Demi Moore's grotesque new movie 'The Substance' has a shocking, bloody, and unhinged ending — and Oscar buzz

• Demi Moore stars as an aging Hollywood star named Elisabeth Sparkle in "The Substance."
• Ousted from her fitness show, she takes a serum to create a younger, "better" version of herself.
• The satirical body horror film goes off the rails in the third act. Here's what happens.

Demi Moore gives the wildest performance of her career in French filmmaker Coralie Fargeat's provocative new movie "The Substance."

The film has sustained buzz since its world premiere at Cannes Film Festival in May, where it competed for the Palme d'Or (the festival's biggest prize) and Fargeat won the best screenplay award.

Now that the film is in theaters, the buzz has reached a fever pitch. Moviegoers have taken to social media to share their reactions to seeing Moore and Qualley hash it out in this sharp, satirical fable about aging in Hollywood and women's internalized self-hatred — and the film's shocking ending is getting a lot of hype.

Here's what to know about the movie's plot, and that wild (and bloody) third act.

Warning: Major spoilers ahead for "The Substance," including the ending.

What is "The Substance" about?

"The Substance" centers on Elisabeth Sparkle (Moore), a former A-lister whose career is on the decline. Elisabeth won an Oscar early in her career and has a star on the Hollywood Walk of Fame, but for the last several decades of her career she's mostly become known for hosting a fitness TV show. It's clear that she loves her job and derives much of her self-worth from it.

Unfortunately, that all gets shot to hell when she's abruptly laid off on her 50th birthday because her show's misogynist producer, Harvey (Dennis Quaid, who's remarkably hideous in this role) thinks she's too old. He wants to find the next fresh young starlet to host the show.

Soon after, a distracted and distraught Elisabeth gets into a car accident when she sees her billboard being taken down. She sustains only minor injuries, but when she's in the hospital, a handsome young male nurse pokes at her back and mysteriously mentions she's a "good candidate." (For what, he doesn't say.)

The nurse slips Elisabeth a USB stick labeled "The Substance" with a note that says it changed his life.

Elisabeth plugs in the USB stick at home, and on it is a video that reveals The Substance is a mysterious "biological process" through which you can spawn — from your own body — a better, more perfect (i.e., younger) version of yourself. The details or possible drawbacks of this apparent metamorphosis aren't made clear, and Elisabeth initially writes the procedure off. But her depression over her declining career eventually leads her to order The Substance, which is delivered to Elisabeth (dubbed customer number 503) via a random drop-off location.

Elisabeth reads through the instructions for using The Substance, which is a vial of neon yellow-green liquid meant to be injected. (This "activator" is also prominently labeled single-use only, which will be important later.) According to the rules, once you inject The Substance and transform, you need to switch back to your original self every seven days. (Or else what? Unclear.)

Throwing caution to the wind, Elisabeth injects the serum into herself and convulses as her body creates, and then expels out of her split-open back, her younger, more perfect self, played by Margaret Qualley. This hideous pseudo-birth leaves Elisabeth's original self (Moore) catatonic on the floor, where Qualley's character needs to hook her up to an IV with nutrients to keep her alive. However, there are only seven days' worth of nutrients available.

The younger double's first order of business is to head over to the auditions for Elisabeth's replacement. Calling herself "Sue," she impresses Harvey, who has no idea Elisabeth and Sue are really the same person. She lands the gig, becoming the new host of the fitness show — even with the stipulation that she can only work every other week (which she says is because she has to care for her ailing mother).

Sue gets the hang of the daily process to maintain herself, which requires her to "stabilize" once a day. Stabilizing means collecting fluid via a syringe from Elisabeth's spine and injecting it into herself. If Sue doesn't stabilize strictly once a day, she starts getting side effects like nose bleeds and headaches.

Very quickly, Elisabeth and Sue begin to despise one another. Elisabeth, depressed while she's in control and Sue is catatonic, sits around overeating and drinking alcohol while watching TV, haunted by the specter of her younger self (in the form of giant portraits hanging all over her apartment) and resenting Sue's skyrocketing fame. Sue, meanwhile, feels that only seven days isn't enough for her, and resents Elisabeth for wasting her own awake time. She even creates a secret room inside her bathroom to hide away Elisabeth's catatonic body (and all of Elisabeth's things) while she's in control — out of sight, out of mind.

