Here’s everything you need to know about the film’s plot and its wild (and bloody) third act.

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

What is “The Substance” about?

The Substance follows Elisabeth Sparkle (Moore), a former A-list actress 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 in the last few decades of her career she has become best known as the host of a fitness TV show. It’s clear that she loves her job and derives much of her self-esteem from it.

Unfortunately, all of that is ruined when she is suddenly fired on her 50th birthday because her show’s misogynistic producer, Harvey (Dennis Quaid, remarkably ugly in the role), thinks she’s too old. He wants to find the next fresh young starlet to host the show.

Shortly thereafter, a distracted and desperate Elisabeth is involved in a car accident while watching her billboard being taken down. She only sustains minor injuries, but as she lies in hospital, an attractive young male nurse nudges her on the back and mysteriously mentions that she is a “good candidate.” (For what, he doesn’t say.)

The nurse gives Elisabeth a USB stick with the inscription “The Substance” and writes that it has changed his life.

Elisabeth plugs in the USB stick at home, and on it is a video revealing that The Substance is a mysterious “biological process” through which one can create – from one’s own body – a better, more perfect (i.e. younger) version of oneself. The details or potential downsides of this apparent metamorphosis are not made clear, and Elisabeth initially dismisses the procedure. But her depression over the decline of her career eventually leads her to order The Substance, which is delivered to Elisabeth (customer number 503) via a random drop-off location.

Elisabeth reads the instructions for the substance, a vial of neon yellow-green liquid that is to be injected. (This “activator” is also clearly marked as single use only, which will be important later.) According to the rules, once you inject the substance and transform, you must transform back into your original self every seven days. (Or what else? Unclear.)

Throwing caution to the wind, Elisabeth injects herself with the serum and begins to convulse as her body pushes her younger, more perfect self, played by Margaret Qualley, out of her split back. This horrific pseudo-birth leaves Elisabeth’s original self (Moore) catatonic on the floor, where Qualley’s character must hook her up to an IV of nutrients to keep her alive. However, there are only seven days’ worth of nutrients available.

First, the younger double has to go to the auditions for Elisabeth’s successor. By calling herself “Sue”, she impresses Harvey, who has no idea that Elisabeth and Sue are actually the same person. She gets the job and becomes the new presenter of the fitness show – but with the condition that she can only work every other week (because, according to her, she has to look after her sick mother).

Sue is getting the hang of maintaining herself on a daily basis, which requires her to “stabilize” herself once a day. Stabilizing means she uses a syringe to take fluid from Elisabeth’s spine and inject it into herself. If Sue doesn’t strictly stabilize herself once a day, she experiences side effects like nosebleeds and headaches.

Very quickly, Elisabeth and Sue begin to despise each other. Elisabeth, depressed while in control, and Sue, catatonic, sit around, overeating and drinking alcohol while watching TV, haunted by the ghost of her younger self (in the form of giant portraits hanging all over her apartment) and resenting Sue’s skyrocketing fame. Sue, in turn, feels that just seven days is not enough for her, and resents Elisabeth for wasting her own waking time. She even creates a secret room in her bathroom to hide Elisabeth’s catatonic body (and all of Elisabeth’s belongings) while she is in control – out of sight, out of mind.

Things go haywire (and gross) when Sue, in order to extend her control period and have sex with a guy she brought home, drains Elisabeth’s fluids for a day. When they switch again the next morning, Elisabeth is horrified to discover that one of her fingers has grown old and frail. This, it turns out, is the side effect of Sue draining Elisabeth’s fluids beyond the seven-day limit before the fluid has had a chance to properly regenerate.

Elisabeth is upset but unwilling to end the experiment, even when the mysterious voice on the other end of The Substance’s customer service center offers her the chance to do so (on the condition that the damage to her finger is irreversible).

When it’s Sue’s turn, she continues to push the boundaries, leaving Elisabeth a little older and more depressed and withdrawn. When Harvey finally tells Sue that she’s got a job as the host of the network’s live televised New Year’s Eve event, which will require intensive rehearsals, Sue decides to stay active for three months.

Sue’s good times with her boyfriend end when she tries to drain more stabilizer fluid and finds that Elisabeth isn’t producing any more. In a panic, she calls the customer service number, which leaves her with no choice but to switch back – the only way to regenerate the fluid Sue needs for stabilization.

She does so, and Elizabeth awakens to find that Sue’s life-force drain has turned her into an absolute monster – she is hunchbacked, with thin strands of hair and completely unrecognizable.

The end of “The Substance” explained

Finally, Elisabeth finds herself on the brink of collapse and calls the company that sent her The Substance. She says she wants to stop the experiment, even though she knows that the process that happened to her cannot be reversed and that she cannot restore her appearance.

The Company sends her a termination fluid to kill her doppelgänger while Sue is catatonic, but Elisabeth changes her mind right after the injection and revives Sue by injecting her own blood into Sue’s heart.

Sue wakes up, the connection is broken and the two face each other for the first time. When Sue realizes almost immediately that Elisabeth tried to kill her, she flies into a rage and brutally beats Elisabeth, practically trampling her to death in the apartment.

The only problem is, as The Substance’s instructional video makes very clear, the two are one. They can’t survive separately because, with Elizabeth dead, there’s no stabilizing fluid, and without stabilizing fluid, Sue can’t go on living. So (while she’s on her way to the big New Year’s Eve performance) Sue’s body starts to deteriorate: her teeth start to fall out and a nail breaks off.

Fighting to stay unharmed and without help from The Substance suppliers, a desperate Sue goes home and injects herself with leftover Substance fluid from months ago, with the goal of creating 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 it was for single use only. The monstrosity that emerges from Sue’s back is a deformed genetic mutation that combines parts of Sue and Elisabeth (called the “Monstro Elisasue”). The monster cannot speak intelligibly, but has Elisabeth’s original face on its back, frozen in a silent scream, amidst a pile of flesh and body parts.

The monster tears down a photo of Elisabeth’s younger face and pastes it on. Then it dresses itself up with carefully placed earrings and some makeup. Once it’s “ready,” it goes into the studio and attempts to perform the New Year’s Eve show in front of a live audience and amidst a crowd of topless dancing women.

The image fades away and the audience is horrified to see Elisasue – especially when the monster, trying to express that she is still the same Elisabeth, spits out a single disembodied breast on the stage.

The crowd is horrified and frightened and attacks the monster. It is decapitated, but its head regenerates and the creature begins to spray blood, drenching the audience and the stage.

Eventually, the monster escapes the studio and makes its way outside, where it collapses into a pile of body parts. Elisabeth’s original face on a pile of flesh is all that remains. The face wriggles away and comes to rest contentedly on its own star on the Walk of Fame before dissolving, the resulting goo being unceremoniously washed away by a street cleaner the next morning.

“The Substance” is now in theaters.