touches of both humor and poignancy, her capacity to take charge. She’s robbed and must use her wiles to survive by conning philanderers and indulging in other minor scams. On a train she meets a blowhard entrepreneur who claims to be spearheading a project to set up tours of UFO-sighting sites but proves to be a fraud; she, on the other hand, experiences such a sighting herself. She uses her wiles to steal a motorcycle when she needs one. And she finally locates Bin, only to find a changed and desolate man.

The third act of “Ash” leaps to 2018, and Qiao and Bin have effected a reconciliation of sorts, but on very different terms than those of two decades earlier. She is now presiding over the mah-jong parlor, while he sits dejectedly in a corner a shadow of his former self, treated without the earlier respect from his colleagues. Yet Qiao, still true to the old code even in a society that has taken a sharp turn, holds to what she considers her responsibility even now. The English title refers to the pure ash produced by the tremendous heat emitted by active volcanoes, which acts as a symbol of the absolute fidelity to the old values Qiao maintains even after enduring two decades of wrenching change in her life—and the society around her.

“Ash is Purest White” is, like Jia’s earlier films, very much a personal reflection on what has been happening to modern China, employing gangster conventions—and sequences involving radical shifts in popular music, dance styles and outdoor entertainment—as an element of that rumination, rather than surrendering to them. It’s technically assured—with classically composed cinematography by Eric Gautier and editing by Mathieu Laclau and Lin Xudong entirely in sync with Jia’s lapidary rhythms.

The linchpin of the film, however, is the stunningly nuanced and vital performance by Zhao, who convincingly charts Qiao’s two-decade journey. She’s ably abetted by Liao, whose sharp features convey strength, but who’s equally effective as the broken shell he becomes in the picture’s latter stages. The supporting cast includes cameos by directors Feng Xiaogang, Diao Yinan, Zhang Yibai and Xu Zheng, which comes across as more a stunt that will appeal to buffs than an artistic necessity.

At nearly two-and-a-half hours with subtitles, this is hardly a film for the megaplex crowd. But it’s an important addition to the canon of one of today’s truly notable filmmakers.