Things start to go south (and get gross) when, in a bid to extend her time in control to have sex with a guy she brought home, Sue extracts an extra day of fluid from Elisabeth. When they switch back the next morning, a horrified Elisabeth finds that one of her fingers has become old and decrepit. This, it turns out, is the side effect of Sue taking more fluid from Elisabeth beyond the seven-day limit, before the fluid had a chance to properly regenerate.

Elisabeth is upset but not willing to end the experience, even when the mysterious voice on the other line of The Substance's customer service center offers her the chance (with the stipulation that the damage to her finger is irreversible).

When it's Sue's turn, she keeps pushing the limit, causing Elisabeth to age slightly more and become more depressed and withdrawn. Eventually, when Harvey tells Sue that she's landed a job as the host of the network's New Year's Eve live televised event, which will require rigorous rehearsing, Sue decides to stay active for three straight months.

Sue's good time, complete with a boyfriend, comes to an end when she goes to extract more stabilizer fluid and realizes Elisabeth isn't producing any more. In a panic, she calls the customer service number, who offers her no other option but to switch back — the only way to regenerate the fluid needed for Sue to stabilize.

She does, and Elisabeth awakens to find that the effects of Sue draining her lifeforce have resulted in her becoming utterly monstrous — she's a hunchback, with wispy thin strands of hair, and completely unrecognizable.

"The Substance" ending, explained

Finally pushed to the brink, Elisabeth calls the company who sent her The Substance and says she wants to end the experience, even though she's aware there's no reversing the process of what's happened to her to restore her looks.

The company sends her a termination liquid to kill her double while Sue is catatonic, but Elisabeth changes her mind right after injecting it and resuscitates Sue by injecting her own blood into Sue's heart.

Sue wakes up, the connection severed, and the two come face to face for the first time. When Sue almost immediately realizes Elisabeth tried to kill her, she flies into a rage and brutally beats Elisabeth, essentially curb-stomping her to death in the apartment.

The only problem is, as The Substance's instructional video made very clear, the two are one. They can't survive separately because with Elisabeth dead, there's no more stabilizer fluid, and without stabilizer fluid, Sue can't continue to exist. So Sue's body (while she's en route to the big New Year's Eve performance) starts deteriorating: her teeth begin coming out, and a nail pops off.

As she's scrambling to stay in one piece, and without any help offered by The Substance supplier, a desperate Sue goes home and injects the leftover Substance liquid from months ago into herself. The goal is to spawn a new, better version of herself that can take over in time for the New Year's Eve show.

Unfortunately, they weren't kidding when they said single-use only. The resulting monstrosity that emerges from Sue's back is a deformed genetic mutation, combining parts of Sue and Elisabeth (dubbed "Monstro Elisasue"). The monster can't speak intelligibly but has Elisabeth's original face on its back, fixed in a silent scream, amid a bunch of flesh and body parts.

The monster rips down a photo of Elisabeth's younger face and tapes it on, then dolls itself up with carefully placed earrings and some makeup. Once "ready," it goes to the studio and tries to perform the New Year's Eve show in front of a live audience and amid a crowd of topless female dancers.

The picture slides off and onlookers are horrified when they see Elisasue — especially when the monster, who's trying to express that she's still the same Elisabeth, regurgitates a single disembodied breast on stage.

The crowd is aghast and terrified and attacks the monster. It's decapitated, but its head regenerates and the creature starts spraying blood, soaking the audience and the stage.

Eventually, the monster escapes the studio and makes its way outside, where it collapses into a heap of body parts. Elisabeth's original face on a heap of flesh is all that remains. The face squirms away and comes to a satisfied rest on her own walk of fame star before disintegrating, where the resultant goo is unceremoniously washed away by a street cleaner the next morning.

"The Substance" is in theaters now.