“The Substance” blowtorches a single idea for two hours and 20 minutes, and narratively it turns to ashes by minute 40. It’s easier for a movie to become a talking point that way. Film festival buzz, which “The Substance” enjoyed at its Cannes world premiere, tends to emanate from movies that dictate a point, early and often, rather than aim a lot of complementary or even contradictory ideas at each other.

The Cannes buzz regarding Demi Moore is a different matter. The film wouldn’t work at all without her, probably, and doesn’t really work with her, because she’s so much better than her material. The technique and sly emotional detail Moore brings to “The Substance” is the film’s substance.

French writer-director Coralie Fargeat has created a fairy tale of one woman’s miraculous if horrific career revival, with Moore portraying 50-year-old Elisabeth Sparkle, one-time Oscar winner and former A-lister, a survivor of a business built on the marketing of female flesh. For years, she has hosted a popular-ish aerobics fitness TV show. But the leg warmers suggest ’80s-era Jane Fonda, as does Elisabeth. Her producer — a manic sleaze played by a rip-roaring Dennis Quaid — decides to go in a different direction with the show. Somewhere fresher, younger, in the 18-to-30-year-old range.

Out of a job and alone in her glassy Hollywood hillside home, Elisabeth spies a newspaper ad promising a way to develop a brighter and tighter edition of herself. Her miraculous makeover uses a fountain-of-youth elixir, with a difference. After taking the substance, Elisabeth has Thing Two growing inside of her, emerging in a messy, painful birth as a fully formed and ready-to-go star in the making. The new arrival, nicknamed Sue, is played by Margaret Qualley.

Her new self lands the replacement host gig on the fitness program. Meantime the secretive stage management involving Elisabeth and Sue sharing the same planet is tricky at best, disastrous at worst. The two Elisabeths must stick to a strict weekly schedule, one week on, one off, taking turns stuffed in a closet, hooked up to a life support system (the movie grinds through a lot of tedious explanation here). This is filmmaker Fargeat’s most subtly damning notion: The patriarchy can’t possibly accommodate more than one side of any one female at a time.

Elisabeth the elder realizes that Sue the younger is having all the fun, or at least all the conspicuous admiration, so she starts messing around with the drug regimen. This turns “The Substance” into a cautionary tale of envy, in this case envy of self, which is a category of narcissism almost too rich for the industry Elisabeth knows so well. If the movie sounds strange, and bracing, well, the trailer makes it look that way, too. But it’s more of a plodder, asking the same two questions throughout. What price beauty? What price Hollywood?

Fargeat and her cinematographer, Benjamin Kracun, treat Hollywood as a sleek, scary collection of long hallways and fish-eye lensing. It’s a look, or a collection of looks, and influences, without a sharp sense of provocation beyond its body-horror explicitness. For investigations of female trauma in the name of beautification, and women’s bodies as both metaphor and vessels of desire, I’ll take Jennifer Reeder’s films any day. Or “Raw,” a terrific 2016 debut feature from Julia Ducournau that folds zombie cannibalism into a coming-of-age story set in a school of veterinary medicine.

The nudity in “The Substance,” frank, female-centered and utterly dispassionate, does not flinch or glamorize, at least not without a cost. In her director’s statement, Fargeat explains that her film is “about women’s bodies. About how women’s bodies are scrutinized, fantasized about, criticized in the public space … and how impossible it is for women to escape this, no matter how educated, strong-minded and independent we may be.”

Frustratingly, though, especially for a satirically tinged nightmare, there’s very little wit. Moore fills in the blanks with some compelling nonverbal passages of despair, or discovery of the new her, or jealousy on the cusp of revenge. She’s gripping in ways the rest of the picture is not, transcending the thesis points and comic exaggerations simply by playing against the comic extremes and holding a card or two, always, in reserve. She reminds us here how good, and tough, she is at her best, when she gets half a chance.

And now it’s time she gets a whole one.

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'THE SUBSTANCE'

2 stars (out of 4)

MPA rating: R (for strong bloody violent content, gore, graphic nudity and language)

Running time: 2:20

How to watch: In theaters Sept. 20

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