Seven years after she rehabilitated the disreputable rape-revenge genre by leaning into its excesses with her debut Revenge, French writer/director Coralie Fargeat has made her English language debut with The Substance, bringing Demi Moore back into the spotlight with what might be her career best performance, and certainly her most daring.

Fargeat's second film is an initially ingenious body-horror that borrows elements from a variety of sources - The Picture of Dorian Gray, The Wasp Woman, All About Eve, Seconds, Tootsie - only to ultimately mash them into a mushy, inedible paste in a Frank Henenlotter blender. It's a love letter to b-movies, but one that is desperate to be seen as an important prestige production with "something to say", and at 140 minutes it could certainly learn a lot about economical storytelling from the filmmakers its aping (Roger Corman only needed 66 minutes to do all this with The Wasp Woman).

Fargeat opens with her film with a bravura sequence that displays exactly the sort of economy that's absent from the film that follows. From an overhead shot we watch a time-lapse of a star being laid on the Hollywood walk of fame, initially greeted by flashing camera bulbs and clamouring tourists, only to fade with the seasons as it becomes covered in ketchup stains. It's a sequence that only takes a minute to make the same point that will be stretched out over the following 139.

For at least half of those 139 minutes we're dazzled by Fargeat's ingenuity as she presents us with an idea that might borrow from previous sources but is wholly original. The walk of fame star belongs to our protagonist, Elizabeth Sparkle (Moore), a one-time Oscar winning actress who pivoted to hosting a morning fitness show (it's as though Jane Fonda had decided to quit acting and focus on her workout videos). On her 50th birthday Elizabeth receives the news from her on-the-nose-named sleazy producer Harvey (Dennis Quaid) that she is to be replaced. The network wants a younger woman to take over her show.

While being treated for injuries sustained in a car crash (she was distracted at the wheel by seeing her image being removed from a billboard), Elizabeth is examined by a shady doctor who suggests she's the perfect candidate for an experimental treatment called "The Substance." From a video contained on the USB the doctor presented her with, Elizabeth learns exactly what "The Substance" involves. It's a process that will allow her to give birth to a new, younger version of herself. The catch is that she can only inhabit this body every other week. So for one week she will be her 50-year-old self, the next week her younger self, and so on. Agreeing to take part, Elizabeth is directed to a warehouse hidden away in a seedy part of town, where a box awaits her. The box is a perfect parody of the sort you might receive from some hipster start-up company, containing basic instructions spelled out in massive fonts and a variety of tools and food packages. The initial birthing of Elizabeth's younger alter-ego (which adopts the name Sue and is played by Margaret Qualley) is a striking piece of body-horror, with Sue literally emerging from Elizabeth's body in an effect not unlike that seen in the recent Aussie horror The Demon Disorder.

Leaving Elisabeth's lifeless body on the bathroom floor, Sue heads out into the world determined to make the most of her new body. Pulling a Tootsie, Sue auditions for the role of Elizabeth's replacement and wins over the drooling Harvey, who not only gives her the job but agrees to allow her to take every other week off so she can "look after her sick mom."

What follows is a classic "Be careful what you wish for" horror narrative of the type you might find in the pages of an EC comic or as the segment of a horror anthology (it bears some similarities to the "Hair" portion of John Carpenter's Body Bags). Fargeat's storytelling is somewhat confusing on this point, but it eventually becomes clear that Sue has her own consciousness separate from Elizabeth, and resents having to regress to a limp figure every other week. This raises the question of what Elizabeth is getting from "The Substance" if she can't share any of Sue's experiences, or even remember them. Frustratingly, it's something the film refuses to address. As Sue rebels and overstays her time, Elizabeth's body begins to rapidly age like a fleshy portrait of Dorian Gray.

It's at this point that The Substance hits a narrative dead end as after setting up such an intriguing scenario, Fargeat fails to follow through in compelling fashion. The film devolves into clunky bedroom farce as Sue and Elizabeth try to conceal their alter-egos from various prying eyes, and in the final half hour it descends into Henelotter-esque body-horror that is so over the top it's unintentionally laughable. The movie's first 70 minutes fly past as it's laying out its novel scenario, but the remaining 70 minutes drag on interminably, which might itself be a meta commentary on the aging process.

The biggest problem with The Substance is the dated nature of its commentary on stardom. Its suggestion that aging stars are binned for younger models doesn't resonate in 2024, when Hollywood's top draws are still Tom, Nicole, Meryl, Brad et al, when Messi and Ronaldo are still the biggest names in football, and when music fans would rather fork out hundreds to see The Rolling Stones rather than take a chance on some up and coming music act. If anything, the problem with stardom today is that the old guard refuses to make way for new talent. Far from still being obsessed with unattainable beauty standards, we now put obese bodies on the covers of fitness magazines. Despite being set in the present day, the milieu of The Substance seems stuck in the 1980s, with no acknowledgement of how media is actually consumed in 2024. Wouldn't Sue be an online influencer rather than a TV host today?

It also doesn't help that Fargeat is often guilty of indulging in the very thing she's ostensibly critiquing. Her camera fetishes Qualley's young body, circling it with the same snaking movements she used so effectively to portray her Revenge heroine as a vengeful warrior, but she films Moore's naked form in a coldly dispassionate way that's keen to remove any sexuality (there isn't enough of a gulf in attractiveness between Moore and Qualley for this to be effective; they both have ridiculously enviable figures), and as the body-horror increases Fargeat asks us to be repulsed by the very idea of an aging female body; too much of the humour relies on us laughing at Elizabeth rather than the system of which she's a victim. At a certain point The Substance's obsession with highlighting its heroine's deteriorating form veers into ableism and it becomes a sort of anti-Elephant Man.

Ironically, the movie's most effective scene is its most grounded. Now aware of her deteriorating form, Elizabeth agrees to a date with a man to whom she earlier gave the brush off. All dressed up and looking a million dollars, Elizabeth catches sight of Sue's image on a giant billboard outside her window. Returning to her bathroom, she begins examining herself, suddenly cognisant of supposed "flaws" she hadn't previously noted, to the point where her self-worth is so shattered she cancels the date (with a man who wouldn't have given a hoot about any of her imagined imperfections). Amid all the rubbery effects and body-horror, it's this small moment that will resonate most with audiences (as a younger man I would often spend the week looking forward to a night out at the weekend, only to cancel it at the last minute having caught my unflattering reflection as I left home). When it comes to looking "our best," we're not competing with societal expectations so much as our own mirrors.

Directed by: Coralie Fargeat; Starring: Demi Moore, Margaret Qualley, Dennis Quaid, Hugo Diego Garcia