Elisabeth Sparkle, brilliantly portrayed by Demi Moore, is the dazzling star of a cherished aerobics television show. However, during her fiftieth birthday, she is confronted with a harsh truth: she has been fired from her job. The situation worsens as she watches her massive billboard come crashing down, which spirals into a tragic car accident that lands her in the hospital. Here, fate introduces her to a mysterious flash drive labeled "The Substance," containing revelations about a ground-breaking serum from an anonymous source, offering the promise of youth and extraordinary beauty. But there's a sinister twist: "[they] are one."
After weighing her options, Elisabeth boldly opts for the single-use "Activator", leading to a shocking transformation. A jagged split appears on her back and from it emerges a younger, captivating alter ego named Sue, played by Margaret Qualley. The enigmatic supplier provides explicit orders for Sue: she must receive daily injections of the "Stabilizer" serum, taken from the unconscious Elisabeth, in order to sustain her youthful appearance.
The two women must navigate a grueling seven-day cycle, alternating their consciousness; one must thrive in the outside world while the other sleeps. Sue quickly steps into Elisabeth’s shoes, reclaiming her role on the aerobics show and enjoying newfound fame and adoration. However, as Elisabeth is forced back into her life, she struggles with profound feelings of inadequacy and self-doubt.
As time goes on, the divide between these two entities grows ominous. Elisabeth, in despair, turns to binge eating and alcohol, while Sue becomes dangerously hooked on the Stabilizer serum, frantic to maintain control over her vibrant existence. This reckless dance only accelerates Elisabeth's descent into aging.
In a moment of desperation, Elisabeth reaches out to the supplier, who gives her a stark choice: follow the prescribed cycles or eliminate "The Substance" entirely. Conflicted, Elisabeth chooses to sneakily manipulate the situation, using the serum to extend her time as the vivacious Sue, all while hiding her own withering self-worth.
As weeks slip by, Elisabeth's decline becomes painfully evident. Sue's ever-growing need for Stabilizer leaves Elisabeth a mere husk of her former self. When an important New Year's Eve special looms, Sue acquires enough serum to prolong her youthful allure for three months; yet tragedy strikes when she runs out just before the big event, forcing a harrowing return to Elisabeth, who now appears grotesque and warped.
Pushed past her limits, Elisabeth decides to break free from "The Substance" once and for all. A lethal concoction known as “Termination” arrives, promising to eradicate Sue forever. However, at the last moment, Elisabeth, overcome by remorse, revives Sue, shattering their entangled fates.
Enraged at discovering a near-empty vial of Stabilizer—an emblem of Elisabeth's betrayal—Sue retaliates with brutal vengeance, taking Elisabeth's life before disappearing into the wild night to seize her moment on stage during the New Year’s Eve celebration.
The aftermath of Elisabeth's demise crushes Sue, culminating in her physical form's swift decay. In a frantic bid to preserve her existence, Sue disregards the supplier's warnings, attempting to manipulate the Activator to create a new version of herself. The result is nightmarish: "Monstro Elisasue," a monstrous blend of herself and Elisabeth, shocks the live audience with its grotesque visage. Chaos erupts among the horrified spectators, leading to a frenzied decapitation of the creature. Yet, in an unsettling turn, the severed head regenerates rapidly, erupting in a rain of blood. As the audience is enveloped in this horrific spectacle, Monstro Elisasue seizes its chance and escapes into the night, collapsing into a gruesome puddle of its own making.
Emerging from this twisted form, Elisabeth's face finds its way back to her neglected star on the Hollywood Walk of Fame. Finding solace in its familiarity, she positions herself upon it, merging with the star as she fades away. The next day, the remnants of this macabre scene are ruthlessly scrubbed away by a mechanical floor scrubber, erasing all traces of the bloody extravaganza.
