“I won’t cry, I promise,” vows the ethereal, long-living Maquia (voiced by Manaka Iwami) to the little mortal boy she is raising as her son. “I’m a mother!” she says, beating her fist lightly against her belly in a gesture of defiance that makes the boy smile. But mother or no, it’s a promise any viewer of this gorgeously rendered, acutely sentimental animated phantasmagoria would be foolhardy to make.

“Maquia: When the Promised Flower Blooms” is the directorial debut of prolific and successful anime screenwriter Mari Okada (“Anohana: The Flower We Saw That Day”), and though set in a medieval-styled, distinctly “Game of Thrones”-esque fantasy world of dying dragons, imprisoned princesses, warring kingdoms, and mystical cloth into which is woven the stories of our lives, the trembling, overflowing heart of the film is a story of motherhood, self-sacrifice, and forgiveness that is informed by Okada’s own fraught relationship with her mother. It is exquisite in every way — sometimes almost too exquisite in its precious sensitivity to the hardships of life as an outcast single mother — but against such intricate magic-hour backdrops, the only thing not beautiful here is the ugly-cry its devastating, happy-sad finale induces.

Maquia is a timid 15-year-old hailing from the Clan of the Separated, AKA the Iolph, an ancient enclave of blond-haired mystics who weave Hibiol, a fine, translucent cloth that marks the passage of time and contains messages only other Iolph can read. They are also blessed/cursed with extreme longevity, and Maquia, at 15, is as old as she is ever going to look. Lonely despite her friendship with the beautiful, adventurous Leilia (Ai Kayano), Maquia is cautioned by the tribal Elder that she will be lonelier still if she ever falls in love with an outsider, as she will be destined to vastly outlive her partner.

In another film, from a different storytelling tradition, perhaps, this would cue up some tragic love affair. But when the Iolph settlement is attacked by armored soldiers from the nearby kingdom of Mezarte riding ferocious flying dragons, and Maquia accidentally ends up miles from home with no way back, it’s not a romantic interest she encounters but a squalling baby, still clutched in his dead mother’s arms. Maquia resolves to raise him, though she’s still a child herself, and calls him Eriel (Yuuki Sakurai).

The first half of the film mostly deals with the hardships Maquia faces as a child-mother, though she’s befriended by kind young widow Mido (Rina Sato) who is raising two little boys of her own, who become Eriel’s playmates. But this idyllic interlude must end, as the villagers are beginning to note with suspicion that while Eriel is growing into a normal, rambunctious kid, Maquia (her telltale blonde hair dyed red) stays as youthful as she was when they arrived. And so the pair settle into a rhythm of moving every few years, sustained by mutual devotion that is only really tested when Eriel reaches those difficult teen years, begins to understand that Maquia is not his “real” mother and now has to pretend they are brother and sister instead.

At times, Maquia can be a little too drippy a character to really invest in. One can grow a little impatient with the close-ups on her expressions of puzzlement, her naive enormous eyes, non-existent nose, and tiny, tremulous chin. But if at times the prettiness of the film’s core message about motherly sacrifice becomes a bit cloying, there are surprisingly rich subplots that add life and texture. In particular, a parallel story unfolds about Leilia, kidnapped during that same attack and forced to marry the Prince of Mezarte in the belief their children will have the Iolph longevity. Her captivity is echoed in the story of the dragons, themselves the last remnants of a once-proud race who have dwindled in numbers having been, essentially, enslaved by the Mezarte warriors. It amounts to some cunningly persuasive commentary on the exploitation and subjugation of other races by colonizing powers, as well as a critique of internecine warfare which is particularly provocative given Japan’s thorny history of militarism.

And of course, those bellicose, masculine ideals provide neat counterpoint to the nurturing, yielding yet tenacious womanhood that is glorified here, in artwork so sumptuous, under a recurring musical theme (from Kenji Kawaii) so epic that it sweeps us through the more shamelessly manipulative moments. Indeed, it’s possible that the film’s passing pleasures are so rich that we don’t even notice how deep Okada has driven her storytelling dagger until she pulls it out in the end, and the tears come, adding, to the bitterness and sweetness of this moving and strange little fable, a hefty dose of salt.